Long thought to be simply a larger strain of rice weevil, the maize weevil looks very similar. It is slightly larger, up to 1/8-inch long, and like the rice weevil, it is dull reddish brown to black with four red-yellow spots on its back.

Mar 07, 2011 You can see where the weevils chewed through the plastic. Due to past insect problems, we’re diligent about storing all foods in glass jars or plastic zip-top bags. Unfortunately, grain weevils (Sitophilus granarius, also called granary weevils or wheat weevils) can chew through paper and plastic packaging. Oct 05, 2019 These little pantry pests are actually beetles; very small beetles that rarely get any larger than ¼ of an inch. Female weevils use their little snouts to drill holes in the casings of grains, such as wheat, oats, rye, rice, corn, barley, and various seeds and beans.

ENTFACT-145: Controlling Insects in Stored Grain | Download PDF

by D.W. Johnson and L.H. Townsend, Extension Entomologists
University of Kentucky College of Agriculture

Over 50% of the grain produced in Kentucky is held in 'on-farm' storage for some period of time. In the past this storage was short-term, from harvest until early spring. However, times have changed drastically. Now, storage periods of 12 months or longer are common. As a result, grain remains undisturbed for long intervals, including the high risk, warm, moist months from late spring to early fall.

The primary causes of grain spoilage in Kentucky are excess moisture and high temperature. However, any grain improperly handled or stored longer than 6 months can be infested by insects. The key to successful storage is to anticipate and prevent potential problems through good bin management practices. Preventing insect problems in stored grain requires 3 steps: 1) sanitation, 2) protection and 3) inspection.

SANITATION

Stored grain insect infestations rarely begin in the field. Most develop from small numbers of pests already present in or around farm storage bins. An effective sanitation program can eliminate or greatly reduce the chance of having serious problems with these insects.

Equipment: Grain handling equipment should be kept clean. The effort involved in thorough preparation of storage bins is of little value if they are filled using contaminated equipment. Combines, trucks, augers and grain driers should be thoroughly cleaned of all old grain.

The Empty Bin: Never put newly harvested grain into bins containing old grain. Empty the bins of all residual grain, then use shovels, brooms and industrial vacuums to complete the job. Be especially careful to clear dust, webbing and fines from around any cracks and crevices, doors, seams, vents and especially under false floors. Even small amounts of old grain left in the bins may harbor enough insects to start a problem. Once the bins are clean, inspect them and repair any cracks or holes which would allow moisture or insects to enter. Then, spray the inside completely with an insecticide. (See Insecticides and Fumigants)

Near the Bin: The area immediately surrounding the bins should be kept clean. Be sure to remove and destroy all spilled grain. Control weeds and grasses, since they can harbor insect and rodent pests. Inspect outside walls and especially the base and roof for damage that could allow pests and moisture to enter. Do not use the area immediately surrounding the grain bins to store cattle feed; keep stock feeders as far away as possible.

The Grain: Store only clean, dry grain. A small percentage difference in moisture content can make a big difference in the probability of a damaging insect infestation. The optimum moisture content for stored grain is 12-13%. Most grain inhabiting insects require 13-15% moisture for maximum feeding and reproduction.

It is also advisable to clean grain before binning. Small pieces of dockage and cracked or split grain provide food for insects not normally found in whole grain. Even though most of these insects will not feed on whole grain, their biological processes produce heat and moisture which can greatly reduce the stored grain's quality.

Level the grain surface once the bin has been filled. This will allow for good air flow and thus improved moisture and temperature control. It also will allow improved access for inspection and treatment.

CHEMICAL PROTECTION

In empty bins, apply a coarse spray of an approved insecticide to run-off at least 2 weeks before binning (See Insecticides and Fumigants). Pay close attention to areas which might hide insects, such as under false floors and vents. Cracks around doors and vents may serve as sources of infestation.

A grain 'protectant' is an insecticide that can be applied to the bulk grain as it goes into a storage bin (See Insecticides and Fumigants). The treatment offers protection for about one season and should be considered if the grain may be held for more than 6 months. Proper calibration of the application system is important to ensure even insecticide distribution throughout the grain. Protectants are effective against both beetle and caterpillar pests.

A surface dressing (Cap Out) may be applied to prevent insects from entering the top of the grain mass and to control surface infestations of Indian meal moth, when a 'protectant' is not used (See Insecticides and Fumigants). Though surface dressing may be useful if a grain protectant is not used, it will not reduce an established infestation within the grain mass. Producers anticipating problems with these pests should follow some specific steps:

Apply the surface dressing as a coarse spray in enough water to obtain adequate coverage. The recommended amount of water differs among products, so check the label of the insecticide you choose. After you spray, incorporate the insecticide into the top 4 inches of grain surface.

To compute how much insecticide you need for top dressing, select the desired product (See Insecticides and Fumigants) and multiply the recommended use rate (example, 1 pt/1000 bu) by the proportion of 1000 bushels contained in the top 4 inches of your bin. Table 1 lists the proportion of 1000 bu in the top 4 inches of some common sized bins. For example, if you wish to top dress a 24 ft bin with a compound at the rate of 1 pt/1000, then 1 pt. x 0.119 = .119 pt or 1.9 oz.

Infestation in the Grain Bulk

Fumigants are gases that penetrate the grain and kill insects both on and in the grain. They are very toxic to man and animals and should be applied only by trained, experienced operators working in pairs.

Once an insect infestation has become established, there are only two treatment options: (1) move the grain and apply a protectant during transfer or (2) fumigation. Both choices have good and bad points. Application of grain protectants during movement will provide some residual protection. But moving grain is costly, time consuming and requires additional bin space. Also, if good control is not achieved, movement will spread the insects throughout the grain mass. On the other hand, fumigation works very well and is relatively cheap. Fumigants are gases that penetrate the grain and kill insects both on and in the grain. They are very toxic to man and animals and should be applied only by trained, experienced operators working in pairs. But because it is very sensitive to poor technique, many failures occur. Also, it is dangerous and provides no residual protection.

Fumigation

Several factors are important in assuring successful fumigation.

• Grain should always be level in the bin to let the fumigant penetrate evenly.
• Any surface caking or crusting should be broken up and removed.
• Grain temperature should be 60°F or higher to ensure proper vaporization.
• Possible leak points such as cracks or holes in the bin should be closed before fumigation since leakage may result in under treatment and poor control.

Causes of Fumigation Failures

Fumigation failures can usually be attributed to one or more of the following:

• Insufficient fumigant--A fumigant's efficiency depends on its attaining a lethal concentration in the grain. Results will not be satisfactory if less than the recommended dosage is used or if the fumigant is applied under unfavorable conditions.
• Storage structure--Leaky bins will not retain fumigants long enough to kill the insects. The depth of grain also affects a fumigation's efficiency. In general, the greater the surface area of the grain in relation to bulk, the greater the difficulties in adequate fumigation. As a result, flat storage bins require higher dosages than do round silo bins. Also, storage structures with a large amount of space over the grain are difficult to fumigate effectively because large amounts of gas escape into that space.
• Moisture--As moisture content increases above 12% a proportionately higher dosage of fumigant is required. It is difficult to effectively fumigate grain having a surface moisture content of 15 to 20% because the fumigant vapors will not penetrate the moist layer.
• Temperature--During fumigation, the gas quickly assumes the temperature of the grain. In general, fumigant activity increases as temperature increases. However, if grain temperature reaches or exceeds 115 °F, the fumigant may vaporize very rapidly and escape from the bin before accomplishing the job. On the lower end of the spectrum, stored grain insects are inactive at temperatures below 60°F. Fumigation should be delayed until the grain temperature increases.

Treatment

Should fumigation become necessary, a variety of compounds are available for treatment (See Insecticides and Fumigants). Fumigants are inherently dangerous and should be applied only by trained operators. Operators should wear a full face gas mask equipped with the proper canister, and have access to self contained breathing apparatus, and another person should always be present during fumigation.

Refer to the training material developed by the manufacturer.

INSPECTION

Grain bin inspection provides important information on the general condition, temperature, moisture and pest activity of stored grain. Inspections allow early detection of problems and enable corrective action to be taken before damage becomes severe See: 'Aeration, Inspection and Sampling of Grain in Storage Bins,' Extension publication AEN-45, provides procedures for checking storage facilities.

Using Probe Traps for Insect

The commercially available 'probe' traps may be the easiest and safest way to monitor for several beetles that can infest bins. These traps are hollow 'plastic' tubes with a series of downward sloping holes all along the sides. The top is a flat cap. The bottom is a pointed piece that screws in place. Insects crawling into the tube through the small holes can accumulate at the pointed end of the trap. A nylon line should be securely attached for easy retrieval from the grain mass.

These traps can be inserted in the grain using a long pole with a cup device on the end. This device is easily made using a paint roller extension handle and some 'PVC'plumbing fixtures. Attach to the extension handle, a PVC 'reducer' that has one side just about the size of the screw on the end of the extension handle while the other side is large enough to serve as a cup over the end of the trap. This will allow you to push the trap into the grain from an inspection hatch, internal ladder, or some other safe place, thus avoiding having to cross the grain surface.

The trap is retrieved using a nylon line which was attached to the trap before it was placed in the grain, and tied off to some convenient location in the bin. The line will also serve to keep traps from being sucked into the grain stream in case they are forgotten at unloading time.

How many traps are needed? The greater the number of trap samples, the greater the probability of detecting insect activity. For grain, the standard is 4 to 5, certainly now fewer than three, per round bin.

Thresholds for a one-week sampling period vary with species sampled and the grain Temperature (Table 2). If grain temperature is below 60o F, the numbers in Table 2. indicate a very high population size.

Economics of Pest Control

The constant fluctuation of grain prices and costs of insecticides, as well as attitudes of local buyers, make it impossible to set a specific cost-return value for treatment of stored grain. In Kentucky, most often the cost of an insect infestation in stored grain is that the buyer pays less for that grain. Find out what local buyers dock for infested grain, and compute the cost of treatment to see which provides the best return. If grain is plentiful, buyers can be choosy so the dock for infested grain will be greater. In that case, treating infested grain is usually more cost effective. On the other hand, if grain is scarce, buyers may not dock at all for insects, so treatment would be cost prohibitive. It is never cost effective to treat non-infested grain.

Insecticides and Fumigants

Labeling and application regulations for fumigants and grain protectants are subject to change. To insure that information on these products is as current as possible lists of insecticides and fumigants for use in stored grain may be found in the appropriate annually revised commodity publications as follows:

ENT-13 Insecticide Recommendations for Soybean
ENT-16 Insecticide Recommendations for Field Corn
ENT-24 Insecticide Recommendations for Grain Sorghum (Milo)
ENT-47 Insecticide Recommendations for Small Grains (Barley, Oats, Wheat)

You may view these publications on line at: http://pest.ca.uky.edu/EXT/Recs/welcomerecs.html

Do not use malathion as residual treatment for empty bins, a protectant or ‘capout' treatment. Malathion labeling is being removed for stored grain. More importantly, several stored grain pests have exhibited tolerance to malathion.

Common Stored Grain Insects In Kentucky

Bin Weevils Bin Cards

A variety of insects can be found in Kentucky stored grain. Fortunately, only a few are responsible for most problems and most of them are relatively easy to identify. These pests are divided into two main groups the beetles and the caterpillars. The caterpillars are easily identified. However, stored grain beetle pests are quite small so a hand lens is needed to see enough features to determine if important pests are present.

Indian meal moth (right) is one of the most common and troublesome insects attacking most any type of stored grain. The adult is a moth with a coppery color on the outer two-thirds of the front wings. Presence of the adult, though not damaging, can indicate the presence of an infestation. The moths lay eggs on the gain surface. These eggs hatch into the caterpillars (worms) that cause damage. The worm is about ½' long when grown and whitish in color. IMM rarely causes extensive kernel damage, or grain discount at the time of sale. However, the larvae may completely web over the grain surface, thus preventing proper air movement for aeration and fumigation and causing surface grain moisture accumulation and 'top crusting'.

The most common beetles, often called 'bran bugs' (pictured left), do not feed on whole sound kernels, but rather live on broken kernels, dust, and trash. There are two major types in Kentucky the 'flour beetles' and the Cryptolestes beetles. These insects are likely to be found in every bin of stored grain in the state. The real question is are there enough to do and damage?

There are two flour beetles (pictured right), Red flour beetle and the Confused flour beetle. They look very much alike and are separated by examination of the last three segments on the antennae. However, these beetles are usually found together in an infestation and do about the same damage.

There are also two common Cryptolestes beetles. They are the flat grain beetle and the rusty grain beetle. They look very much alike and are about one-half the size of the flour beetles. They are very flat in appearance and have beaded antennae about one-half as long as the body. They are often found in mixed infestations with flour beetles and do a similar type damage, but it takes two to three times as many flat/rusty grain beetles to do the same amount of damage as the flour beetles.

Perhaps the two most dangerous insects in Kentucky stored grain are the 'weevils', which are usually found in corn, and the lesser grain borer which is more often a problem in wheat.

The weevils (pictured left) are relatively easy to tell apart from other insects because of their 'snout'. These are long, thin, downward curving mouth parts sticking out from the head.

The lesser grain borer also has a distinctive shape. However, it is the overall look of this insect that allows the identification. Lesser grain borer are very cylindrical in shape and both the head and tail end look very flat. It is kind of like using a straw to take a plug out of an apple only much smaller.

Both the weevils and lesser grain borer are primary feeders. They both feed on whole, sound kernels and their young develop inside the kernel. Infestations of these insects are very important.

Most other insect pests are secondary feeders. They cannot feed on whole sound kernels, but rather do their damage by their presence, and the heat and moisture they generate.

There are a large number of other insects that can infest stored grain. You may find information, pictures of these insects and links to other storage sites at: IPM in Kentucky Farm Stored Grain

REFERENCES

• Oklahoma State University, Stored Grain Management in Oklahoma - F7180.

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.

Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!

I am pleased and honored to be the one to tell you that yes, you have eaten weevils. I know it’s a disgusting thought (or an invigorating one, no judgement), but there’s really nothing to be done about it. Most people are never even aware that it has happened. It might have simply been their eggs. It could, however, have been little bits of exoskeleton, maybe a leg or two, or possibly even the entire nasty weevil . . . or at least its larvae. With something as common as weevils, it’s pretty much unavoidable. They can be found in nearly any prepackaged food you buy that contains any sort of grain. I’m talkin’ cookies, crackers, biscuits, cake mixes, pastas, breakfast cereals, anything. This is due to the fact that weevils lay their eggs in, you guessed it, grains.

The two most common types of pantry weevils are the granary weevil (Sitophilus granarius) and the rice weevil (Sitophilus oryzae). They are often referred to as snout weevils. These little pantry pests are actually beetles; very small beetles that rarely get any larger than ¼ of an inch. Female weevils use their little snouts to drill holes in the casings of grains, such as wheat, oats, rye, rice, corn, barley, and various seeds and beans. Once the hole is drilled, she deposits an egg and seals the hole back up with a gelatinous glue-like substance that she creates all by her little self. The egg hatches after a few days, and the larvae uses its surroundings as a lunch box for about the next month. For this reason, it can be difficult to detect an infestation. Not only is it slow to start but the larvae are in hiding. Once grain weevils (a.k.a. flour weevils) or rice weevils present themselves, you’ve got a problem. A single female can lay up to 400 eggs, which means up to 400 more hungry bugs lookin’ for a snack. If you’ve experienced something like this, you’ll need to know how to control and get rid of weevils.

Weevil Identification

Rice Weevil

• 1/8- to 3/16-inch long
• Dull reddish brownRound pits on thorax
• Four light spots on carapace
• Able to fly
• Found more commonly in warmer states

Granary Weevil

• 1/8- to 3/16-inch long
• Shiny reddish brown
• Elongated pits on thorax
• Can’t fly
• Found more commonly in cooler states

Best Methods of Weevil Control

Remove contaminated food products.

Sounds like a no-brainer, but this might be the single most important step in getting rid of weevils. Look through your ENTIRE pantry and every single cupboard for any food products that might be contaminated. If you find something that you aren’t sure of, pitch it. There’s no point in risking reinfestation. It could very well cost you more money in the long run. Once you have it all gathered up, take it outside to the trash immediately and move the can as far away from your house as possible.

Pull a Cinderella.

Conduct the most thorough cleaning of your pantry and cupboards that you have ever done. Start by taking absolutely everything off the shelves and vacuuming them. Make sure to get all the cracks where flour or other food bits might be hiding. If you have contact paper lining the shelves, remove it (you can put new stuff on later). Use a rag and some hot sudsy water to do the rest of the cleaning.

Kill weevils with cold.

To avoid future problems with weevils, it is advisable to freeze your food. You can do this to flour, oats, cookies, corn meal, grits, whatever. If you have the space in your freezer, you should just keep the stuff in there full time. If you have a small freezer and can’t afford the space, set the freezer as cold as it gets and leave the food in there for at least four days. That will kill any eggs, larvae, or weevils. Also, that’s an added 4 days of shelf life!

Kill weevils with heat.

If you don’t feel like freezing everything, a little heat exposure will accomplish the same thing. Spread your food or seeds thinly on a baking sheet, preheat the oven to 120°F, and leave it in there for 1 hour. If you’re impatient, you can do 130°F for ½ hour. If you want to use the microwave, spread the stuff on a glass dish or plate, and run it for 5 minutes. Keep in mind that if you are heating seeds for gardening, the heat may destroy the seed’s chances for germination. Also, don’t heat fine-grained things, like flour, in case of combustion.

Further weevil prevention.

Start by cleaning the cupboards and pantry regularly. If you spill something, clean it up immediately and thoroughly. In the pantry and cupboards, you may want to consider sealing any shelving cracks with caulk to keep food from getting trapped down there in the future. Buy your goods in smaller amounts that you can use quickly. Rotate your stock and don’t mix new food with old. Finally, store all of your perishables in tight-lidded glass, tin, steel, or plastic containers. If you have limited cupboard space, Rubbermaid modular containers (sold at Amazon) may be a good plan.

Control Weevils with Pyrethrins

With as common a pantry pest as weevils are, you would think that there would be quite a number of pesticides that target them directly. Unfortunately, this is far from the case. The reason is simple: weevils live and breed in your food. Since you would like to eat that food, contaminating it with insecticides probably isn’t the best idea. For this reason, you should probably just stick with pyrethrins. There are quite a few different pyrethrin sprays and aerosols available that are sold to be used indoors and in food handling areas. You should still avoid spraying the food directly. Once your pantry and cupboards are completely empty, spray them down. I would recommend you leave the cupboards empty and open for at least a few hours before filling them back up. Look for products such as CB-80 Extra, Riptide Waterbased Pyrethrin ULV, and Konk Too.

Best Natural Weevil Control Methods

Bay leaves.

Bay leaves are a natural weevil deterrent. After freezing or heating your grains, drop a bay leaf or two into the storage container on top of the food product you want to be protected. You may also wish to randomly scatter a few leaves around the cupboard or pantry. Amazon sells a bundle of ’em here.

Bin Weevils Blog

Cloves.

Cloves are another natural deterrent for weevils. Treat your grains with cloves the same way you would with bay leaves. After treating your food with hot or cold, drop a clove on top of it before storing. Scatter a couple around the cupboards and pantry too. Hell, go crazy and try a couple of each.

Matchbooks.

Sounds odd, I know, but I read this suggestion in a number of places. Take a plain old matchbook, open it up, and set it in with your grains or pastas. Apparently the sulfur smell from the matches acts as a weevil deterrent.