Some Insect Pests of Important Seed Crops

F. V. LIEBERMAN, F. F. DICKE, AND ORIN A. HILLS.

THE HARMFUL insects that seek food and shelter in forage legumes, grains, and sugarbeets must be controlled if crops of seeds are to be produced profitably.

Modern chemicals have made it reasonably easy to prevent much of the damage they inflict, particularly when the crop leavings are not to be used as feed. For controlling others, we rely on helpful farm practices and resistant varieties. Parasites, predators, and the diseases that attack the insects often help.

Most of the harmful insects attack the plants when they are in the vegetative, or growth, stage. Controlling these root and foliage feeders is the first order of business in protecting a seed crop, for a healthy plant with normal growth is the best foundation for successful seed production. In fact, in combined effect the insects feeding on the roots, leaves, and stems often are responsible indirectly for most of the loss in yield and also reduce the quality of seeds.

Different conditions over the country influence the choice of chemical treatment. The stage of development of an insect and the particular pest complex present at the time of treatment also must be considered.

County agricultural agents give advice on the insecticides to use and suggest the dosage that gives good control. They also know the restrictions placed on the use of various chemicals to protect the health of people. Some of our most effective insecticides must not be used on a seed crop if they leave a residue on any part (chaff, straw, stalks, leftover seeds) that is to be fed to livestock.

Some insects that feed on plants in the early stages of development, like the beet leafhopper (Circulifer tenellus) and the green peach aphid (Myzus persicae) on sugarbeets, carry plant diseases.

The insects that feed on the buds, flowers, or seeds can more truly be called seed-crop pests. Many are the same ones that earlier fed on the stems or leaves or, in another developmental stage, on the roots. They are best controlled therefore before they directly attack the fruiting parts.

Some destructive insects specifically seek the buds, flowers, or seeds. They ordinarily cause damage out of proportion to their numbers and their feeding. Their damage is often swift or hard to detect. Some are thought to inject toxins into the plants. Controlling them is further complicated by the need to protect pollinating, bees from destruction while they are visiting flowering fields or flying to and from their hives or nests.

EVERYWHERE where seed alfalfa is grown, the lygus bugs must be controlled. Their tremendous importance was first shown by C. J. Sorenson at the Utah State Agricultural Experiment Station before 1932. Growers of alfalfa seed generally did not regard them as their chief tormentors until after DDT was developed, and the great benefit of their control was demonstrated. Now they are recognized as the chief pests of seed crops of alfalfa and sugarbeets. They do great damage in cotton, other legumes, and a number of vegetables.

Three species of the genus Lygus, L. hesperus, L. elisus, and L. lineolaris, are important. The last is sometimes called the tarnished plant bug. The lygus bugs are pale-green to reddish-or dark-brown sucking insects about three-sixteenths inch long. Their young are yellowish or bluish green. The adults insert their eggs into the top growth of plants they attack.

No distinction is made in a control program among the Lygus species. We do know, however, that differences exist in their relative abundance from place to place and from crop to crop. L. lineolaris is the common species in the eastern half of the United States. Hesperus dominates in the Southwest, but on seed sugarbeets lineolaris is equally abundant. L. elisus tends to be most abundant in the Northwest. Within these regions, the composition of the lygus bug population on cultivated crops may vary considerably; often it is influenced locally by the abundance of various wild host plants.

The damage the several species do also varies. L. hesperus and lineolaris favor the seeds of alfalfa and sugar-beets and can do more damage to them than elisus, which prefers to feed on the buds and flowers.

Lygus bugs live and feed on many kinds of crops, weeds, and native vegetation. The adults are strong fliers and have an uncanny knack of seeking out alfalfa at just the right time to lay their eggs so that hatching begins when the first buds appear. The bugs have been known to destroy 99 percent of the potential crop in a field of alfalfa by blasting the flower buds as fast as they are formed. When not enough of them are present to prevent flowering, they will feed on the blossoms and cause most of them to drop. Then they suck the juice from the seeds.

DDT was first tried against lygus bugs in 1944. The results on alfalfa were spectacular. In Utah, for example, a DDT-treated plot yielded 24 times the amount of seed produced on an untreated check plot. Soon all producing areas were utilizing DDT to control lygus bugs. Yields started an upward climb. The average yield was tripled by 1956. A part of the gain was due to better control of other insect pests, better crop management, and increased use of honey bees for pollination. But the lygus bug had to be checked before such gains could be registered.

During the late 1930's, when the large-scale production of seed of sugar-beets was getting started in this country, growers were troubled by the occurrence of a high percentage of nonviable seedballs. The corky balls would contain two to four seeds; all would be dead; but the grower might not know that until harvesttime.

At first the dead seed was attributed to adverse weather, insufficient water, or alkaline soil factors that do cause seed embryos to abort. But researchers of the Department of Agriculture questioned that the severe losses were caused entirely by soil or moisture conditions. They suspected insects.

At Phoenix, Ariz., in 1938 various harmful insects found in seed beet fields were caged alone on developing seedballs. Then the balls were dissected to appraise any damage done. It was learned that the same three species of lygus bugs that damage seed alfalfa were the main offenders. Both adults and the nymphs inserted their mouth parts through the balls and into the seeds, drawing out the sap and destroying the embryos. The nymphs and the female adults of all three species damaged more seeds than the males.

Additional cage tests showed that the greatest amount of damage done to the beet seed crop by lygus bugs was to the soft, newly formed seeds. Little or no damage was done to the crop before or after this stage.

Efforts to develop an insecticide treatment that would kill the bugs before the crop of beet seed attained the soft-seed stage were rewarded when DDT was tested. By application of DDT, growers could consistently produce good yields of good seed.

MOST CHALCIDS are beneficial insects and destroy many common pests. A few chalcids attack crops.

One species destroys the seeds of alfalfa and certain clovers and trefoils. It is a tiny, black wasp that lays its eggs singly in their seeds. It does not attack any other part.

No differences were observed for many years among the chalcids in alfalfa, clovers, and trefoils.

A. N. Kolobova, a Russian scientist, in 1950 demonstrated that chalcids raised on alfalfa would not lay eggs on clover, and vice versa.

Now we know that there are at least three biological races of legume seed chalcids. One infests alfalfa and other Medicago species. One develops on red clover and other Trifolium species. A third lives on birdsfoot trefoil and other Lotus species. They are all called the clover seed chalcid (Bruchophagus gibbis).

This pest has attracted special attention in California since 1957. Kern County has a relatively new certified alfalfa seed industry. It started in 1949 when the seed supply of certified varieties was short, and it was learned that seed of new varieties could safely be grown outside their region of adaptation. Good crop and pollination management and insect control provided excellent yields. As it was a new area, chalcid damage was not a problem at first. By 1957, however, the clover seed chalcid was cutting deeply into yields of seed despite good practices conducive to its control, and it became clear that better control measures were urgently needed. Greatest hope lies in new systemic insecticides or in the development of resistant varieties. Research workers undertook studies of both.

Each egg of the variety of seed chalcid that lives on alfalfa is laid inside a partly developed seed of alfalfa or burclover. The newly hatched larva gradually devours the contents of the seed, pupates, and changes to an adult wasp. The wasp chews its way out through the seedcoat and the pod. It leaves a hole in each.

One to several generations may develop, depending on the latitude and altitude of the area, each season. The winter is spent as full-grown larvae within the hollowed-out seedcoats.

These infested seeds are the source of infestation for the new year wherever they may be--in cleaning plant or warehouse, in the thresher, on the ground, or on volunteer or other uncut plants that have gone to seed.

Control measures recommended in 1961 were based on destroying as many of the larva-bearing seeds as possible. To do so, one should clean all seed carefully and destroy or use the cleanings, prevent seed from forming on volunteer plants, clear the field after harvest, work all chaff into the soil, and, if necessary, provide moisture to encourage the growth of fungi present in the soil that will kill the overwintering larvae in the seeds.

STINK BUGS, particularly the Say stink bug (Chlorochroa sayi), occasionally cause great damage in the West to alfalfa and sugarbeet seed crops as well as to small grains.

An outbreak of stink bugs usually depends on unusually good conditions for their development in the uncultivated areas. During wet years on the deserts, a lush growth of host plants can produce a huge population. The adults fly to cultivated areas, often far away, when the desert plants dry up. They usually attack grains first, but may fly directly to alfalfa or sugarbeets. They suck the sap from the immature seeds and have been known to destroy excellent seed sets completely. In sugar-beet fields they can damage mature seed and may even do damage after the seed growth is cut and windrowed.

Of the many organic insecticides that have been used to kill stink bugs, toxaphene, benzene hexachloride, and dieldrin are particularly effective.