Applying Insecticides

Using Insecticides Effectively

E. J. Newcomer, W. E. Westlake, B. J. Landis.

Merely to secure effective insecticides and apply them is not enough. The best material may fail if it is used at the wrong time or in the wrong way. The user will do well to learn something about the habits of the pests he wants to get rid of, the physical properties of insecticides, and the influence of weather and type of plant growth on their effectiveness.

The contact insecticides; those that kill only the insects they touch must be applied thoroughly. Because many materials retain their effectiveness for only a short time, the chance that the insects in moving about will later come into contact with them is not great. Also, many kinds of pests, such as some aphids and scale insects, cannot move about, although if the insecticide has some fumigating action it may suffice to place it within a- short distance of the insect. Nicotine, for example, in hot weather will kill aphids not actually wet by it.

Thorough application of the stomach poisons, those that kill the insects that eat them, is less necessary. Many leaf-feeding insects move about. Larvae or caterpillars may travel from leaf to leaf on a plant: flying insects travel from plant to plant. Killing them at the earliest possible time may be unimportant. An application that reaches only one surface of the foliage or perhaps reaches only a part of the foliage may be enough. For a boring insect, such as the larva of the codling moth, however, thoroughness is essential in order to destroy it before it gets out of reach of the poison.

Consideration should be given to possible residual action of insecticides. Some are effective only for 24 hours or less. They evaporate or decompose rapidly, and often are particularly useful if a pest has to be dispatched shortly before harvest, as they do not leave harmful residues.

Other insecticides disappear more slowly and are effective for a week or more. They are of value in arid farming areas, for example, where row crops and orchards must be irrigated at intervals, during which time it may not be possible to apply insecticides. Still other insecticides, particularly the stomach poisons, may retain their effectiveness indefinitely.

Thus, if the period during which control is needed is long, frequent applications of an insecticide like tetraethyl pyrophosphate (TEPP), which decomposes rapidly, may be necessary. On the other hand, a single application of DDT in the soil has been known to control some soil-inhabiting pests for 5 years or more.

THE PHYSICAL CONDITION of insecticides affects their efficiency.

Emulsions droplets of oil suspended in water are an example. The size of the droplets largely is governed by the kind and amount of emulsifier used in the mixture. If the size is large, the emulsion will not be stable; the oil tends to rise and float on the surface. The droplets may be kept mixed with the water by vigorous agitation, but they separate rapidly when agitation is discontinued. Separation is likely to occur in the spray tank or in the supply lines to the nozzles, and the result is that sometimes almost no oil will be present in the spray, whereas practically undiluted oil may be sprayed when the tank is nearly empty.

But if the oil droplets are very small, the emulsion may be so stable that virtually no separation will occur, even after standing for hours. For insecticidal use we require something in between. It is desirable to have an emulsion that is sufficiently stable to remain in suspension in the spray tank and supply lines but unstable enough to break immediately after leaving the nozzle, or upon contact with the surface being treated. Such an emulsion gives a much higher deposit of the oil than the more stable type.

Wettable powders are widely used as insecticides. The insecticide and diluent are combined into a dry powder, which is mixed with water before use to form a suspension. The particles must be less than io microns in size in order to give maximum efficiency. Larger particles do not adhere well to plant foliage and give less complete coverage, as there are fewer of them in a given quantity of material. Some commercial preparations even have an average particle size of 1 micron or less.

The physical properties of insecticidal dusts, such as particle size, bulk density, and flowability, are important. A dust, carried to the plants by a blast of air, is easily diverted by natural air currents. Also, the adherence of the particles to the surface being treated depends partly on the velocity at which they strike the surfaces. Very small particles are not desirable in dusts since they tend to drift with the air currents and lose their velocity quickly after leaving the outlet of the machine. As a rule, dusts should pass through a 325-mesh screen, by washing with a suitable medium. Such a screen will pass particles as large as 44 microns in diameter. The usual dust mixture will contain particles much smaller than this, of course, but the average size may be far above that desirable for use in sprays. Particles in excess of about 40 microns in diameter are not desirable because they will not adhere well to the foliage.

Bulk density, the weight of the uncompacted powder per unit volume, may be expressed in pounds per cubic foot. The greater the weight the less tendency there will be for the dust to float or drift. Bulk density is particularly important in aircraft application because then the dust settles quickly to the ground to avoid excessive drift. Dusts so used should have a bulk density of not less than 40 pounds per cubic foot. For application by ground equipment, 30 pounds per cubic foot may be enough.

Mineral oil at the rate of 1 to 2 percent by weight aids in reducing the drift of dusts and probably increases adherence to plants. The addition of mineral oil to dusts for aircraft application is required in some areas.

Dusts must resist packing in the hopper well enough to permit them to flow freely and uniformly into the feed mechanism. A dust that packs will not give a uniform rate of flow from the machine and the coverage will be uneven. Flowability is determined by the diluent used. Some of the better diluents from the standpoint of bulk density do not flow well while some free-flowing materials are too light. Often a mixture of a small amount of a free-flowing material added to one of high bulk density is used to give the desired properties.

Proper timing is important. If put on too early insecticides may be dissipated before the pest is present or in a susceptible condition. If they are applied too late, the pest may already have caused injury. Timing is sometime determined by the stage of growth of the crop. For example, it is not desirable to treat crops when they are in bloom. Bees visiting the blossoms may be killed and the yield of fruit or seed crops seriously reduced because of lack of pollination. The blossoms themselves may be. injured. Sometimes a compromise must be made. For example, eggs of the European red mite often hatch just as apple trees are coming into bloom. An application of lime-sulfur just before blossoming is apt to be too early; one made afterwards is too late for the early-hatching mites. If thorough control is desired at this time both applications must be made.

Some pests, such as the green peach aphid or the two-spotted spider mite, which attack numerous crops, are present during most of the growing season. Effective and economical control of such pests depends on proper spacing of applications so that the crops are protected from injury with not too many sprayings or dustings. Control may be needed over a longer period in an early season than in a late one because the active period of the pests is lengthened.

The location of the pests on the plants also must be considered. If they feed chiefly on the lower surfaces of the leaves, those surfaces may have to be reached, especially if a contact material is being used. If they are on the roots, a soil insecticide is needed. If they climb the trees to feed on the foliage, a treatment of the trunk may be indicated, or perhaps merely a mechanical barrier will suffice.

AS TO WEATHER: The United States Weather Bureau has numerous stations and cooperative observers and can supply weather information for practically any farming area in the country. Information on temperature, rainfall, and wind is especially useful. Much of this information is given in the Yearbook of Agriculture for 1941, Climate and Man. By studying his local conditions over a period of years, a grower can learn to avoid bad weather or take advantage of good weather to a great extent when controlling insect pests.

Wind often limits the application of insecticides. If it is blowing more than 5 or 6 miles an hour, spraying is interfered with, although at times spraying may be necessary in windy weather in order to provide some protection from immediate insect attack. A study of the occurrence of wind may help to determine when to spray or dust. In the Yakima Valley of Washington, for example, the average percentages of good spraying weather during daylight hours in the spring when dormant sprays need to be applied to fruit trees are: March 1-15, 35 percent; March 16-31, 22 percent; April 1-15, 15 percent; and April 16-30, 14 percent. Thus it is advantageous to get this spraying done early in this valley, because good spraying weather is less frequent as the season advances.

Application of dusts by aircraft should be made in the early morning or evening. Usually, after the sun has warmed the air, rising currents tend to carry the dust away from the crop. The same limitation often holds when dusting with machines on the ground. Perfectly calm weather is not necessary, and a slight drift may be advantageous. An air movement of 1 to 8 miles an hour is preferred when aerosols are used.

Temperature inversion, which occurs most often early in the morning or after sunset, should be taken advantage of when using aerosols. The temperature near the ground is then slightly cooler than at a height of several feet; the cloud of insecticide is kept from rising and therefore coats the plants more thoroughly.