The Pea Weevil

T. A. Brindley, Joseph C. Chamberlin.

The pea weevil depends entirely on edible and field peas for its existence. It occurs nearly everywhere peas are cultivated.

It is a pest in all pea-growing areas of the United States and Canada. It is especially abundant in places where peas are grown for the dry seeds. In some localities the upper Snake River Valley of eastern Idaho and parts of Montana, among them it is held in check by the long, cold winters. Heavy and long continued rains, such as occur in parts of the coastal areas of Oregon and Washington, also reduce winter survival.

Until 1920 or so the pea weevil was considered primarily a pest of dry or seed peas and, indeed, the main limiting factor in their production. In vain attempts to evade its ravages, the industry moved steadily westward from one growing area to another, until the now great pea-growing areas of the West principally Idaho, Oregon, and Washington, were reached. When the production of green peas for processing was begun, the problem became even more acute because the weevily or "wormy" peas are unfit for human use.

To meet this challenge and to assist the new industry in controlling weevils so as to prevent the contamination of canned or processed products, the agricultural experiment stations of Oregon, Washington, and Idaho and the Department of Agriculture in 1930 began a cooperative research program to develop a solution.

The damage done by the pea weevil is due entirely to the feeding of the grubs or larvae within the growing seeds. Almost always a single larva completes its development in one seed, but so many eggs are laid that every pea may be infested. In the Willamette Valley of Oregon infestations in 70 to 90 percent of the total crop have been observed and an average loss of 30 to 70 percent may occur annually if steps are not taken to control it. In other sections, such as the Palouse area of eastern Washington and northern Idaho, the damage is less, although infestations of 5 to 50 percent have been recorded.

THE PEA WEEVIL is one of several types of weevils of the family Bruchidae, which exist by feeding on seeds of leguminous plants. The adult is small, generally mottled, grayish or brownish gray, and about one-fifth of an inch long. It has a small head with short threadlike antennae, a "round-shouldered" oval body, and long hind legs. The weevils are active insects, running and flying quickly during warm weather but in a rather clumsy fashion. When disturbed they often fall to the ground and play dead.

The life history of the pea weevil is simple. The winter is passed in the adult stage. The weevils fly into the pea fields at the end of the hibernation period at a time that coincides rather closely with the blossoming period of the peas. Mating follows a relatively brief period during which the weevils feed on the nectar and pollen of the pea flowers and to some extent on the petals or foliage. The female does not produce viable eggs unless she has fed on pollen. By the time the pea pods have dropped their blossoms, she is ready to lay eggs. The orange or lemon-colored eggs are oval and less than one-sixteenth inch long. They are laid singly or sometimes in pairs, one above the other, on the outside of the pea pod, to which they are firmly attached by means of a transparent glue-like substance. Within a few days the darkening head of the developing larva may be seen through the translucent shell. Hatching time varies with temperature, being shortest during warm periods. The incubation period may be as short as 4 days or as long as 2 weeks or more. It averages about 9 days.

The tiny grub when fully developed cuts its way through the thin eggshell. Then it eventually finds its way through the young pea pod into one of the embryonic peas within it. Several grubs may enter a single pea but usually only one grub lives to complete its development. The hole made by the larva in entering the pea soon heals, but its site is usually visible as a tiny dark spot or "sting."

Once safely within the pea the larva soon molts. Having no further need for them, it then loses its legs and the peculiar tomblike structure on the back of the first segment of the thorax, which no doubt helped the newly hatched grub in getting from the egg into the pea. The larva is now white or creamy yellow, except for the strong, dark-brown, jaw-like mouth parts. For the rest of its larval life it eats and grows rapidly and does most damage to the pea. The endosperm and often the germ of the developing seed are almost completely eaten. Infested seed either die or at best produce weakened sprouts.

The larval stages, four in all, take 4 to 8 weeks (depending on temperature) to reach maturity. Development proceeds fastest in warm weather.

The full-grown larva cuts through or nearly through the seed coat leaving a circular cap or "window" through which the future adult may easily force its way. Afterwards the larva becomes nearly inactive for a short period the prepupal stage. Then it molts, giving rise to the pupal stage, which averages 12 to 14 days. The pupa looks like the adult. During the prepupal and pupal stages, by a process as mysterious as the development from egg to larva, the internal tissues regroup themselves to form the adult. The adult beetle is first soft and pallid, but the skin soon hardens and within hours assumes the normal adult colors. The size of the adult varies with the amount of food it had during the larval stage. Because no one larva feeds in more than a single seed, the size of the pea affects somewhat the size of the emerging adult. Small or undeveloped peas may be entirely consumed except for the seed coat; from them come dwarfed adult weevils. The normal adult is strong, vigorous, and well provided with relatively enormous stored energy in the form of fat. That is important because no further feeding occurs until the following spring.

ENVIRONMENTAL FACTORS as well as available food supply govern the abundance of the pea weevil. Among them are suitable climatic and cropping conditions for development and successful hibernation.

The pea weevil in its spread to new areas has mostly left behind the parasitic and predatory insects that presumably evolved along with it in its native habitat. Or possibly such natural enemies as did succeed in accompanying it found the new conditions unsuitable to their survival. In any case, the pea weevil in this country is practically free from the attack of parasitic insects. Attempts to introduce parasites have not been successful.

Beginning with the emergence of the adult weevils from the peas, there are four critical periods to consider: The period during the summer-to-fall emergence of the weevil with the succeeding dispersal flights to hibernation quarters; the hibernation period; the period of dispersal flights in the spring from hibernation quarters to the pea fields; and the period of oviposition.

The adult weevil appears about the time the pea in which it feeds becomes fully developed. Under some conditions many peas shatter from the pods and become scattered over the ground. Additional peas are lost by shattering during the harvesting process. Weevils from any of the infested peas begin to emerge within a few weeks. The rate of emergence is stimulated by moisture from heavy dew or rainfall. The emerging weevils take flight and drift about with the prevailing winds until by chance they arrive at locations which offer more or less suitable shelter in which to pass the winter. These seemingly indirect flights of the pest may take it 3 miles or more from the place it emerged from the pea. In Northwestern States, fall emergence flights start about mid-July and early August and continue until early October.

The adult weevil is a daytime flier and its critical activities are governed by temperature. Flights rarely start earlier than 8 a. m. and continue later than 4 to 6 p. m. Peak flights occur near or shortly after noon. The minimum observed temperature at which summer and fall flights take place is 60 -61 F.; peak flights occur mostly at temperatures between 66 and 79 .

The number of weevils coming from shattered peas in unharvested, abandoned fields or from volunteer pea stands in other crops is large, but still is smaller than the number present in the harvested seeds. The weevils in green peas are destroyed in processing and therefore do not contribute to weevil populations.

If the peas are stored in tight sacks or other tight containers, many weevils may remain in the dry seed and emerge only in limited numbers or not at all until the seeds are planted. Then, activated by the soil moisture, the weevils emerge from the seed and soil. If infested peas are kept in dry containers long enough, many adults never emerge at all but die in peas. In a few cases, weevils kept under such conditions have survived two full winters before finally succumbing.

The fall migration period is one of peril to the pea weevil. Many are destroyed by predatory insects, animals, and birds. The greatest danger, however, results from various farming operations. Livestock and poultry foraging in the field after harvest may destroy weevil-inhabited peas in large numbers. Deep plowing and packing the soil immediately following harvest kill many weevils. Finally, the treatment of the dry peas by fumigation, heat treatment, or other methods shortly after harvest takes an extra toll.

It would almost seem that man might spell the doom of the insect by applying cultural practices and fumigation. But that has never been the case, for enough weevils invariably escape to perpetuate the pest mainly because of the practical impossibility of securing community-wide adherence to the needed practices and because cultivation and fumigation are not completely effective under the conditions that obtain in pea production. Such practices are nevertheless of great help in reducing subsequent infestations and are a vital part of any good control program.

The hibernation shelter of the weevil might be almost any crevice in natural or artificial objects or debris of any kind into which the insect can crawl and be protected from the weather. The most favored quarters are those that are nearest the place where the weevils came into being. Checked and split posts, old sheds, barns, warehouses, or other structures offer favorable situations; so do crevices in the bark of trees, brush piles, weedy or brushy field edges, moss and lichens on trees, and forest litter. In some places such as the rolling, treeless areas of eastern Washington and northern Idaho, no hibernation quarters may be nearby, and many of the insects fly far up the mountain slopes, where they find suitable quarters under the bark of trees or in the forest duff, especially in the ponderosa pine groves.