Control of Forage Pests

by W. A. BAKER

IN THE ever-present battle to control insects attacking cereal and forage crops in the field, the use of insecticides has been restricted. One limitation was the generally low per-acre value of the crops compared to the costs of the insecticides that have been available. That limitation is being overcome by recent discoveries of new insecticides and more efficient ways to apply them.

But it must always be stressed that the application of poisonous materials on crops to be eaten by man or animals is complicated by the hazards of residues remaining on the crops. There is constant need for precautions or adaptations in their use. Up to now our experience with the new insecticides has been too limited to permit a thorough evaluation of the problems of residues, and our recommendations for their use on cereal and forage crops must be qualified accordingly. Several of the insecticides have given satisfactory control of various insects against which they have been tested, and we hope further work will reveal entirely safe ways to use them.

The first of the newer materials to come to our attention was DDT. Late in 1942 we included DDT in laboratory tests against the European corn borer (Pyrausta nubdalis (Hbn.). Its performance was outstanding. In subsequent field tests we found it highly effective against the corn borer and several other insect pests of cereals and forage crops.

An outstanding exception was the sugarcane borer (Diatraea saccharalis (F.), a serious pest of sugarcane. Populations of this insect actually were higher in some DDT-treated plots than in plots receiving no treatment, probably the result of the adverse effect of the insecticide on parasites and predators that otherwise help keep the borer population down.

With grasshoppers, various dust and spray formulations of DDT were effective, but the dosages required to obtain the results were so heavy that we cannot now recommend its use to control grasshoppers : The cost is high, but, more important, there are possible hazards of poisonous residues if livestock graze in treated fields or are fed hay or other forage harvested therefrom.

We have had promising experimental results from DDT against the European corn borer; the corn earworm (Heliothis armigera (Hbn.); the white-fringed beetle (Pantomorus leucoloma (Boh.); and the chinch bug (Blissus leucopterus (Say). Both spray and dust preparations have been effective against the corn borer infesting market sweet corn; treatments with dilute sprays applied with ground equipment at a dosage of about three-fourths pound of DDT per acre provided the maximum protection. Good results also were obtained with dust mixtures containing 6 percent of DDT in pyrophyllite applied at a rate of 40 pounds of mixed dust, or 2.4 pounds of DDT per acre application. Airplane applications of the dust at the same rate, or of a concentrated spray containing DDT in an oil carrier (the latter applied at a rate of 2.75 gallons containing about 2 pounds of DDT per acre application), also gave promising results in preliminary experiments.

Most of our tests with DDT for corn borer control have included four applications of each treatment at approximately 5-day intervals, beginning with the first hatching of borer eggs. Results obtained in a number of experiments, however, indicate that a satisfactory degree of control can be had with fewer applications. The cost would then be lower, and there is the possibility of extending the method to sweet corn grown for canning and to field corn in localities where the borer causes losses.

We got much better control of the corn earworm in sweet corn and dent seed corn with injections of mineral-oil solutions or emulsions of DDT into the developing corn ears at about the time the silks were beginning to turn brown, than with preparations containing pyrethrum or styrene dibromide. The injection of DDT preparations into ears of sweet corn that are to be used for food is not considered safe, however. Very satisfactory protection from earworm attack has also been obtained in preliminary tests by atomizing DDT in oil solutions or emulsions onto the ears and silks of sweet corn. With further development of formulations and application equipment, this method promises to reduce greatly the cost and inconvenience of controlling this insect on corn.

DDT is the most potent insecticide we have tested against adults of the white-fringed beetle, a destructive pest of many crops in the South. Both sprays and dusts are effective as stomach and contact poisons. As a stomach poison, DDT in dust form was 69 to 74 times more toxic than sodium fluoaluminate, and a spray containing one-eighth pound of DDT per 100 gallons of water was about as effective as one containing 8 pounds of synthetic cryolite (85.4 percent sodium fluoaluminate). As a contact poison, DDT in dust form was effective when applied directly to the beetles or on the soil surface, or when mixed with soil, DDT is also highly toxic to white-fringed beetle larvae; dosages of 2 1/2 pounds of DDT or more per acre, mixed in the upper 3 inches of soil, killed all newly hatched larvae. Somewhat higher dosages are necessary to give complete kill of mature larvae. Solutions of 0.5 to 5 pounds of DDT in 7.5 gallons of kerosene have given net mortalities of white-fringed beetle eggs in excess of 99 percent when the eggs were dipped for an instant in the liquid, and were equally as effective as 1 pound of 2, 4-dinitrocyclohexylphenol in 7.5 gallons of kerosene.

Although DDT has been found effective as a contact poison against chinch bug adults and nymphs, the dosages required to get satisfactory kills have been too great to be of practical value with formulations we have tested thus far. But when used as a barrier line on the ground against migrating bugs, dust mixtures of 5 percent or less of DDT in, pyrophyllite compared favorably with dust mixtures containing 4 percent of dinitro-o-cresol in halting the bugs; a mixture containing 3 percent of DDT gave almost perfect protection from the young nymphs.