Postharvest Control of Insects and Fungi

LYMAN S. HENDERSON AND CLYDE M. CHRISTENSEN.

INSECTS and fungi attack seed in storage to the extent that its viability will be reduced or completely destroyed if preventive steps are not taken. It is foolish to make the considerable investment in all that is required to produce good seed and then allow it to become inferior through the damage inflicted by insects and fungi during storage.

Only since about 1940 have we become fully aware of the extensive storage losses that were occurring. In the meantime we have learned much about how to reduce the losses.

It turns out to be a fortunate coincidence that the same conditions required to prevent insect or fungus attack on seeds are ones that are favorable for the preservation of other qualities of seeds.

The important facts can be oversimplified in the statement that dry, clean seeds should be stored under cool, dry, clean conditions. This chapter expands on these ideas, discusses other preventive and control measures, and gives some additional background information.

Insects can be one of the most important single causes of damage to seeds after harvest if infestation is not prevented.

The kinds of insects that attack stored seeds are primarily the same that infest stored grain, cereal products, and a number of other commodities. They are commonly called stored-product insects and mostly are general feeders on plant products.

About a dozen species are responsible for most of the damage. Another 50 kinds frequently infest stored products, including seeds.

The insects found most commonly in stored seeds are rice weevil, Sitophilus oryza; granary weevil, Sitophilus granaries; lesser grain borer, Rhyzopertha dominica; Angoumois grain moth, Sitotroga cerealella; cadelle, Tenebroides mauritanicus; saw-toothed grain beetle, Oryzaephilus surinamensis; flat grain beetle, Cryptolestes pusillus; flour beetles, Tribolium species; dermestids, Trogoderma species; bruchids, several bean and cowpea weevils; Indian-meal moth, Plodia interpunctella; and almond moth, Ephestia cautella.

These insects cause several kinds of damage to seeds. The most serious is that they chew out the germ, which is rich in food value and is often the first part of the seed attacked by some of the moths and beetles. Then, of course, the seed cannot germinate.

Some insects live inside the seeds and feed primarily on the endosperm. The germ may not be damaged directly, but partial or complete reduction of the endosperm lowers the vitality of the seed, or the seedlings may be weak and unable to survive.

The moth larvae spin large amounts of webbing that entangles the seeds in ropy masses. The webbing may build up into unsightly films over the surface of bags of seed or on top of seed stored in bulk. It is a sign of potential damage to the germ. Some of the seed-infesting insects chew holes through fabric, film, and paper containers.

Most of the insect infestation originates after the seed is placed in storage. The kinds of insects involved are widely distributed, abundant, and feed on a wide variety of grains, cereal products, animal feeds, and other commodities. They occur commonly in warehouses, storage bins, barns, feed sheds, and homes. Seeds may become infested during temporary storage on the farm or during warehouse storage if it is near infested products or put into infested bins or structures.

Many of the insects are strong fliers and can move into storage structures to start infestations if preventive measures are not carried out. Used bags can also be a source of infestation if they are not cleaned thoroughly or fumigated before refilling.

Bagged or packaged seeds carried over from one season to the next provide an infestation hazard. The presence of insects inside the containers may not be detected. Adult beetles or moths lay their eggs near or on packages. The tiny, newly hatched larvae can crawl in through minute openings. The first external evidence of trouble may be when the mature insects cut holes in the package through which to emerge after their development is completed. By this time extensive damage may have been caused and moth larvae will have spun webbing.

The necessary reserve stocks of seeds are another source of difficulty. The longer storage period provides time for the development of more generations of insects and possibly a tremendous increase in numbers. The older seeds may also become more susceptible to attack if storage conditions have not been proper.

Seeds may become infested in the field before harvest in the Southern States, notably by the rice weevil and the Angoumois grain moth. They can complete their development and continue to reproduce after the seed is put into storage. Damage from field infestation can be kept to a minimum by prompt harvesting and proper handling, which may include drying or fumigation, or both.

Field infestation by stored-product insects is not common in the Northern States where much of our seed is produced. An exception is the attack of bruchid beetles on beans, peas, and other legumes. Most of the species continue to reproduce after the seed is harvested and stored. Insecticides should be applied in the field to prevent or reduce this infestation. Proper harvesting and handling procedures will stop further damage.

Temperature and moisture are important factors in relation to infestation. As the temperature and moisture become lower, the rate of insect activity, feeding, development, and reproduction is reduced. Low moisture and temperature also help to keep seed in the dormant condition that is necessary to maintain its quality. We can take advantage of these conditions to preserve seeds and at the same time prevent insect damage.

The amount of dockage in seed also influences its susceptibility to attack. Clean seed is of better quality and is better able to resist infestation.

A further preventive measure is to clean the storage structures and bins thoroughly so they are free of infestation before seed is placed in them.

Protestants applied directly to the seeds are especially useful for preventing infestation in bulk storage. Insect-resistant packages protect seeds after they are placed in containers of various types.

Control or corrective treatments are required when seed becomes infested before protective measures can be applied, or when such measures have been inadequate. These treatments may include fumigation, the application of an insecticidal spray or dust, or the use of controlled heat. We discuss them in more detail later.

FUNGI probably always have caused losses in stored seeds of all kinds when storage conditions were improper. The losses were overlooked for centuries. Now we have found out how common and significant they are. We know how to detect them and recognize the causative organisms. We have learned how to avoid the damage.

About a dozen species of Aspergillus and several species of Penicillium comprise the storage fungi. They invade seeds almost solely after harvest. They are different from the field fungi, such as Alternaria, Fusarium, and Helminthosporium, that cause seedborne diseases and infest the seeds as they are developing on the plants in the field. The moisture content of harvested seed is so low that these fungi do not continue to grow. They are dormant and do not cause losses of seeds during storage. They cause root rots or blights of the germinating seeds or diseases of the growing plants.

The primary damage caused by storage fungi is their effect on the germination of seed. Invasion of seeds by storage fungi can weaken or kill them. The result is slow or erratic germination when the seeds are planted or a low percentage of germination.

Stored seed may be damaged by heat as a result of the activity of storage fungi. Heating is common in baled cotton or wool, hay, grain, or seeds stored in fairly large bulks. At one time the heating was thought to be due to the natural respiration of the grain or seed itself. Many practical storage men still believe this to be true. They do not know that moist grain or seed is killed in a few hours by temperatures slightly above 100 F., and that after death there is no respiration. There is no evidence that seeds or grain stored at moisture contents below 18 percent will respire rapidly enough to cause any great increase in temperature.

Insect infestation will cause an increase in, temperature to as high as 108 . Storage fungi accompanying or following insect infestation, or developing independently, will raise the temperature up to 130 . In the process, they may produce enough moisture to permit thermophilic (heat-loving) bacteria to grow. They may further raise the temperature to about 175 . If conditions are right, compounds produced by the growth of fungi and bacteria may undergo oxidation that will cause heating up to the point of spontaneous combustion.

Mustiness is another result of activity of storage fungi. When fungi have extensively invaded and partly consumed seeds, they have produced masses of spores. The seeds have a musty odor and a moldy appearance. They may be caked together by the fungi. This is the first evidence of fungi to many who store seeds, and unless it reaches this stage they are unaware of the infestation. It is, however, the final stage of spoilage. The preliminary stages that precede damage can be detected only by special techniques.

"Germ damage" is a term applied to dark brown or black embryos or germs in small grains and corn caused by storage fungi. This type of damage is important in grain or seed that is to be processed into flour, meal, or other food. The damaged germs may end up as dark particles in the final product. Damaged germs usually contain large quantities of fatty acids and often are extremely moldy. They impart undesirable flavors to the food. Damaged germs reduce the market grade and price of food grains. They are an indication of possible poor quality in seed.

Spores of storage fungi occur in small numbers on the outside of seeds at harvesttime. There also may be some slight and superficial infection of the outer parts of the seeds. Even when plants are subjected to continued moist weather at harvest, however, the fungi do not invade seeds sufficiently to cause any reduction in germination. Damaging infection occurs later, after the seeds have been stored, and only if the conditions of storage are such as to permit the inoculum naturally present to grow.

We have accumulated abundant evidence from thousands of tests on many kinds of seeds, including all the common cereal grains, that storage fungi do not infect seeds to more than a very minor degree before harvest.