The large silken webs or tents in the branches of trees this time of year are the work of the eastern tent caterpillar. These 2 inch long, white and blue marked caterpillars produce this tent primarily in wild cherry trees, but will also infest other fruit tree and shade trees along roadsides. When the tents are large and containmany caterpillars they can eat all the leaves on the tree and do some severe damage.
These insects overwinter as eggs, which appear as gall-like masses on twigs and branches of trees. The caterpillars hatch in early spring--when wild cherry leaves first appear. The begin their "tent" in the crotch of the tree and will expand it as the caterpillars grow larger. The eastern tent caterpillar is most active in late May and early June in Virginia. Full grown caterpillars leave the tree and find secure places to pupate. Adults emerge in early summer to mate and lay the eggs that will hatch the following spring. There is but one generation per year. Fruit and shade trees may be protected from these pests with an application of carbaryl (Sevin), but there may have to be several applications.
It seems that many Americans are fearful of what they know least about--virus-caused diseases, nuclear power plants, and pesticides used on food crops and in and around the house. In reality, we are at greater risk driving our car, smoking cigarettes, and eating fatty foods--but perception is reality!
The potential for an insecticide to become a water contaminant or to come into direct contact with people is what seems to concern the public the most, and this may depend on the insecticide's persistence and fate in the environment. Perhaps a brief review of these two aspects will help prepare Agents, Master Gardeners, and others to understand the problem and to answer questions.
BASICS. The ideal insecticide is applied to a limited surface, contacts immediately or the dried residue is contacted by and quickly kills the target pest and then breaks down in harmless byproducts--usually carbon dioxide, water, and simple minerals. A few household and turf insecticides come close to this ideal, but most persist for several days to months after application. Certainly, in the case of termiticides and perhaps cockroach control chemicals a long residual activity is desirable, but for other insecticides it may not be.
WHAT HAPPENS TO PESTICIDES APPLIED TO TURF/SOIL. Their fate is influenced by exposure to sunlight, heat, rainwater, and soil microorganisms. Each of these and other aspects of the outdoor environment act to degrade insecticides.
Volatilization. Loss to volatilization depends on the pesticide (its specific vapor pressure) and on climatic conditions, especially temperature. Most pesticides have a very low vapor pressure and do not evaporate easily. However, if the temperature is high enough nearly all will evaporate into the atmosphere to some degree. Evaporation usually occurs within the first 4-5 days of application and declines to nothing quickly after that (as the surface residue is degraded by other factors). When air containing the insecticide is inhaled, the chemical may be absorbed through the lungs and enter the blood-stream; however, the amounts are very small and rarely present a risk to humans. Much of the "smell" associated with an insecticide is due to one of the components of the solvent system and not the actual pesticide. In fact, many of the modern insecticides have no odor.
Photodegredation. When an insecticide residue is exposed to direct sunlight, it can absorb energy from ultraviolet light, and that energy can break chemical bonds and degrade the material. Of course, this may limit the use of some insecticides outdoors--and most of those available for turf and ornamental use are fairly photostable. Insecticides used indoors may be more sensitive to light, but they are usually out of direct light--and those that may be sensitive to light are often used in baits where they can be protected.
Absorption by Plants. Some insecticides are readily absorbed by plant foliage, some are not. Once absorbed by the plant the insecticide is usually not transported from the site it was applied. It is usually metabolized and converted into an inactive chemical. Some insecticides are designed to be systemic and their efficacy depends on being transported throughout the plant to come in contact with pest organisms.
Adsorption on Thatch in Turf. The thatch layer may serve as a "trap" for many insecticides applied to turfgrass. When washed off turfgrass leaves, an insecticide next encounters the layer of thatch that accumulates on top of the soil. This layer of dead leaves, stems and other organic matter provides sites for insecticides to attach and become immobilized.
Sorption by Soil. The soil environment provides many obstacles to the residual effectiveness of insecticides. These include adsorption onto soil particles, metabolism and breakdown by microorganisms, and chemical degredations (by soil pH and other chemicals). Soils contain many organic and mineral components which can attract and bind organic molecules such as insecticides. When bound to soil particles, an insecticide is removed from solution and may not be easily moved from that site. The tendency of an insecticide to bind with organic matter is expressed in terms of Koc. A large Koc indicates strong tendancy for an insecticide to bind with organic matter. For example, chlorpyrifos (Dursban) is used as a termiticide and applied to the soil around the perimeter of structures, it has a Koc of 2500-14800; diazinon is not used as a termiticide, it has a Koc of 40-570.
Metabolism by Soil Microbes. Once in the soil, an insecticide can be absorbed by some of the microorganisms present there. Inside microbe cells the insecticide can be degraded into nontoxic components, or simply taken out of action and can no longer exert it toxic effects. Soils high in organic matter normally are rich in soil microbes, and they may have an increased capacity to inactivate insecticides.
Leaching in Percolate Water. Insecticide molecules that escape the potential fates described above and remain in solution in soil percolation water may leach through the soil. There is the possibility that some of the insecticide would make its way into the ground water. However, the likelihood that an insecticide will be transported from the application site to contaminate ground or surface water is subject to many environmental variables. As soon as it is applied the process of degredation and inactivation begin, the intrinsic properties of insecticides, plant and soil substrates make predicting the amount of transport and leaching difficult.