Insects on Field Crops

Cereal and Forage Insects

C. M. Packard.

For many years the available methods of controlling the insect pests of cereal and forage crops have consisted mainly of modifications in cultural practices to prevent or reduce infestations. Such measures as rotation of crops, thorough and timely tillage, variations in time of planting or harvesting, destruction of crop residues, weeds, trash, and volunteer crop plants, and fertilization to promote rapid, vigorous crop growth can usually be applied for the control of insects with little or no addition to the cost of crop production. Low cost of application is particularly important with respect to cereal and forage crops, which generally are of such low value per acre as to make the use of expensive methods of control impractical.

More recently, increases in yields per acre and market or farm value of cereal and forage crops have raised the permissible limit on expenditures for control of the insects that attack them. Also, more efficient insecticides and insecticide application equipment than were formerly available have now been discovered or devised. In 1950, for example, the average acre-value of corn in the United States was $42 higher than it was in 1940, the acre-value of No. 1 baled alfalfa hay increased $10 or more, that of alfalfa seed increased about $33, and that of red clover seed increased approximately $14. Furthermore, properly applied insecticidal treatments costing about $3 to $6 an acre have produced returns in increased yields amounting to several times their cost.

On the other hand, cultural control measures still continue to be the best if not the only methods of control for some of the insects. Because the general subject of cultural control methods has been discussed in another article (p. 437) , only those that have been worked out and recommended during the past few years are mentioned here.

THE ALFALFA WEEVIL, an immigrant from Europe, is prevalent in the Rocky Mountain and Pacific Coast States. Its control by early cutting of the first and second alfalfa crops of the season is a development in cultural control. Early cutting means harvesting when most of the alfalfa plants in a field are in the flower-bud stage of growth, when there is only a sprinkling of first bloom and only the earliest shoots of the next crop have appeared at the crowns. Clean cutting at that time and prompt removal of the hay leave the weevil larvae on the bare field where nearly all of them, together with eggs and pupae, die of starvation or exposure to heat. Thus, the growth of the second-crop shoots is unhindered by the weevil and comparatively few adults of the new generation will be produced to carry the species over winter.

To keep the alfalfa weevil under control by this means, the first and second crops should be cut early every year even though they are not actually being injured. If this practice is not followed, enough weevils may be produced to cause damage the following year. In areas where the alfalfa weevil does not occur or never becomes a serious pest, early cutting is not advisable; continual early cutting tends to reduce the stand.

Another practice was found to be beneficial in the alfalfa-seed-growing districts of southwestern Arizona for the control of lygus bugs, the little greenish-brown sucking bugs about three-sixteenths inch long and winged when full-grown. They greatly reduce yields of alfalfa seed and hay. Under Arizona conditions they can be fairly well controlled by a community-wide program of cleanly mowing or pasturing off all growths of alfalfa and weed hosts in the winter, early cutting of the first crop of alfalfa for hay in all fields within a 10-day period in late April, growing the second crop for seed and starting it as nearly as possible on the same date in all fields, and regulating irrigation so as to avoid extremely succulent vegetative growth of the seed crop. This system kills off most of the overwintering lygus bugs, and during the growing season prevents the survivors or their progeny from multiplying by moving back and forth among fields in different stages of growth. Thus the insect is kept under control by starvation. This method is less practicable in other areas where there is greater diversity of crops and a larger proportional acreage and variety of wild host plants.

THE WHEAT STEM SAWFLY has caused serious losses of wheat in Montana and North Dakota. It is a little, wasplike insect. In its worm stage it mines up and down within the growing wheat stem. In making its overwintering cell in the base of the stem, it cuts a groove around the inside at about ground level. As the heads become heavy with grain, the infested stems break over in the wind and fall to the ground, so that the harvester cannot pick up many of the heads. The only methods yet developed for reducing the losses are cultural. They consist of early harvesting before many of the stems have fallen; shallow cultivation of the stubble as soon after harvest as possible to throw it out on the surface of the ground where many of the saw-fly worms that overwinter in it will die from lack of moisture; plowing the stubble under deeply where wind and water erosion is not a serious factor, so that the sawflies cannot emerge from it the following summer, rotation of wheat with crops that the sawfly does not attack, such as barley, oats, flax, corn, or mustard; and, in areas to which it is suited, the use of the Rescue variety of wheat, which is resistant to the sawfly. For most effective results these control measures must be applied throughout whole communities.

Cultural measures have been found helpful in the control of the wheat midge in the Pacific Northwest. Fall-sown wheat grown on uplands matures early enough to escape infestation by the midge, but spring wheat becomes infested unless it is sown very early. If sown by the first week in April it usually escapes injury. Either winter or spring wheat sown on low, wet peat land, however, may mature so slowly that it becomes infested. The stubble of infested wheat should be turned under if possible, to bury the midges that overwinter in it so that they cannot emerge the following June to infest the new crop.

The pale western cutworm is a serious pest in the small-grain areas of the southern Great Plains. It is of longstanding importance in the spring-wheat region farther north. It works mostly underground and therefore cannot be controlled satisfactorily with the poison baits that are effective against most species of cutworms. In the spring-wheat region it can be controlled by early spring starvation of the newly hatched worms. That is accomplished by thorough cultivation of the wheat-stubble fields to destroy all green vegetation early in the spring as soon as the weeds and volunteer grain show 1 to 2 inches of growth, followed by a delay of 10 days before seeding to a spring grain crop. In the southern Great Plains, however, where fall-sown wheat is the chief crop, the starvation method of control is not feasible except possibly where a spring grain crop such as barley is to be sown on wheat-stubble land.

Under the conditions prevailing there, winter wheat sown on land that has been cleanly fallowed during the preceding summer and that had been planted during the previous year to a row crop such as sorghum, almost always escapes serious injury. Alternation of winter wheat with clean summer fallow also is an effective way to prevent injury. In applying this method, the stubble of the year's crop is left undisturbed until the following spring, when the ground is cultivated and kept clean throughout the summer until wheat-seeding time the following year. The spring cultivation should start as early as possible without incurring danger of spring soil blowing, preferably before April 15. From the standpoint of soil conservation, subsurface cultivation to produce a trashy mulch may be more desirable than clean fallow, but its effectiveness as a substitute for clean fallow in controlling the pale western cutworm has not been determined.

ROTATION OF CROPS is a good way to control the so-called white-fringed beetles, which are several closely related South American insects and are abundant in some parts of the Southeastern States. Their larval (grub) stages attack many summer crops, especially peanuts, soybeans, velvetbeans, crotalaria, corn, cotton, and vegetables. The adult beetles feed on the legumes, cotton, and various broad-leaved vegetable crops, shrubs, flowers, and weeds. On the other hand, grasses and winter grains are unfavorable to both larvae and adults. Although these insects can be controlled by insecticides, the crop losses they cause can be prevented by the following cultural practices:

1. Plant oats or other small grains for grain and grazing on the heavily infested portion of the farm.

2. Do not plant more than one-fourth of the cropland each year to such summer legumes as peanuts, soybeans, velvetbeans, or other plants that are favorable food for the adult beetles. Do not plant them on the same land oftener than once in 3 or 4 years.

3. Do not intercrop corn with peanuts, soybeans, crotalaria, or velvet-beans, and do not permit broadleaved weeds, such as cocklebur, to grow among the corn.

4. Fertilize cotton and corn heavily with a commercial fertilizer, and use a winter-grown manure crop that can be turned under before the cotton or corn is planted.

MOST OF us are familiar with the large, white grubs with brown heads and curved bodies that live in the soil and feed on plant roots. They are pests of bluegrass, timothy, corn, and several other crops. When they are full-grown they turn into the large, brown beetles commonly known as May beetles or June beetles. They often ruin bluegrass pastures in the Northeastern and North Central States. Grub populations can be reduced on infested farms in those States by planting deep-rooted legumes such as sweetclover, alfalfa, and red clover (which are unfavorable to them) in rotation with the more susceptible crops. The legumes are most effective if they are planted in the years of major beetle flights, which come in 3-year cycles and have been determined in advance for the infested areas.