Root Weevils: Troublesome Rhododendron Pests

Nodule weevil — control measures

Journal ARS Article

Vol. 55: No. 4: Year 2001

Root Weevils: Troublesome Rhododendron Pests

Hank Helm
Bainbridge Island, Washington

Perhaps the worst insect pests that attack rhododendrons are the several species of weevil, found in nearly all areas of the world. Weevils attack over 100 different plant species in addition to rhododendrons. They attack yews (Taxus), hemlock (Tsuga), strawberry (Fragaria), various vines and flowers and even potted houseplants. In the Pacific Northwest, weevils also feed on salal (Galtheria shallon) and huckleberry (Vaccinium).

The most common and most significant of weevil species are: black vine weevil, Otiorhynchus sulcatus; obscure root weevil, Nemocestes incomptus; woods weevil, Sciopithes obscures; strawberry root weevil, Otiorhynchus ovatus; rough strawberry root weevil, O. rugosostriatus; obscure root weevil, Sciopithes obscures; clay-colored weevil, Otiorhynchus singularis; and (no common name) Dyslobus ssp. Collectively all of these species are commonly called root weevils. Varying in length from about ј inch (0.6 cm) to Ѕ inch (1.3 cm) long, they have a short broad snout with antennae attached. They vary in color from slate gray to blackish brown and generally have some small yellow or white flecks on their backs.

The obvious indications that there are weevils present are the conspicuous notches around the edges of the leaves of the plant. These notches can be few or in some cases so extensive that the entire leaf edges end up being jagged and very unsightly! Usually the damage is aesthetic and plant health is not in jeopardy. The plants may have only a few notches in some leaves; however, many plants do end up with their leaves extensively chewed. In container grown plants, the damage can be particularly bad, and an infestation in nurseries can prevent plants from being saleable.

There are some plants that are particularly susceptible including ‘Blue Peter’, ‘Elizabeth’, ‘Ruby Hart’, ‘Creeping Jenny’, and ‘Scarlet Wonder’. Less susceptible varieties attacked include ‘Sappho’, ‘Sir Charles Lemon’, ‘Pink Petticoats’, ‘Golden Torch’ and Nobleanum Album Group. Generally, hybrids are more susceptible than species; however, among the species I have observed with weevil damage are R. piercei, R. campanulatum, R. chaetomallum, R. degronianum var. heptamerum and R. celebicum. Young new growth is the particular foliage of choice. Thinner foliage is more susceptible than foliage that is thick. Leaves that are recurved at the edges are bothered less. My observations do not indicate that indumentum is any deterrent.

Weevils are nighttime feeders, so spend the day in the loose ground, mulch or leaves beneath or near the plants. The adults are wingless so do not fly but crawl from plant to plant, container to garden and even garden to garden. If gardens are adjacent to woodlands, they will travel from there into the plant beds. Because they feed and develop on a variety of plants (especially salal or huckleberry thickets), infestation can come from many directions. Weevils must crawl up from the ground or from adjacent plants or structures into the plant to feed.

The adults emerge from pupation chambers in May to June and feed on foliage from twenty-one to forty-five days before they lay their eggs. All root weevils are apparently egg-laying females. There have been no males found in the United States. They generally feed on new or current year growth. Weevils emerge in the spring over a four to six week period and then feed for another four weeks or so before beginning to lay eggs. They lay several eggs each day into the soil or leaf litter. Adults live for ninety to one hundred days and may lay up to two hundred eggs in this time. The eggs hatch in two to three weeks to larva.

The not so obvious damage caused by the weevils is from the larvae which are white grubs. These grubs live in the soil and feed on the plant rootlets and stem at the base of the plant. The damage caused by the larvae can kill plants and are particularly damaging to container grown plants. Complete girdling of the stem and/or fatal root damage can occur.

The larvae are C-shaped and legless as shown above. The color is dirty white with a dark head. They grow slowly over the summer, molting five to six times. By late fall they will be about 5/8 inch (1.6 cm) long. Winter is spent in the soil in and around roots. The change from larvae to adult is pupation. Pupation takes place in the spring when the soil warms up. The pupae are about the same size as the adult and are white. They are soft and have the outline of the parts of the adult weevil. Pupae are inactive and do not feed. The damage caused to the roots by the larvae generally is not life threatening to the plant unless it is in a pot.

In some species, the winter may be spent as adults; however, they are inactive during cold weather. Milder winters encourage this over-wintering. There is an overlap in different stages of growth in woods weevil. It is possible to find adults, larvae and pupae all at one time in one location. The greatest numbers of adults of all species occurs in late autumn. The life cycle begins anew in the spring when the adult weevil emerges.

Weevil Control
Prevention of weevil damage can be done in two ways:

• Keeping plants healthy and happy is always the first line of defense. Stressed plants suffer damage more often than happy plants and seem to attract pests of all kinds. Proper watering, a mulch to retain moisture, fertilizing appropriately and good housekeeping will help.

• Pruning off the lower leaves that touch the ground, structures or other plants will reduce access to the plant.

PHYSICAL CONTROL. Determination of the proper time to begin physical or chemical control can be done by observation. At night and sometimes after sundown or on cloudy days the weevils can be found on the leaves or hidden somewhere on the plant. A white cloth can be spread beneath the plant and the branches shaken to dislodge the weevils. They will fall on the cloth where they can easily be seen. Observations should be made frequently (once a week) when the weather warms or in early May in the Northwest. New leaves can be observed for damage as the growing season begins.

Weevils can be picked from the leaves and plant at night if one has the patience and is willing to spend time well after dark with a flashlight. Close examination will reveal the weevils, and they can be picked or scraped into a container for disposal. Because the weevils must travel up into the plants, it is necessary to spend some time looking and they may not be visible early in the evening. During a recent hunting trip I picked seven weevils from plants in a short span of about fifteen minutes. Although I have been able to use this method with some success, I confess I become weary of the late hours and do not spend the time I should to really have an impact on control!

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Sticky barriers such as Tanglefoot, Tangletrap, Tack Trap or StickEm will prevent adult weevils from traveling up the trunk to the leaves. These barriers must be applied to the trunk so there are no unprotected avenues of travel up the stalk. The weevils either will choose not to travel up the stem or will become stuck in the barrier and thus cannot move into the canopy to feed. There are some indications that prolonged use of these materials can be somewhat damaging to the bark and stem, so a strip of polyethylene, waterproof tape or thin plastic can be fitted tightly to the stem and the sticky material applied to it. Plastic tape, etc. , must be removed before stem growth is restricted. The barrier materials can be difficult to remove from hands or tools. Flat sticks can be used for application and then disposed of. Paint thinner may be required to remove these materials from tools. This method of control will not work if there are access routes other than the stem to the plant such as other plants, fences, etc.

Teflon barrier tape such as SureFire Insect Barrier Tape can be used in place of sticky barriers. The tape must be wrapped snugly around the trunk with the sticky side adhering to the bark leaving no avenue for the weevils to crawl under it. Make sure that you do not constrict plant growth. Another disadvantage of these products is the fact that the tape is white and so is visible on the plant.

Traps can be made from burlap or coiled corrugated paper and placed under plants. Take burlap and make folds in it as you place it at the base of the plant. The weevils will hide in the burlap or paper during the day and can thus be trapped. Before the weevils move into the plants in the evening, the traps can be removed and the weevils destroyed.

CHEMICAL CONTROL. Chemical control for weevils should be used with extreme caution and only as a last resort! Some of these chemicals will kill beneficial insects such as predacious ground beetles and bees. Chemical sprays should never be sprayed on flowers. Be sure to check to see what chemicals are registered or are permitted in your area for use as a control for weevils. When using pesticides, follow all label instructions and apply only as directed. Use proper protective masks and safety equipment per directions and as appropriate. Store pesticides in their original containers and keep out of the reach of children, pets and livestock. Always read labels before using. Insecticides work the best on adult weevils and are normally applied to foliage. Timing is very important, or the spraying will be worthless. Foliar applications should begin at the first appearance of adults in early summer and must be reapplied regularly (according to label instructions). All available insecticides have a short residual activity (three days to three weeks depending on which one is used).

Foliar insecticides that are used in the Northwest and have been registered for weevil control include azadirachtin (Azatin EL), acephate (Orthene), bendiocarb (Turcam), bifenthrin, (Talstar), chlorpyrofis (Dursban, Pageant), and cyfluthrin (Tempo 2). A recent visit to a large warehouse store in the Northwest found Orthene, Bug-B-Gone Dust, Lindane 20 and Tempo 2 on the shelves with their labels listing weevils as one of the insects controlled. Azadiractin is a growth regulator. Pyrethroids (Talstar and Tempo 2) may be repellent, particularly at low application rates, but weevils may simply move to untreated areas.

Coffee grounds spread around the base of plants will deter weevils. The disadvantages of this control are the amount of coffee grounds required for large plants or many plants and repeated applications. The author has used this method on a single plant with some success, only to see the weevils return the following year. The coffee grounds are a good mulch but will add to the acidity of the soil. Some may not like the color of the grounds and a thick covering may become moldy.

BIOLOGICAL CONTROL. Biological control is non-chemical and is not toxic to plants, humans, pets or beneficial insects. Beneficial nematodes are a biological control that is a relatively new pest control product. They are effective against a number of serious pests that live in the soil, including weevil larvae. Several nurseries have begun use of beneficial nematodes as an effective control measure for weevils.

Nematodes are simple roundworms and are colorless, unsegmented and lack appendages. There are free-living, predaceous and parasitic nematodes; however, the beneficial species of nematodes that are insect-parasitic and possess the biological control attributes for control of weevils are in the genera Steinernema and Heterorhabditis. These nematodes are multi-cellular metazoans. Nematodes are formulated and applied as infective free-swimming juveniles. This is the only free-living and environmentally tolerant stage of the above genera of beneficial nematodes. Infective juveniles range from 0.4 to 1.1 millimeter in length. When a host (weevil) is found, the nematode enters into the insect via a body cavity. Once inside, a bacterium is released which multiplies rapidly and causes death to the weevil.

Unlike chemicals, nematode applications do not require masks or other safety equipment to apply. Nematodes do require specific conditions to be effective. A specific temperature range (soil temperature must be above 55 F, 13°C) is required. Because the nematodes are free swimming, the ground must be moist. It is advisable to apply water before and after applying. Applications must be made in accordance with the date listed on the container and application rates must be strictly adhered to. Beneficial nematodes are not found naturally in the soils of most areas, so must be applied yearly. They are only effective against the larvae, and the most beneficial time for application is in the fall. Applications in late summer can only be effective if there are larvae present. Some of the commercially available brands are BioSafe, Exhibit, Biovector, Nemesis and Scanmask. These products can be found in many garden stores and through mail-order catalogues or on the World Wide Web.

Those who have used nematodes stress the importance of following the directions supplied with the product, particularly concerning soil temperature and expiration time. Soil temperature can be measured with a soil thermometer available at most garden stores. As with other control measures, the applications will have to be done on a yearly basis.

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Hank Helm is a member of the Kitsap Chapter.


There are many pantry pests which can infest homes and businesses. Though Meal Moths, Grain Beetles and Flour Beetles are very common, Rice Weevils are just as likely to be the unwanted insect in such areas. Rice Weevils are small and easy to kill, but they can complete their life cycle quickly. Adult females will start laying eggs almost immediately so once you have some activity in a structure, it is sure to blossom into a problem which will need attention. Found throughout the world, Rice Weevils are a pest which can be controlled like many other pantry pests. Find the route of entry, discard infested food or belongings and treat with both residual insecticides and traps. The article below will cover this process in detail but if there is anything we don’t cover, give us a call or email with your questions and we’ll try to further assist.


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Rice weevils are a small insect, about 1/16 to 1/8 of an inch long. They are mostly brown to black in color and can have spots on their thorax and abdomen. As the name implies, they like rice. However, Rice weevils can be found eating just about anything in the home. They typically like seeds of all types, dried beans, cotton, nuts, cereal, any wheat product, corn, flour, pasta, bread and other grain products found in most any home. But their tastes don’t end in the pantry.


Rice Weevils will readily feed on cotton fabric found in furniture and clothing. They love dried flowers and plants, decorative ornaments and dried flower arrangements as well. In fact its not uncommon to find them living in or on Christmas ornaments, dried flower collections and stored clothing. And unlike other pantry pests, rice weevil adults love apples, pears, grapes and other fruit. Since they fly well, it is not uncommon for them to enter a home attracted by the scent of some fruit or grain.


Rice weevils start their life as a small caterpillar (larvae) which typically hatches on a food like wheat grain, seeds or nuts. Eggs will hatch within a couple of days of being laid and feed immediately. Feeding will occur for 1-3 weeks and then larvae will spin a cocoon in which they will pupate to an adult.


Pupation takes about 1-2 weeks and when complete, adults will emerge. Rice weevils present more of a nuisance compared to other pantry pests because adults feed just as much as their larva preferring a wide range of things they’ll eat. Since they fly well and are small, rice weevils can move throughout the home easily finding all kinds of food. Adult females can start laying eggs within 3 days of emerging from their pupa cocoons. Most will lay 300-600 eggs during their 3-6 month life span so just one female can cause a huge problem. Since rice weevils can grow from egg to reproducing adults in less than one month, they can be a formidable foe in and around the home. If you suspect you have rice weevil activity, there are several things needed to knock out current activity and get them under control.


Here are guidelines and treatments needed to control a local rice weevil problem.

First, empty all cabinets, shelves and closets where they’ve been seen or thought to exist. Any food stuff which has them active must be discarded in sealed plastic bags before throwing them out. This will contain them until the garbage is picked up.

If you are unsure something has activity, store it in a plastic bag and check it every week. If there are rice weevils in it they will try to get out within a few weeks. If some are found, throw it away immediately; if you don’t see any live activity after 3 weeks you can assume to safe to keep.

Since this pest is temperature tolerant, don’t waste your time trying to freeze adults, eggs or larva. Though you can certainly kill some by freezing, eggs and pupae will only go dormant until it gets warm enough for them to continue developing. So even if you store something for months in the freezer, once you take it out any eggs or pupae would just continue on as if nothing ever delay ed them. In the end, you could end up getting a whole new problem by saving something heavily infested which was hard to notice because you froze its contents.


Once the infested product is discarded, you’re just about ready to treat. We always suggest to first vacuum all closets, shelves, cupboards, pantries and baseboards about to be sprayed. This will help by removing eggs and some pupae. It will also remove hard to see food their young would eat. Rice Weevils lay eggs with a glue like excretion which helps protect their brood by attaching them to surfaces where food is likely to be available. So even though vacuuming will help, it will not be a solve all.


Once everything has been removed and you’ve done a good vacuuming of the areas with activity, you’ll be ready to spray. We have several products that work well for cabinets, pantries and areas with lots of cracks and crevices.

The first option is FS MP AEROSOL. This product is odorless and fast acting. If you know where they’re focused, FS MP will work well. Use it in pantries, cabinets, under the sink and even in baseboards. The can comes with a straw attachment making it ideal for cracks and crevices where adults and larva like to hide. Be sure to treat all your cabinets too – not just where you think the weeviles may be hiding.

Remember, rice weevils are small, fast and quick to hide when ever people are around disturbing them. And since they fly, its easy to miss where they might be active so its important to treat every cabinet.. Let the treatment dry for 1 hour and after that, all dishes and food stuff go back without hazard to people or pets. Expect to treat every 2 weeks if you see continued activity at that time. If not, treat every 2-3 months to insure they don’t come back.


For most problems limited to a cabinet or two, FS MP Aerosol will solve the issue. But if you’re finding weevils throughout the house, aerosols are not practical to apply. And they won’t get the random travelers unless you spray every surface which isn’t practical.

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Since rice weevils fly, they can get in laundry rooms, garages, basements and other areas where pet food and grain products like grass seed are stored. If this is happening in your home, go with BITHOR.

Bithor is very unique in that it combines two actives. One lasts a few days and effectively kills and agitates all stages anywhere close to where you spray. This forces them out into the open and onto the treatment. As the one active wears off, the second active takes over. This second active is very stealthy. Weevils (and other insects) won’t know its there. They’ll walk over it, pick some up and die in 1-2 days. Before they die they can even share the active with other weevils so the net effect is being able to kill off hidden nests.

The second active will translocate meaning it will move around the home, covering 50-100% of the walls and even ceiling where this pest might forage. The amount of movement is minimal but enough to affect them. And since the active is not hazardous to people (its actually used as a systemic to protect vegetable and fruit plants), you can get amazing results throughout the home with no risk to people or pets.

Mix 1 oz per gallon of water and apply the mixed solution to baseboards, carpeting and any room suspected of harboring this pest. Bithor can be spot sprayed and even broadcast over large areas so its safe enough for use throughout the home. 1 gallon of mixed solution will cover up to 800 sq/ft and you would expect to treat monthly for normal problems. But if they come back quickly within 1-2 weeks, spray more frequently until they subside. In most cases, 1 treatment solves the problem using 1-2 gallons but following up within a month is advised.


For extra protection, add NYGUARD GROWTH REGULATOR with the Bithor. Its designed to be used at the same time as the Bithor, in the same sprayer, so it won’t take extra time to apply it.

Now its important to understand what Nyguard does. Basically it won’t affect adult weevils but it works specifically on eggs and larvae. Nothing can technically “kill” weevil eggs but Nyguard will prevent the hatching larvae from developing normally. Basically Nyguard mimics a very important protein the young weevils need during their growth. And when over exposed to this protein, they won’t grow right. This interruption causes them to stall during their development, mutate and then die. This in turn will help end the infestation.

Adding Nyguard will enable treatments to last for several months compared to using just Bithor alone. So if you have a warehouse or bad infestation to treat, using Nyguard with Bithor would be the smart way to proceed – especially if the problem is in a restaurant or food processing plant. Add 4-12 ml along with the 1 oz of Bithor per gallon of water.

For small needs, we have it in a 1 oz and 4 oz bottle too. This is less concentrated but works just as well. Use 1 oz of this concentrate per gallon of water along with the 1 of Bithor.

To apply the Bithor and/or Nylar, you’ll need a good PUMP SPRAYER. Our sprayer is made to our specifications and can produce a fine fan pattern ideal for a controlled application that’s neat, tidy, not over sprayed but consistent with its coverage.


Lastly, install RICE WEEVIL PHEROMONE TRAPS in any area where you have or suspect activity. These traps use strong pheromones or sex attractants to lure adults. Once they crawl or fly into the holding tray, the thick catching oil will hold them for good. These traps should be installed a day after you treat. And they should be used as a way to monitor the problem. In most cases, they may only capture a few if you’ve sprayed well. But if you continue to trap weevils for weeks after a treatment, its a sure sign you missed an important area and will have to spray again.

Set these triangular traps in the back of any shelf or cabinet area. They are quite discreet and easy to conceal. Use at least one trap per cabinet. The expected life of these are 1-2 months. But if they fill up sooner, replace them immediately.

For open floors and areas with a lot of activity, the XLURE TRAP will be better suited for the job. The Xlure trap has a protective, hard plastic body with a “catching gel” and pheromone bait located deep inside and out of sight. Rice weevils will readily find these traps and once they enter, they’ll get caught but because of it’s design.


Rice Weevils can be a persistent pest once established in the home. To break their cycle, you’ll need to remove all infested food. Next, you need to clean and vacuum all cabinets or closets where they’ve been found or seen. Treat with FS MP or Phantom Aerosol to kill off hatching larva and migrating adults. If you have them throughout the home, treat with Bithor to get better coverage. Finally, set some Pheromone based traps out in cabinets and pantries where activity has been seen. Be sure to keep the traps fresh so they are always helping by catching adults before they mate and reproduce. This program will help break the cycle in 3-6 weeks under normal conditions. After they’re gone, treat every 2-3 months to insure they don’t reinfest the structure.


Give us a call if you need further help. Our toll free is 1-800-877-7290 and we’re open Monday through Thursday, 8:00 AM to 7:00 PM. On Friday, 8:30 AM to 5:00 PM and on Saturday, 9:00 AM to 2:00 PM (Eastern Standard Time).

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kimberly bell says

Rice weevils have infested my home. I never knew they existed. Thanks to your webpage I’m going to try your suggestions to get rid of them.

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