Bean Leaf Beetle, Pests, Soybean, Integrated Pest Management, IPM Field Crops, Purdue University
Bean Leaf Beetle
- 1 Bean Leaf Beetle
- 2 Cerotoma trifurcata Forster
- 3 Insect Pests of Canola
- 4 The Crop
- 5 Crucifer Flea Beetle and Striped Flea Beetle
- 6 Bertha Armyworm
- 7 Lygus Bugs
- 8 Diamondback Moth
- 9 Emerging IPM Issues
Cerotoma trifurcata Forster
Appearance and Life History
The bean leaf beetle can be a destructive pest of soybean in the Midwest. This beetle is native to North America and was first described as a pest of edible beans when the continent was first colonized. As soybean production increased, the bean leaf beetle adapted to feeding on soybean foliage and pods.
Bean leaf beetle adults are found in soybean throughout most of the season causing damage to soybean foliage and/or developing pods. They are about 1/4 inch (6 mm) long and usually yellow, tan, or red in color. The beetle is distinguished by a distinct black triangular mark on its wing covers directly behind the thorax (the «neck» area of the insect). Often two black spots are present on each wing cover and a distinct black band extends around the outer edges. Sometimes the wing covers have no markings, except for the black triangle. When disturbed, the bean leaf beetle typically folds its legs and falls to the ground where it may remain motionless for some time.
The bean leaf beetle larva is white and distinctly segmented, with a brown head and a brown hardened area at the posterior end of its body. To the naked eye, it looks almost identical to the corn rootworm larva. It feeds on soybean nitrogen fixing nodules and, to a lesser degree, soybean roots.
A heavy bean leaf beetle infestation may cause serious leaf and pod damage. Overwintering adults colonize early emerging soybean, feeding on the cotyledons, stems, unifoliate leaves, and emerging trifoliolates. Stand losses are possible in soybean at this stage of growth. First and second generation beetles chew small round holes in soybean leaves during mid and late season. Once the maturing foliage becomes less attractive to beetles, they feed on the green tissue of pods, leaving a thin membrane over the seed. During pod maturation, this membrane often cracks leaving an entry hole for air borne plant pathogens. These pathogens may cause discolored, moldy, shriveled, and/or diseased beans.
As noted above, the larvae feed on root nodules and girdle roots. The amount of damage that normally occurs has not been shown to significantly reduce yields. Therefore, no specific sampling technique has been developed to determine larval population levels.
There are 3 important times to evaluate fields for bean leaf beetle activity:
- Early in the season it is important to determine the amount of cotyledon, stem, and foliage feeding damage caused by the overwintering adults. Since plants are small at this time, it’s easy to determine the presence of beetles and their damage. While scouting, determine the degree of cotyledon or stem damage and/or the average percentage defoliation level for plants in each of 5 areas of the field and note the number of beetles per plant. Also, note whether the growing point is being severely damaged or killed on any of the plants. Watch for areas where replanting may be necessary due to seriously damaged or dead plants.
- Mid and late season defoliation from first and second generation beetles must be assessed to determine whether a control application is necessary. Determine the percentage defoliation level for individual plants in 5 separate areas of the field and estimate the average percentage defoliation level for the field as a whole. Assess the beetle population, and other defoliators, in these same areas to determine their density and activity. It is important to approach the sampling area with caution since the beetles will drop from the foliage to the ground and hide with the least bit of plant movement. Mid-morning (after the dew has dried from the foliage) and early evening sampling will give an accurate picture of the population since beetles seek cooler areas such as cracks in the ground during the heat of the day.
- Late in the season observe whether beetles are feeding on the pods. This damage results in a white paper thin membrane which covers the seed. Seldom does feeding damage extend into the seed. Do not confuse this with grasshopper damage which usually extends through the pod to the seeds. Randomly select 2 plants in each of 5 areas of the field and count the number of pods per plant and the number that show insect damage (10 total plants). Figure the percentage of damaged pods per plant for the field as a whole. Note if the pods are green, beginning to turn yellow, or are yellow/brown. Also determine the number of beetles per sweep using an insect sweep net. Take 5 sets of 20 sweeps in the field. Determine the number of bean leaf beetles per sweep. Additionally, note whether beetles are still actively feeding while surveying the field.
Soybean Insect Control Recommendations: E-series 77-W (PDF)
- For VE — VC stage soybean, refer to the following thresholds:
- For defoliation in V1 — R6 stage soybean, consult the following table; also take into consideration whether the number of beetles present is increasing, decreasing, or appears to be constant.
- For pod feeding in R5 — R7 soybean, see the following table:
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.
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Insect Pests of Canola
Michael J. Weiss 1 , Janet J. Knodel 2 and Denise Olson 2
1 Technical Service Agronomist, Golden Harvest Seeds, Decorah, IA
2 Department of Entomology North Dakota State University, Fargo, ND
Oilseed rape is the term used for three major species of Brassica, B. rapa L. (campestris), B. juncea (L.) Czern., and B. napus L. There are both spring and winter types, with the spring type planted primarily in the Northern Great Plains of North America and the winter types planted in the Mid-West, southern and Pacific Northwest of the United States. The oil from oilseed rape has been used for greater than two millennia as lamp oil, industrial lubricant, and more recently, as edible oil. The transformation to edible oil was accomplished when Keith Downey and Baldur Stefansson in Canada identified a natural occurring mutation that produced a seed-oil that was low in glucosinolates and the fatty acid, euric acid. Oil and subsequent meal with high levels of glucosinolates and euric acid reduces palatability and/or are toxic to some animals, and euric acid has been shown to be a potential health hazard to humans. Cultivars that exhibit low levels of both glucosinolates and euric acid are often referred to as double low oilseed rape or canola. The word canola is a backronym from CANadian Oil Low Acid. Today, most of the canola planted in North America is hybrids of the spring-type of B. napus which usually reaches maturity in 95 days. The majority of the United States acreage of canola is in North Dakota, Minnesota, Montana and the Pacific Northwest, and in the Prairie Provinces of Canada.
Canola is usually grown in rotation with small grain crops and because of the threat of disease (e.g., blackleg) it is recommended that it be produced in a rotation with other non-host crops. In addition, rotation with crops susceptible to white mold (e.g., sunflower, dry pea, dry bean) is not recommended as canola is susceptible to white mold infection as well. For spring types, planting usually occurs in late April to mid-May. Early planting is highly recommended because canola is very susceptible to heat and drought stress during flowering, with yields being reduced if planted after mid-May. Canola is frost tolerant with the plant being able to withstand temperatures of -4 °C (25°F) . As a rule of thumb, canola should be seeded between 5.6 and 9 kg/ha (5 and 8 lbs/acre), to establish a plant stand of 175 plants/m2 (16 plants/ft2) or 1.5 million plants/ha (600,000 plants/acre). Oilseed rape is a very compensatory plant with respect to yield and plant stand, often lower plant populations will yield equal to that of higher plant populations; however, competition from weeds can be more severe in lower plant densities. Sulfur can be a limiting nutrient in canola production and a soil test for sulfur is highly recommended. Good weed management is essential, as canola seedlings are very susceptible to competition from weeds. In addition, wild mustard can be a very serious contaminate of canola and producers can receive significant discounts if the canola is highly contaminated. In the northern Great Plains, the crop is usually swathed prior to harvesting. Swathing is recommended when 30 to 40% of the seeds on the main stem have turned from green to brown to reduce shattering.
The major insect pests of canola in the United States are the cabbage seedpod weevil (CBSW), Ceutorhynchus assimilis (Paykull) (syn. Ceutorhynchus obstrictus [Marsham]), Bertha armyworm (BAW), Mamestra configurata Walker, the flea beetles; crucifer flea beetle, Phyllotreta cruciferae Goeze and striped flea beetle, Phyllotreta striolata (F.), the tarnished plant bug (TPB) Lygus lineolaris (Palisot de Beauvois), and diamondback moth (DBM), Plutella xylostella (Linnaeus). Minor pests include aphids; cabbage aphid, Brevicoryne brassicae (Linnaeus) and turnip aphid, Hyadaphis erysimi (Kaltenbach) and grasshoppers.
Introduced from Europe to British Columbia approximately 70 years ago, it is currently found in North America in the Pacific Northwest to Saskatchewan and northern Montana and in the East (Ontario) and South (Georgia), it has not been reported in the canola production areas of North Dakota or Minnesota; although climate prediction models indicate that the cabbage seedpod weevil can survive in the prairie providences of Manitoba. The adult is light-grey and approximately 3 to 4 mm (1/6 in) long.
Cabbage Seedpod Weevil. Photo courtesy the Canola Council of Canada
Adults migrate in September and late November to sheltered areas where they overwinter in the soil under plant debris. In the spring, the adults emerge from wintering sites over several weeks when soil temperatures rise above 12°C (54°F), and can be found on early flowering hosts (wild mustard, flixweed, hoary cress, stinkweed, and volunteer canola). Weevils move to canola fields when the crop is in the bud to early flower stage and fed on pollen and buds. After a pollen meal, mating occurs on the plant. When small pods develop, the female will deposit an egg through the pod wall onto, or adjacent to a developing seed. In areas where both winter and spring canola are grown (Pacific Northwest), the adult weevils move, as flowering progresses, from the winter to spring canola. Adults are found most commonly on the canola buds and flowers, but during windy days, they move to sheltered areas within the plant canopy
The egg is very small, oval, and opaque white. Most often, only a single egg is deposited per pod; however, two or more eggs can be laid per pod when cabbage seedpod weevil densities are high. Eggs hatch in about six or seven days. Larvae are white and grub-like and consist of four larval instars. The first instar larva feeds on the cuticle on the outside of the pod. The second instar bores into the pod and feeds on the developing seeds. Larval development takes approximately six weeks, and during this time, a single larva consumes about five canola seeds.
Cabbage Seedpod Weevil larva. Photo courtesy the Canola Council of Canada
Mature larvae chew small, circular exit holes in the pod walls and drop to the soil surface, burrow in, and pupate within earthen cells. New generation adults emerge about 10 days later and feed on immature canola or other green cruciferous plants until late in the season when they migrate to overwintering sites. In some areas there are two generations per year. Second generation adults feed on green pods by penetrating the pod wall with their snout and sucking out the seed tissue. Late flowering spring canola is prone to damage from this summer generation of weevils.
The cabbage seedpod weevil is a pest of both fall and spring planted canola. Crop injury is caused by adults feeding on buds, reducing yield potential, particularly in dry years when the canola is stressed and can not compensate for bud loss. Larval feeding on the seeds is the most severe type of injury and infested pods are more prone to shattering that causes seeds to be un-harvestable. Infested pods are often misshapen as a result of the larval feeding. Yields decrease by about 1.7% for each percentage point above 23% infested pods, with pod infestation rates of 22% or below not representing a measurable yield loss. Research on a winter type of canola in the southeastern United States indicated that pods infested with one larva per pod reduced seed weight by about 20% on a per pod basis, and three larvae per pod reduced seed weight by about 52% on per pod basis. When second generation adults feed directly on the canola seeds, the damage response by the plant is reflected in reduced seed yield, seed oil content, seed weight, and seed germination. Economic losses of up to 70% of the potential yield have been reported for second generation feeding and losses of 20 to 40% are common.
Preventative Management Tactics
A high number of adults in the fall may indicate the potential for economic infestations the following spring. A trap crop border of an early maturing Brassica (e.g. B. rapa) can be planted seven to ten days prior to the canola crop to attract and concentrate adult weevils in the field borders. The trap crop should be monitored for the presence of weevils and when weevil populations are high an insecticide can be applied to the trap crop prior to bud formation in the canola field. Although a parasitoid, Microctonus melanopus (Ruthe), has been found to attack adult weevils, the impact of the parasitoid as a control agent has not been quantified.
Responsive Management Tactics
Scouting for the cabbage seedpod weevil when the crop is at the bud stage through flowering is critical for successful management of this insect pest. Infested fields are often noticeable when flocks of birds are present foraging on the adult weevils. Sweep net samples should be taken at ten locations within the field with ten 180° sweeps per location. Samples should be taken in the field perimeter as well as through out the field. Adults will invade fields from the margins and if infestations are high in the borders, application of an insecticide to the field margins may be effective in reducing the population to levels below which economic injury will occur. An additional benefit to monitoring for cabbage seedpod weevil is that Lygus bugs can be sampled, provided the sampling continues thorough the early pod stage. An insecticide application is recommended when three to four weevils per sweep are collected. Fields with a high yield potential will warrant control at the lower end of this range. Timing of the insecticide when the crop is in the 10 to 20% bloom stage (2-4 days after flowering starts) has been shown to be the most effective. To reduce the impact on pollinators, consider making insecticide applications late in the day.
Crucifer Flea Beetle and Striped Flea Beetle
Description and Life History
The crucifer flea beetle was introduced from Eurasia in the 1920’s and the striped flea beetle was introduced in the early 1800’s. Both species are small, elliptical or oval-shaped and less than 2.5 mm (0.1 in) long. Adult crucifer flea beetles are uniformly black with a metallic bluish sheen.
Crucifer Flea Beetle. Photo courtesy the Canola Council of Canada
The elytra are randomly punctuated and the enlarged hind legs are a dark amber color. The striped flea beetle has two pale yellow strips on the elytra.
Both species have a single generation per year. However, two adult populations occur during a single growing season, overwintering or spring population, and the summer generation. Development from egg to adult takes about seven weeks. Flea beetles overwinter as adults on the soil surface under the leaf litter, grass and debris in sheltered areas and close to last year’s canola stubble and volunteer cruciferous plants. The overwintering adults begin to emerge as the mean ground temperature rises to 10-12°C (50-54°F) and emergence peaks when soil temperatures reach 14-15°C (57-59°F). Depending on temperature fluctuations, emergence can occur over a three to four week period. The striped flea beetle emerges slightly before the crucifer flea beetle.
Under cool conditions, flea beetles walk or hop to host plants (e.g., cruciferous crops or weeds). As air temperatures reach 10°C (50°F), they will move to canola fields. When temperatures exceed 14°C (57°F) (early to mid-May) and wind speed is minimal, adults will disperse throughout the entire field. During cool and windy conditions flea beetle flight activity is reduced and the beetles will be found concentrated at the field margins. Upon feeding the males produce an aggregation pheromone, found in the frass, attracting large numbers of both sexes. Mating occurs after adults locate host plants. Oviposition usually begins in late May and continues for about 30 days. Females deposit an average of 100 yellow elongated eggs, singly or in small groups in the soil near host plant roots. Soil moisture is very important in maintaining egg viability. Egg hatch occurs in about 10 to 12 days.
Larvae are dull white with a brown head and anal plate. There are three instars and the larvae complete development in approximately 30 days. Larvae feed on host plant roots and pupate in earthen cells. Pupae can be found in the soil during July and are a bright white, with dark eye spots that darken as development progresses. The pupal stage is usually completed in one week. Emergence of the new or summer generation adults occurs from mid-July until early September. These adults feed on the canola pods and stems causing pod shattering and green seeds in pods when populations are high. After feeding, adult beetles begin to move into overwintering sites during August and September.
The most significant injury and subsequent yield loss is caused by adults in the early spring (May) when the plants are in the cotyledon to the two-leaf stage. High populations can result in significant reductions in plant density and reduced plant growth resulting in uneven development later in the season and lower seed yields. Adults feed on surface of the cotyledons and true leaves resulting in a pitting appearance on the surface. Margins of the feeding pits turn necrotic, causing a shot-hole appearance. Larval feeding has been reported to cause yield loss (about 5%) when larval densities reach 1 per square inch.
Crucifer Flea Beetle injury. Photo courtesy the Canola Council of Canada
The summer generation of adults can reach extremely high densities on late developing canola; however, economic injury during this period in the production season is usually negligible.
Preventative Management Tactics
Flea beetle infestations are highest in conventional, high disturbance tillage systems and less in no tillage or minimal disturbance tillage systems. Flea beetles prefer warm, dry soil conditions. These conditions commonly occur with cultivation and high soil disturbance tillage systems.
Canola that is planted as early as possible may germinate and establish before the hibernating beetles emerge and, larger seedlings are more tolerant to injury then smaller plants. Although later spring-planted canola also results in lower feeding injury from flea beetles, its use may have limited potential for flea beetle management, because canola planted in late May typically has lower yields due to the susceptibility of the canola crop to heat stress during flower development. The planting rate can be increased to the maximum recommended rate to mitigate yield loss due to flea beetle injury because canola can compensate for reduction in plant density early in the growing season. In addition, planting using wider row spacing has been shown to lower injury on a per plant basis.
Good weed control of cruciferous weeds or volunteer canola in canola fields after the crop is in the fourth true leaf stage may reduce the migration of the flea beetles to the canola crop. Early planted trap strips (volunteer canola or dormant seeded canola) near overwintering sites may be effective in attracting adults. A foliar insecticide can be applied to the trap crop prior to adults migrating to canola fields which may result in lower densities in the canola crop.
Dormant planting (planting in the fall when soil temperatures are below the germination threshold for canola) has reduced the need for a foliar application of an insecticide as the plant is significantly larger (true leaf stage), then if the crop is planted in the spring (cotyledon stage), when the flea beetles emerge from overwintering sites. Dormant seeding is a high-risk practice in areas with unstable winter temperatures and soils, which have the potential to dry out significantly during the winter. The greatest challenge has been getting even and well-established stands.
Responsive Management Tactics
The overwintered flea beetle populations do not reach economically damaging densities every year and there is no method available for predicting densities that will result in economic injury. Summer flea beetle populations that are extremely high can indicate that an insecticide seed treatment may be warranted as a precautionary measure against potential economic feeding injury in next year’s crop. Because spring flea beetle feeding injury can be significant, can occur over a very short time period, and can not be predicted prior to planting, insecticide seed treatment is the most common management tactic against the flea beetles. In areas where flea beetle population levels are high, populations of late emerging flea beetles are high, or spring feeding periods are extended due to cool weather conditions, the residue of seed treatments can be diminished and significant feeding injury can occur. In these cases, a foliar insecticide in addition to the seed treatment may be necessary to reduce further feeding injury and subsequent yield loss.
Flea beetles are most active during the warm period of the day when temperatures are 14°C (57°F) or above, and this is the best time to sample for adults. On a daily basis while the canola crop is in the cotyledon to four leaf stage, plants should be checked at random across the field in several locations for shot-holing or pitting in the cotyledons and first true leaves. Adult flea beetles are usually found at higher populations along the field margins and in areas within the field that are adjacent to overwintering sites and these areas should be checked as an early warning of the extent of the potential injury. Although canola seedlings can tolerate up to 50% tissue loss, flea beetle populations can increase quickly; therefore, intervention with a foliar insecticide is recommended when defoliation is 25% and adults are present. Intervention may be necessary at less than 25% defoliation if conditions are hot and dry. Canola plants growing in these conditions are stressed and more susceptible to flea beetle feeding injury.
Description and Life History
The Bertha armyworm is a native species and occurs throughout the Northern Great Plains of North America. In most years, population densities are relatively low as a result of weather conditions and natural enemies; however, outbreaks can occur and densities can be relatively high over large areas.
The moth has a wing span of about 4 cm (1.5 in) with the forewing being predominantly gray, and with patches of black, brown, olive and white scales. Near the middle of the forewing, towards the leading wing margin (front), there is a white, kidney-shaped marking defined with a ring of white scales. Near the tip of the forewing, there is a conspicuous white and olive-colored, irregular transverse marking.
Bertha Armyworm. Photo courtesy the Canola Council of Canada
Eggs are deposited in clusters of about 50 to 500 eggs on the lower surface of the host plant leaves. When first deposited, they are pale white but become darker as they mature with hatching usually occurring within a week.
Bertha Armyworm eggs. Photo courtesy the Canola Council of Canada
The larvae have six instars and, depending on temperatures, complete development in about six weeks. Newly enclosed larvae are about 0.3 cm (1/10 in) long and are pale green with a pale yellow stripe along each side. Upon disturbance, the small larvae use a silk thread to drop off the leaves. Young diamondback moth larvae exhibit a similar behavior, and therefore, this pest is often times mistakenly identified as the Bertha armyworm. In the later instars, the larval color becomes extremely variable, ranging from green, brown to black, The final instar is about 4 cm (1.5 in) long, with a light brown head and a broad, yellowish-orange stripe along each side. The black larvae have three narrow, broken white lines on the dorsal.
Bertha Armyworm larvae. Photo courtesy the Canola Council of Canada
The final instar burrows into the soil, to a depth of 5 to 15 cm (2 to 6 in) and pupates. Reddish brown pupae can be found in the soil in mid- to late August. Adults first emerge in mid- to late June and continue until early August. Adult moths are attracted to blooming fields to feed on pollen. Mating occurs within a 5 day period after emergence and the average fecundity is about 2000 eggs per female.
Environmental conditions can have a significant impact on Bertha armyworm densities. During winters when snowfall is minimal and soil temperatures drop below 10°C (50°F), significant pupal mortality can occur. In areas where reduced and no-till systems are practiced, more snow accumulates on the soil thus insulating the pupae and increasing survival.
Larvae feed on a variety of broad leaf crops and weeds. Canola, mustard, alfalfa, lamb’s quarters and related plants are preferred, and larvae have been found to attack flax, dry peas and potato. The impact of injury is dependent on the growth stage of the plant in relation to the developmental stage of the larvae. In most cases, the most significant injury occurs between late July and late August, depending on the geographical location.
Early larval instars feed on the underside of the leaves, chewing irregularly-shaped holes in the leaves, with this injury being minimal. The most significant injury occurs during the fifth and sixth larval instars, when the larvae consume 80 to 90% of the needed plant material to complete their development.
Bertha Armyworm early larval instars. Photo courtesy the Canola Council of Canada
If the plant begins to senescence with onset of leaf drop and larvae have not completed development, larvae will feed on the seed pods. Larvae typically chew holes in the pod and consume the immature seeds, but at high densities the entire pod will be consumed. Only the slightest feeding on the pod epidermis can cause significant yield loss as pods may shatter prematurely.
Preventative Management Tactics
Larval density is not a reliable predictor of population levels in the next growing season. Because, the impact of harsh winters and natural enemies (parasitoids and diseases) on larval populations can vary from extremely high in one year to very low the following year. During outbreak cycles, consideration should be given to planting alternative crops (e.g., small grains), controlling host weeds such as lamb’s quarter, early swathing of canola, and fall cultivation to destroy overwintering pupae.
Direct destruction of pupae can be accomplished with fall tillage to reduce the stubble and decrease the accumulation of snow, thus exposing pupae to more severe winter temperatures resulting in greater mortality. Since adults can fly great distances, tillage over the infested area will be more effective than tillage over single or isolated fields. However, tillage can result in wind erosion and less available soil moisture and these effects should be considered. The two major parasitoids of the bertha armyworm are the tachinid, Athrycia cinerea (Coquillett) and the ichneumonid, Banchus flavescens Cresson.
Responsive Management Tactics
Monitoring for the adults has been used to develop risk indices for damaging larval populations. Pheromone traps (unit trap bucket design) are used to monitor adults during a six-week flight period of mid-June through mid-July. When moth populations range from 0 to 300, the risk of a larval infestation is low; 300 to 900, the risk is uncertain, but fields that are flowering during peak flights should be checked; 900 to 1200, the risk is moderate and fields should be checked for larvae and plant injury; greater than 1200, there is a very high risk of an economic larvae infestation. All canola fields in a potential infestation area should be checked for larvae and their injury beginning two weeks after peak moth captures as indicated by the pheromone traps, as infestations can be variable between fields.
Sample at least three locations within a field with a minimum of 50 m (50 yards) between samples. Samples should be taken at least 50 m (50 yards) from the field borders and through out the entire field. Field edges and areas within the crop that are not representative of the field should not be sampled. Beat the plants within a 1 m2 (1 yd 2 ) area to dislodge larvae and search the litter for the larvae.
A larval density of 20/m 2 (17/yd 2 ) will reduce yields by 65 kg/ha (58 lbs/acre), thus a treatment guideline can be determined using the estimated market price per kg or pound and the cost of an insecticide plus application. For example, an estimated market price of $0.22/kg ($0.10/pound) and a larval density of 20/m 2 (17/yd 2 ) would result in a $14.80/ha ($5.80/acre) loss. When the treatment costs exceed the expected loss, an insecticide application would not be economically warranted. If an insecticide is warranted, make the application during the later part of the day if the crop is in the bloom stage to reduce honeybee mortality and apply high volumes of water when the canopy is dense to ensure coverage and penetration into the canopy.
Description and Life History
The tarnished plant bug is the most common pest species of the several species belonging to the genus Lygus that attack canola. Tarnished plant bug adults are about 5 mm (1/4 in) long and 2.5 mm (1/8 in) wide, oval, and slightly flattened. Variation in color ranges from a pale dull green to reddish brown with a “V” marking about one-third of the distance down the dorsum, just in front of the wings.
Lygus Bug. Photo courtesy the Canola Council of Canada
The young nymphs are pale-green in color while the older nymphs are more variable in color, similar to the adult. Young nymphs can be confused for aphids. Unlike aphids, Lygus bugs are active and move rapidly when disturbed.
Lygus Bug nymph. Photo courtesy the Canola Council of Canada
Lygus bugs have a wide host range and feed on many weeds (e.g., chickweed, red root pigweed) and crops (e.g., alfalfa, canola). Adults overwinter in plant debris along fence rows and shelterbelts. Adults begin emerging from overwintering sites in mid-April to late May and begin feeding on weed hosts. The eggs are deposited on the stems and petioles of the host plants. Hatching begins about 10 days after oviposition. The first generation adults appear in mid-June to mid-July and disperse to host crops or new weed hosts. Second generation nymphs feed on the host plants during August. Adults produced by this generation emerge in late August and September and during some years a third generation may emerge in late September.
Adults and nymphs cause plant injury by injecting toxic saliva into the plant tissues and causing blasting of flowers or buds, and shriveled seeds. Both will feed on all plant parts, but economic injury occurs when feeding is focused on the reproductive parts of the plant. Blasted flowers turn white within 24 hours and quickly fall to the ground. The small and damaged seeds are lost during harvest. The most significant injury occurs during the flowering to early pod stage. Under high population densities yield losses can approach 10 to 35%.
Preventative Management Tactics
Lygus populations fluctuate widely and are difficult to forecast. Early sampling of weed hosts can be used to obtain a relative population density. Usually in years with above average spring moisture, allowing for maximum growth of weed hosts, populations can be higher then in years where spring moisture is limited. Reduction of weedy areas that are adjacent to cropping fields may reduce overwintering sites for the adults.
Responsive Management Tactics
Sampling is recommended from flower bud formation to early pod development during late June through July to assess the risk of economic injury. Sampling earlier then this period is usually ineffective as the majority of the adults and nymphs are lower in the plant canopy, and capturing with a sweep net is almost impossible. During flowering to early pod development, both nymphs and adults are feeding in the upper canopy on the developing flower buds or seeds in the pods. Treatment with an insecticide is usually recommended when an average of 15 to 20 bugs (nymphs or adults) are captured per 10 sweep net sample taken in ten locations in the field (150 to 200 bugs per 100 sweeps). If soil moisture is good, canola plants usually compensate for Lygus bug feeding injury in the flowering stages. However, if populations are high, control during the early pod ripening stage usually is the most economical.
Description and Life History
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The migrating diamondback moth usually arrives in late May or early June in northern Great Plains. Its life cycle is about 32 days from egg to adult. There are several generations during a single growing season, so different life stages (eggs, larvae, pupae, adults) can be found in the field at the same time. The adult is small, about 1.3 cm (½ inch) long, drab brown in color and at rest the forewings of the male moth form three diamonds — hence the name diamondback.
Diamondback Moth. Photo courtesy the Canola Council of Canada
Females lay their eggs during the night and can lay up to 160 eggs during their life period. Eggs hatch in five to six days into pale yellowish-green caterpillars with a forked posterior. The newly emerged larvae burrow into the leaf and mine the leaf for several days to a week. Then, the larvae exit the leaf and feed externally for an additional 7 to 14 days.
Diamondback Moth larva. Photo courtesy the Canola Council of Canada
When disturbed, the larvae violently thrash backwards and often drop from the plant using a strand of silk. The larvae pupate for 5 to 15 days in a white net-like cocoon attached to the leaves, stems or pods.
Diamondback Moth pupae. Photo courtesy the Canola Council of Canada
Larvae feed on the leaves, buds, flowers, seed pods, the green outer layer of the stems, and occasionally, the developing seeds. The amount of damage will depend on the crop stage and the larva densities and size. Extensive feeding on the flowers causes flowers abort, delayed plant maturity, uneven crop development, and reduced seed yield. As leaves wilt and drop in late July to early August, the larvae feed on the stem, pods and developing seeds. Damaged seeds will not fill completely and may shatter. Severely damaged pods appear whitish in contrast to the normal yellowing and browning of ripening pods.
Preventative Management Tactics
A number of natural factors can minimize diamondback moth populations. For example, heavy rainfalls can drown many larvae of the first generation, and prevent build-up of high population densities. Humid conditions associated with rainfall can favor the development of fatal fungal diseases (Entomophthorales). In addition, there are several parasitic wasps and predators (flies, lacewings, minute pirate bugs, spiders and birds) that prey on the larvae of diamondback moth.
Responsive Management Tactics
SEQ CHAPTER h r 1
Sex pheromone traps are useful tools for detecting the flights of adult diamondback moth. The recommended trap design is the wing trap or delta traps with sticky inserts to capture the moths. Traps should be suspended near the crop at the field’s, and provide an early indication of a possible infestation. There are two to three generations in the Northern Great Plains. The second generation is the most important, because it is usually present when the crop is susceptible to injury, blooming to early pod development. The third generation is usually too late to injure most canola crops except for the late-planted fields.
If high numbers of adults (>100 moths per trap week) are being captured in the traps during bloom to early pod development, fields should be monitored for larvae. Beat plants to dislodge the larvae and count larvae on the ground or dangling from plants on silk threads. Check several locations per field. The action threshold for canola at the pod stage is about 20 to 30 per 1/10 th of m 2 , or two to three larvae per plant. No threshold has been established for the early flowering stage; however, insecticide applications are likely required at larval densities of 10 to 15 per 1/10 th of m 2 , or one to two larvae per plant. Early monitoring of adults and larvae, and judicious use of insecticides, only when fields are above thresholds, are the best pest management practices for preventing losses from diamondback moth on canola.
Emerging IPM Issues
Canola acreage has expanded relatively rapidly in the Northern Great Plains of the United States (North Dakota, northwestern Minnesota, and Montana) from a relatively obscure amount in the early 1990’s to over 1 million acres in 2005 in North Dakota alone. The epidemic of fusarium head blight (Fusarium graminearum Schwabe [teleomorph Gibberella zeae (Schweinitz) Petch]), attacking wheat and barley and the orange wheat blossom midge (Sitodiplosis mosellana Géhin) attacking wheat beginning in the 1990’s in the Northern Great Plains states, undoubtedly increased the adoption of other crops like canola that are not host to these small grain pests.
Currently, spring canola types are well suited for the Northern Great Plains. The spring types of B. napus and B. rapa are the predominate types produced in the United States and Canada. Both of these species are susceptible to heat stress during pollination that can result in pollen death with the ultimate impact of reduced yields. This has reduced the adoption of canola production in areas of the Mid-West south of the current major canola production areas. The winter canola types are susceptible to frost in the spring after winter dormancy terminates and growth begins. This has limited the acceptability of canola in the central part of the United State’s Mid-West. However, if canola types are modified for more heat and drought tolerance from Brassica juncea, there exists the potential to expand canola acreage further south. And, because the oil from oilseed rape can be used as bio-fuel (e.g., bio-diesel) there exists the possibility that B. juncea may expand southward for use as an industrial oil.
The increase and possible southward migration of canola acreage will undoubtedly result in increased pest management issues. In addition, for a crop that was hailed as a “sure-fire” solution to an epidemic of wheat and barley pests, canola may result in unintended pest management impacts in other crops. The green peach aphid, Myzus persicae (Sulzer), is a world wide pest because it serves as a vector of plant viruses. In potato, green peach aphid is a vector of potato leafroll virus (PLRV) and potato virus Y (PVY). The green peach aphid has a large host range including canola. It has been speculated that the increase in canola production in North Dakota and Minnesota has resulted in increasing green peach aphid populations and that this pest migrates to potatoes fields after canola has matured. The increase in the population levels of the green peach aphid has resulted in an increase of PLRV and PVY incidence in the seed potato production areas of North Dakota and Minnesota, which has had a severe impact on the seed potato industry.
The tarnished plant bug is a generalist that attacks a wide variety of plants, including canola. Canola is suspected to be a reservoir host of Lygus that infest sugarbeet and confection sunflower fields late in the growing season after canola is harvested. Although research has documented that cutting of alfalfa for forage results in dispersal of Lygus from the harvested alfalfa, dispersal from canola in late summer to other host crops has not been well documented.
In 2000, the swede midge, Contarinia nasturtii (Keiffer), was detected in Ontario and the Northeastern United States. The midge is native to Europe and Asia and it attacks a wide variety of vegetable cole crops and canola. Injury to plants can be difficult to diagnosis without closely examining the plant and injury is often confused with molybdenum deficiency, herbicide injury, or heat or frost injury. The larvae are gall inducers and their feeding causes distortion of plant tissues and flower buds can become swollen and remain closed. Quarantine is enforced in the swede midge infested areas in an attempt to limit the spread of this pest. Economic losses are estimated to be significant should the swede midge become established in the primary canola region of Canada.
The increase in canola acreage may also be attributed to the increasing populations of crucifer flea beetle that attack cole crops in home gardens. Many home gardeners located in the major canola growing areas of North Dakota have complained about high flea beetle populations and that it is impossible to grow cole crops (e.g. cauliflower, broccoli, cabbage) without the use of insecticides.
Canola continues to be an increasingly important oilseed crop in the Northern Great Plains and its southward migration will continue to increase with the development of heat- and drought-tolerate canola types. New pest management issues in canola and those associated between canola and other host crops will continue to develop with the increase and spread in canola production. The reliance on current and new pest management technologies will be necessary to minimize potential economic losses from insect pests of canola and unintended pest impacts on other crops to help maintain the vitality of canola production in the United States.