Insect Pest Management (IPM) in Farm-Stored Grain
David L. Keith
Dept. of Entomology
IANR, University of Nebraska/Lincoln
The quality of grain when harvested and stored will deteriorate unless managed carefully. Insects, moisture and molds must be effectively dealt with to maintain stored grain quality. This publication deals specifically with insect pest management in grain that is to be stored for extended periods on the farm.
Grain may be damaged in the field by a number of pests, including European corn borers, corn earworms and western bean cutworms. Following ear-attacking insects, several species of fungi or molds and secondary insects can invade damaged grain and cause additional losses. Field damage contributes to reduced quality and a shorter "product life" in the bin, particularly if the grain is harvested and stored at relatively high moisture levels.
In Nebraska, our traditional stored grain insect complex does not invade in the field, but rather enters harvested grain in on-farm storage facilities, older infested stored grain or poorly cleaned harvesting and grain handling equipment. While there are over 70 insects that have been associated with stored grain and grain products, only 8 or so are considered major pests in the Midwest.
WHAT IS INTEGRATED PEST MANAGEMENT (IPM)?
Integrated Pest Management, or IPM, is a philosophy of pest control. It includes the use of several management techniques (e.g. bin cleanup, sanitation, facility repair, temperature manipulation, etc.) used together to reduce pest infestation levels below economic thresholds. An Economic Threshold is a given level of insect infestation (e.g. 1 lesser grain borer per quart of wheat) which triggers a preventive treatment, such as an insecticide. The Economic Threshold is the level of insect infestation where treatment must occur to prevent the population from rising to the Economic Injury Level (or EIL). The EIL is that level of infestation where the cost of treatment equals the cost of control measures. Once the EIL is exceeded, the farmer loses money.
Here are some steps in the process of establishing an IPM Program in stored grain:
1. Correct Identification of Insect Species Involved
Because insect habits, life cycles, ability to damage grain and control procedures may differ widely, correct identification of the species infesting the grain is essential.
2. Continuous Sampling/Monitoring, or Scouting
Each structure which contains stored grain must be checked and monitored for evidence of insect infestation on a regular basis. Various traps and other monitoring devices and procedures are available to help evaluate insect buildup and activity. Sampling provides the basis for control procedures.
3. Comparison with Economic Thresholds
Monitoring data are evaluated after each examination to see which species are present, their relative numbers, whether populations are increasing or decreasing, the amount of damage, etc. When levels of pest(s) reach the economic threshold, more management procedures are needed, perhaps in the form of a grain protectant or fumigant.
Some examples of current thresholds include: 1 rice weevil per quart of grain, 1 lesser grain borer/quart of grain, 5 red flour beetles/qt., 5 rusty grain beetles/qt., 5 other bran bugs/qt.
4. Making Decisions/Consider Alternative Methods as First Defense
The backbone of IPM is the use of alternative (non-insecticidal) methods to suppress pests, using chemicals as a last resort.
5. Use Pesticide(s) If Necessary/Apply Safely
Insecticides can be used in limited ways to prevent insects from quickly invading new crop on-farm stored grain. Residual sprays can be used as bin treatments prior to storing the grain. Careful consideration must be given to the treatment needed, the farmers training, willingness and skills in handling essential treatments, which may include hazardous grain fumigants.
6. Evaluate Success of Treatment/Management
After an IPM program is in place and treatment/management decisions have been put in place, periodic evaluations must be made so that the system can be "fine-tuned" and improved. Such a system could result in improved maintenance of stored grain quality and reduced storage losses.
SOME EQUIPMENT USED TO SAMPLE GRAIN FOR STORED GRAIN INSECTS
Grain Trier - A simple slotted probe tube that is pushed into the grain mass at several points on the grain surface. Once inserted, the inner tube is twisted, which opens the "gates" in the tube, allowing grain to flow inside. When complete, the tube is again twisted to seal the grain inside the gates. When withdrawn, the grain from various depths is pooled into a sample which is screened and evaluated for insect presence and activity.
Pelican Sampler - Shaped like a pelicans beak, this metal scoop is inserted one or more times into a flowing grain mass to develop a composite sample. The device is commonly used at elevators to sample loads of grain being purchased by the elevator, in an effort to detect damaging insects, broken kernels and foreign matter (trash, etc.).
Probe Trap - Probe traps are slotted plastic tubes which are inserted into the grain mass from above. They may be placed at various levels in the grain mass, in an effort to detect insects such as lesser grain borers, etc. Random movement of pests results in insects dropping into the probe, from which they cannot escape.
Pheromone Trap - Various types of traps can be used in combination with sex pheromones (i.e. insect "perfumes") to monitor the presence of several species of stored grain pests. Only males of the species are attracted to sex pheromones. These, in fact, can be placed in the probe trap discussed above to increase the efficiency of the trap.
STORED GRAIN INSECTS AND RELATED PESTS
Basic Structure of Insects
Insects are arthropods, with jointed legs and a hard outer shell (exoskeleton). Insects have three body regions (head, thorax and abdomen). The head contains the compound eyes, antennae and mouthparts as well as the insect brain. The thorax contains the muscles for locomotion and bears the three pairs of true, segmented legs and may or not bear wings. Some insects are wingless, while others have one pair or two pair. Occasionally, even when equipped with wings, insects may rarely, if ever, fly. The abdomen has no appendages, but does have paired reproductive structures at the end. This region contains large parts of several of the insect body systems, including the digestive, tracheal, circulatory, excretory and reproductive systems. Paired holes, called spiracles occur on several body segments and these connect to a system of branching, reinforced tubes (tracheae) inside the insect. Air passing through the spiracles moves through successive branches and sub-branches of this system, eventually reaching a very tiny tube (tracheole) that branches to contact individual cells in the insect. Oxygen passes into the system and thus reaches the cells, while CO2 diffused back outside of the system. Insect blood does not normally carry oxygen, since most species lack hemoglobin. Several stored grain insects may have the ability to shut down their spiracles during fumigation, therefore this process makes it essential that we seriously consider the maintenance of fumigant concentration during the fumigation operation.
Stored grain insects begin their life cycles with the egg stage, deposited on or near food by the female parent. All insects must then pass through a series of developmental process referred to as metamorphosis (a change in form). When metamorphosis is complete, the insect has reached the adult stage and is ready to disperse, mate and complete the life cycle. Basically, stored grain insects pass through two types of metamorphosis -- gradual metamorphosis (Cockroach type) and complete metamorphosis (Beetles, flies, moths).
Insects with gradual metamorphosis have the following stages: egg, nymph and adult. The young of these insects looks like the adult, but lacks it size, sexual maturity and wings for dispersal. These usually are found in the same place and eat the same food as the adult. The growth occurs in the nymphal stage, in a series of steps, between molts. The insect between molts is sometimes referred to as an "instar". When an insect hatches from the egg it is in the first instar. Then it feeds, molts its skin and grows in size, to become the second instar. Many insects have 5-6 instars and become increasing like the adult as they proceed through their development.
Insects with Complete Metamorphosis have the following stages: egg, larva, pupa and adult. The young (larva) of these insects looks totally different than the adult, may not even live in the same habitat and often doesn't eat the same food. The insect is referred to as a first instar larva upon emerging from the egg. It passes through a variable number of larval instars, then chooses a quiet place, sometimes weaving a silken cocoon, where it transforms first to the pupa and then to the adult. The significance of the larval stage in these insects is that it allows them to more successfully exploit other food opportunities, or ecological niches. For example, the adults of dermestid beetles such as Trogoderma sp., feed on pollen in flowers while the larval stages can attack stored or spilled grain.
Insect Ecology/Temperature and Moisture
The key to insect growth, reproduction and activity lies in the fact that, as cold-blooded animals, insects must rely on warm conditions to remain active. Most insects are inactive below 55 degrees F. Thus a key to stored grain insect pest management is proper temperature management, so that the grain mass is evenly cooled (fall) and very slowly allowed to warm (spring). The objective is to reduce the temperature differential between the binned grain and the outside air. Temperature maintenance is very sensitive due to the chance of moisture condensation in the grain mass due to extreme temperature gradients within the mass.
Some Important Stored Grain Insect Species
Over 70 insects have been found attacking stored grain and cereal products. A few of the major pests are discussed below. These insects can be grouped according to their feeding habits. Internal feeders are capable of attacking sound, undamaged grain. These insects deposit their eggs on or in the kernel and the larva devours the endosperm, resulting in a hollow grain. External feeders feed on broken or damaged kernels of grain. Some feed on grain dust and flour while other insects (Pests of High Moisture Grain) are highly attracted to wet, moldy grain.
For descriptions, photos of grain storage pests, their damage and life cycles, refer to USDA Agricultural Handbook No. 500, Stored-Grain Insects.
Rice Weevil Granary Weevil
Maize Weevil Lesser Grain Borer
Indian Meal Moth Rusty Grain Beetle
Flour Beetles Saw-Toothed Grain Beetle
Flat Grain Beetle
Insects/Mites Associated with Damaged/High Moisture Grain:
Rusty Grain Beetle Foreign Grain Beetle
Hairy Fungus Beetle Grain Mites
STORED GRAIN MANAGEMENT METHODS
Some Non-Insecticidal Approaches to Stored Grain Insect Management
Temperature Manipulation - The objective here, through proper management of aeration, is to maintain grain storage temperature as low as possible to reduce insect reproduction. Condensation of moisture in the grain mass is prevented by slowly reducing the gradient between the grain mass temperature and the outside (ambient) temperature.
Regulation of Grain Moisture - Most stored grain insects cannot live on extremely dry grain (less than 10 percent), however it is impractical to reduce grain moisture much below minimum moisture levels necessary for long term storage. Insect activity and reproduction are favored by high grain moisture (14 percent or more), especially when condensation and molds occur, and fermentation raises temperature in the grain mass. Such spoilage and internal heating allows insects to remain active even in winter.
Sanitation/Sweeping - The single most effective control or preventive measure for many insects is the use of sanitation procedures. By eliminating spilled, cracked and broken grain and grain flour, along with insects feeding on such material, and by cleaning grain, one can correct conditions that could exacerbate storage problems over time. Since stored grain insects invade new crop grain from infested harvesting and handling equipment (combines, augers, etc.), older stored grain and infested bins, slotted floors and aeration ducts, therefore cleanup and sanitation are essential.
Harborage Removal - Remove infested, unused equipment and other clutter around storage facilities.
Exclusion - Insect-proof structures, plug holes, seal bins by caulking and making general repairs.
Interception - If purchasing older grain for storage with newly harvested grain, keep a close watch for insects in the incoming grain. If infested grain is purchased, say for feeding to livestock, store it away from the new crop and feed it as soon as possible.
Rotation - Grain stocks may be rotated, or moved and a grain protectant applied at the time of turning.
Trapping - Several stored grain pests can be monitored with special traps that utilize various "pheromones", essentially insect odors or "perfumes". These may be used to attract males (sex pheromone) or in some cases, both sexes (aggregation or feeding pheromones).
Storage Practices - Store sound, clean, dry grain. It may be advisable to screen out broken grains, trash and fines to increase the quality of the final storage product. Also, the elimination of trash will enhance fumigation, should this procedure be required.
Landscaping - Cut or otherwise control weeds around grain bins and flat storage where grain is to be stored to prevent providing harborage to insects and rodents. Landscaping should be maintained well back of stored grain facilities. We suggest leaving a 4 foot wide strip of bare gravel around the perimeter of on-farm grain storage bins.
Exterior Lighting - The use of sodium vapor lamps, as opposed to mercury vapor bulbs on farm yard lights will reduce many insects that are attracted to lights. While many of these insects do not directly attack stored grain, they may end up on the grain surface if lights attract them toward storage areas.
Heat - For smaller quantities of grain, heat can be used to eliminate insect pests. Heat is also now being used in large grain mills and processing plants as an alternative to grain fumigants. Heating of grain to 130 degrees F. and holding it at this temperature for up to 24 hours will result in excellent control of stored grain insects.
Freezing - Reducing grain temperature to 0 degrees F. and holding it there for 7 days would reduce or eliminate stored grain insects. Even if feasible in certain situations, it must be noted that problems could arise while rewarming grain from such extreme temperatures.
Vacuuming - An excellent tool to reduce available food and harborage for stored grain insects. Essential for proper cleaning of grain bins.
Some Insecticidal Approaches to Stored Grain Insect Management
General Treatment Around or Outside Grain Bins
Options for perimeter treatments around the outside of grain bins include diazinon, carbaryl (Sevin) and chlorpyrifos (Dursban). Do not confuse and interchange chlorpyrifos (Dursban or the field crop insecticide, Lorsban) with chlorpyrifos-methyl (Reldan) as these are very different chemicals. Never under any circumstances use Dursban or Lorsban formulations directly on stored grain. Only chlorpyrifos-methyl (Reldan) is approved for direct application to grain.
Residual Bin Sprays
Spray inside bin surfaces with one of the following: methoxychlor 25EC (3.75 pts/5 gals. water, apply to point of runoff), malathion 57EC, Premium Grade (1 qt./5 gals. water, apply liberally, use 1 gal finished spray per 1,000 sq. Ft. of surface, chlorypyrifos-methyl (Reldan) 4E (0.5 pt./3 gals spray, apply 1 gal./1,000 sq. Ft., Diatomaceous Earth (0.4-1.0 lb/1,000 sq. ft, see label), cyfluthrin (Tempo 2), (8 ml/gal/1,000 sq. ft., cyfluthrin (Tempo 20WP) (9.5 gm/1,000 sq. ft.)
Vapona (DDVP resin strips). Suspend 1 strip per 1,000 cubic feet of air space above the grain mass. Effective only on the adult stage of the indianmeal moth, lasts about 3 months.
Direct Grain Treatment With Protectants
These materials can be applied to older grain as it is being moved or to new grain coming in from the field and going into long term storage. Grain protectants are applied at the proper rate to the grain surface or to the hopper as the grain enters the auger so that it is mixed well with the protectant as it is binned. Available choices include: Malathion 6D (10 lbs/1,000 bu.), chlorpyrifos-methyl (Reldan 4E, 11.5 fl. Oz. in 1-5 gal. water mixed with 1,000 bu. grain, not registered for use on corn, lesser grain borer not listed on label), chlorpyrifos-methyl (Reldan 3D) (Apply 10 lbs. per 1,000 bu., can apply to grain surface as "top-out" treatment at rate of 7 lb./1,000 sq. ft. of surface area, not registered on corn), pyrimiphos-methyl (Actellic 5E) (Use on stored corn and grain sorghum, not wheat, at 9.2 to 12.3 fl. oz./5 gal water, will treat 30 tons of grain or 1,071 bushels, "suppresses" lesser grain borer), Pyrethrins (dilute one part spray with 29 gal. water, apply 4-5 gal/1,000 bu, gives short-term knockdown), and Diatomaceous Earth (Insecto) (0.4 to 2 lb/ton of grain and 1-3 lb/1,000 sq. ft as a topdress, distribution is critical and grain should be below 13% moisture, also applicator needs respiratory protection).
The following may be used as surface treatments for binned stored grain. If grain is crusted over with silk and debris from feeding of the indianmeal moth, remove damaged grain and/or break up crust so that the protectant can be incorporated. Apply surface application of malathion 57EC, Premium Grade (0.05 pint per 1-2 gals. water/1,000 sq. ft., note that malathion may not be effective for indianmeal moth), Bacillus thuringiensis (Dipel, Thuricide, Bactospeine, Top-Side Dust) Effective only on indianmeal moth (See formulation for proper rate), Pyrethrins (dilute one part product with 19 parts water and apply 2 gals/1,000 sq. ft., chlorpyrifos-methyl (Reldan 3D) (Apply 7 lbs. per 1,000 sq. ft. of grain surface, not registered on corn), Diatomaceous Earth (Insecto) Apply 1-2 lbs./1,000 sq. ft as a topdress, distribution critical and grain should be below 13% moisture, applicator needs respiratory protection. Rake surface-applied protectants into the top 5 - 6 inches of grain.
Fumigation is the process of controlling pest insects in the grain mass by creating a poisonous atmosphere, produced by applying a grain fumigant. Fumigants are dangerous and must be applied only by teams (2 people minimum) of applicators who are trained, certified and experienced and who have the proper clothing and safety equipment. Fumigation is not effective unless the storage structure to be treated is well sealed and the grain temperature is well above 50 degrees F. Follow all instructions to the letter when fumigating, especially those concerning applicator safety.
Cleanout Fumigation - Chloropicrin, a heavier-than-air, non-flammable fumigant can be used to clean insects out beneath subfloors and aeration ducts in empty storage bins. Do not use chloropicrin on feed or seed grain. The product is related to tear gas and is highly corrosive. See label for specific application rates and procedures. Proper training and certification, safety equipment and procedures must be utilized when using this or any fumigant product.
Conventional Fumigation - Just two products are registered for grain fumigation -- methyl bromide (Meth-O-Gas, etc.) and aluminum/magnesium phosphide (Phostoxin, Fumitoxin). Methyl bromide (MB) is contained in pressurized cylinders and the gas is introduced into sealed (essentially airtight) stored grain bins, through a closed system, often one that recirculates the fumigant. MB is heavier than air, so it moves through the grain toward the bottom. This fumigant should be used only by licensed commercial applicators. The toxic gas produced by aluminum or magnesium phosphide pellets or tablets is called phosphine, which is slightly more dense than air, so it moves only very slowly through large grain masses. A closed loop fumigation or recirculation system may be needed to recirculate the fumigant. While there is evidence that carbon dioxide gas used in low concentrations with phosphine may increase the effectiveness of the phosphine fumigant, CO2 alone is not much used in the U.S. This material, which is clear, odorless and dangerous to handle, has the ability to get insects to release their spiracular controls, forcing them to open and take in the companion fumigant more quickly. In Australia, they are having some success with CO2 in tight fumigation chambers.
Cautions When Using Insecticides to Protect Stored Grain
Follow labels exactly to prevent damage to grain and seed, contamination of food and feed, and to prevent personal injury.
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Hagstrum, D., P. Flinn, and S. Fargo, 1995. How to sample grain for insects. In Stored Product Management, Circular Number E-912, Oklahoma State University. 242 pp.
Hedges, S. A. and M. S. Lacey, 1996. Field Guide for the Management of Structure-Infesting Beetles. Volume II, Stored Products Beetles/Occasional and Overwintering Beetles. Published by Franzak and Foster, Cleveland OH. 212 pp.
Higgins, R. and G. Lippert, 1987. Section 3. Insect management in stored grain. Stored Grain Management. The Kansas Wheat Commission, Kansas Corn Commission and Kansas State University. USDA Extension Service Special Projects - IPM.
Noyes, R., R. Weinzierl, G. W. Cuperus and D.E. Maier, 1995. Stored grain management techniques. In Stored Product Management, Circular Number E-912, Oklahoma State University. 242 pp.