Controlling Pests of Stock

by E. W. LAAKE and W. G. BRUCE

OUR SEARCH for improved insecticides and research to make better those already in use have brought developments that promise far-reaching effects on the control of serious pests of livestock. There are many such pests cattle grubs, lice, horn flies, stableflies, mosquitoes, ticks, mites, and other external parasites of domestic animals and every livestock raiser knows how seriously they can cut his profit.

The outstanding development was the discovery of DDT. When experiments showed its effectiveness against insect pests of man, experimental work was begun with it on the various insect pests of livestock.

Our early work with DDT demonstrated its superiority against many pests over insecticides formerly used. Although its initial action is relatively slow, compared to pyrethrum, its continued action for weeks or months after a single application puts it in a class by itself. The duration of effective residual toxicity of the old insecticides when applied to livestock was usually not more than 3 days for cattle lice and at best approximately 1 or 2 days for the horn fly. Daily treatment was required therefore to control horn flies and at least two treatments at a certain interval were necessary to eradicate cattle lice, because the louse eggs were not affected by the materials formerly used and the effective residual toxicity ceased long before all the eggs hatched.

In contrast, rather low concentrations of DDT kill all horn flies on the animals at the time they are treated, and the residue remaining or, the animal continues to kill this species for 2 to 3 weeks. Because that period is longer than the life cycle of the fly from egg to adult, the entire population on a farm may be wiped out with one treatment. Reinfestation will result, however, after the effective residual toxicity ceases if horn flies are introduced on untreated animals or if the treated animals are exposed to infested premises nearby. Therefore, the duration of control would vary with the size of the area treated and would be proportionately longer as the size of the treated area is increased. This advantage no doubt will encourage communities to undertake treatment of a large area with DDT for the control of the horn fly.

The remarkable results to be had in controlling the horn fly with DDT were well established in a series of tests in 1945. They showed that treated cows and calves gained 42.2 to 51 pounds and 46.9 to 70 pounds, respectively, more per animal than comparable untreated cows and calves in approximately 3 months. The gains were obtained at an expenditure of less than 10 cents a head for DDT. On the basis of the results of the tests (which were made on rather small, scattered ranches that required three or four treatments during the season of horn fly abundance), each pound of technical DDT spent yielded a gain of from 1,202 to 2,306 pounds of beef on treated over untreated animals. The formula used contained only 0.2 percent DDT in suspension, applied at the rate of one-half gallon per animal.

Greater concentrations of DDT in suspension have given longer protection and have an advantage where animals cannot be corralled for treatment as often as desired. The results reported from different areas in the United States with DDT for the control of the horn fly apparently differ considerably. They indicate that the low concentrations that give excellent results in some areas are not so effective in other areas and are relatively ineffective in still others.

The amount of rainfall and the intensity of sunshine seem to be the main factors that bear on the effectiveness of DDT. The application to animals of DDT in the form of an aerosol fog has promise of being a desirable method for treating large herds quickly. Dipping animals in suspensions of DDT to control horn flies and lice has given results fully as effective as those obtained with sprays. Where neither spraying nor dipping is feasible, DDT may be dusted on cattle and other domestic animals. Good hand dusters for small herds or a power duster for larger herds are satisfactory. The effectiveness of DDT against the various livestock insects depends upon the concentration used, rather than on the method of application.

For controlling the stablefly, DDT has not given satisfactory results when applied to animals, but excellent control is obtained when it is applied as a spray in concentrations of from 2.5 to 5 percent to barns, fences, and other objects about the barnyard upon which these flies rest. When the interiors of barns and sheds are sprayed, the DDT residue on the treated surface continues to kill stableflies, houseflies, mosquitoes, and other insects for a long time after treatment.

For lice on livestock, DDT is excellent. The motile or crawling stages of all species of lice are easily killed with low concentrations of DDT applied as a dip, spray, or dust; but because the eggs of lice are not affected by DDT, somewhat higher concentrations are needed in order to obtain sufficient duration of residual toxicity to kill all the young lice that hatch after treatment.

No satisfactory control of horseflies and deer flies has yet been obtained with DDT. Because those species usually do not enter barns or other buildings, they cannot be controlled by spraying buildings, and because DDT is so poisonous to fish, its use in streams and ponds, where the horseflies and deer flies spend their immature stages, is not feasible.

For the control of the sheep tick or ked, DDT is apparently at least as effective as any of the old insecticides. But for the true ticks and mites attacking livestock, DDT has not given satisfactory results, except possibly for the brown dog tick, which seems to be less resistant to it than other species of ticks and mites. For the control of the cattle grub, DDT has shown little or no merit even when applied in high concentration. The rotenone-bearing powders derris and cube--are still the only effective materials for control of cattle grubs.