How insecticides work

Grounds-care professionals who use insecticides on turf and ornamentals can benefit from an understanding of how these materials work. Knowledge of these products' mode of action allows you to more effectively select and apply insecticides, as well as understand their potential hazards. Mode of action, route of entry, and movement on and in plants all play important roles in the effectiveness and safety of insecticides. Let's discuss each of these factors.

Chemical mode of action Insecticides are materials designed to kill or otherwise control insects. Most traditional insecticides accomplish this by interfering with the nervous system of the target insect.

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Many insecticides that grounds managers use are in the organophosphate chemical class. For example, chlorpyrifos, diazinon, isofenphos, malathion and trichlorfon are all organophosphates. Other familiar insecticides, such as bendiocarb and carbaryl, are in another well-known chemical class, the carbamates. Insecticides in both of these classes affect insect nervous systems in the same way.

Nerve cells in all animals (including insects and humans) carry impulses from the point of stimulus (for example, touching a hot stove) to the nerve center (the brain) or from the nerve center back to the muscles, to signal a response (move the finger off the hot stove).

Nerve cells do not actually touch each other. When an impulse (a tiny electrical current) gets to the end of one nerve cell, it must jump across a gap, the synapse, which separates that cell from the next. The gap contains acetylcholine (ACh). Molecules of ACh carry the impulse across the gap to the receiving cell, to which they attach themselves. When this happens, the receiving cell sends out a new impulse, continuing the process. After the ACh crosses the synapse and reaches the receiving cell, molecules of another compound, cholinesterase (ChE), attach themselves to the ACh and remove it from the membrane of the receiving cell. This leaves the cell in its original state, able to receive another impulse (see diagram, page C 4). All of this happens in a tiny fraction of a second, but a nerve impulse must pass through hundreds or thousands of cells and gaps before it reaches its final destination.

Organophosphates and carbamates interfere with this process by tying up the ChE, and so we call them cholinesterase inhibitors. When ChE is unavailable to pull the ACh off the receiving cell, the ACh stays attached, providing a constant signal for the cell to keep sending an impulse. This muddles the message the nerve cells send. When this happens, the nerve system is unable to distinguish between real and "imagined" impulses.

While humans and other mammals are sensitive to these chemicals in the same manner as insects, the effects usually are much less severe on larger animals. This is because response to exposure usually relates directly to dose. Larger body size often means that a plant to which you have applied an insecticide is not likely to hold a dose great enough to be toxic to larger animals. However, enough toxin is present to kill the target insect. This is not always the case, however, so respect these materials and always heed warning statements on insecticide labels.

Symptoms of overexposure in humans include headaches, muscle twitching, difficulty breathing or swallowing, sweating and other problems related to muscle control. Fortunately, the effect on the nervous system from overexposure to carbamates and organophosphates is reversible, at least up to a point. If a person is suffering from over-exposure to one of these insecticides, staying away from these insecticides for a time gives the body a chance to recover. In extreme cases, a physician can administer an antidote that overcomes pesticide poisoning.

The synthetic pyrethroids also affect insect nervous systems. However, these appear to affect the nerve cell at a different point. Imidacloprid (Bayer's Merit) represents another insecticide class, the chloronicotinyls. Imidacloprid blocks the receptor sites to which ACh attaches, preventing its removal. Most other traditional insecticides also attack the insect nervous system in some fashion or other. In all of these cases, nerve impulses are unable to travel normally from cell to cell.

Bacillus thuringiensis (Bt) insecticides, which are effective larvicides, use bacterial toxins to kill insects. Various strains of Bt are available for control of a variety of insect pests, and these use a mode of action different from other insecticides. The toxins in Bt insecticides paralyze the gut and rupture cells in the stomach lining of insects that ingest the poison. The insects cease feeding and soon die.

Insect growth regulators Manufacturers are developing new approaches that affect the target insect differently and also are much less likely to be toxic to mammals. One approach is the use of insect growth regulators (IGRs). An IGR interferes with the ability of an insect to develop normally.

IGRs can interfere with the development of an insect in two ways. One is to disrupt the normal molting process. As insects feed and grow, they must shed their skin periodically. This process involves several complex interactions that must proceed smoothly. Chemists have identified compounds that prevent the insect from shedding its skin properly or from forming the new skin at the right time. Both of these processes are critical for normal molting, and an insect eventually will die if it is unable to molt properly. The chemicals that affect the molting process, such as diflubenzuron (Uniroyal's Dimilin) and a new compound, halofenozide (RhoMid's Mach 2), are much less acutely toxic to mammals and other vertebrates. This is because vertebrates do not have a process comparable to molting.

Another kind of compound, as long as it is present, signals the insect that it is not yet time to molt to the adult stage. These IGRs naturally are present in immature insects, such as larvae or nymphs, but usually disappear at the end of the last immature stage. Entomologists often refer to these chemicals as juvenile hormones because they keep insects in a juvenile stage.

Juvenile hormones usually are specific to one insect species or to closely related species. Chemists have identified the juvenile hormones for several different kinds of insects. Development of these materials into commercial products is ongoing, but a few that are useful to grounds-care managers already are available. One example is Novartis' Enstar, which controls whiteflies, aphids and other pests on ornamentals.

Because IGRs are specific to the target species, they help preserve beneficial insect populations. Further, because of their low mammalian toxicity, IGRs are becoming popular alternatives in settings where exposure to pesticides is more difficult to avoid, such as greenhouses. Note, however, that IGRs do not kill target insects immediately; do not expect to see dead insects the day after you apply an IGR. Instead, they have a long-term debilitating effect. It may be 2 or 3 weeks before you notice any effect on the target population.

How insects encounter insecticides Insects may encounter insecticides in several ways. Perhaps the most common way is by direct contact. In this case, insecticide residues remain on the surface of the plant you have treated. The insect comes in contact with the material as it walks across the treated surface. The insecticide enters the insect through its feet and then makes its way to the site of action (for example, nerve cells or hormone sites). If the insect is present at the time you apply the insecticide, the spray also may cover the insect and penetrate its body directly.

In some cases, an insect will feed on a treated leaf surface. The insect ingests the insecticide and absorbs it through the stomach lining. In this case, the insecticide is able to attack the site of action more quickly than when the insect simply walks across the treated surface. Ingestion usually is more toxic to the insect than direct contact, so an ingested insecticide will induce a more severe response than the same amount of material an insect encounters through direct contact, but there are exceptions.

Often, an insect will experience both contact and ingestion, thereby getting a double exposure to the insecticide. For example, sod webworms or cutworms usually come in contact with insecticides as they move from the thatch to the surface to feed and also consume some of the treated turfgrass. The combined effect of contact and ingestion proves difficult for the insect to overcome.

Some insecticides change to a vapor quite readily. These materials, fumigants, enter the insect's breathing apparatus. These kinds of products are useful in enclosed areas where the vapors can remain concentrated, such as greenhouses or storage bins, but usually do not work well in open landscapes. However, some insecticides may create a bit of fumigant activity at the time an insect is moving across the treated surface.

Contact vs. systemic insecticides As we just discussed, target insects walk across or feed on the plant material to which you have applied the insecticide. Insecticides that work in this manner are contact insecticides. They remain where you applied them and do not move on or inside the plant. Most traditional insecticides are primarily contact materials.

A few insecticides have systemic qualities. This means the plant absorbs the material, which then translocates (moves via the vascular system) to other parts of the plant. Certain fungicides and herbicides are systemic, as well.

Some products translocate upward. In this case, material taken up by the roots can move up into above-ground parts. Other materials translocate downward; pesticide entering the leaves moves to lower regions of the plant.

One common turf and ornamental insecticide, acephate (Valent's Orthene), has systemic characteristics. These qualities make this product particularly effective for controlling insects, such as aphids, which suck plant juices. If the plant tissues contain acephate, the aphids will ingest the insecticide directly when they feed.

Imidacloprid (Bayer's Merit) is another systemic insecticide. Root systems of turfgrasses and ornamentals take up this material, and it then moves into the stems and leaves. The value and convenience of such performance is obvious.

Final thoughts Many kinds of insecticides are available that affect insects in a variety of ways. The careful landscape manager will use various methods of control to reduce the chance of insects developing resistance to one particular approach. Insects that develop the ability to break down one insecticide often can use the same mechanism to break down other insecticides of the same chemical class. So keep in mind the chemical class and mode of action of each insecticide and avoid the temptation to keep using the same approach repeatedly. Insects adapt quickly and soon can become resistant to a product.

Consider using some of the new approaches, such as IGRs. Many scientists believe that, because these materials relate so closely to natural processes that insects require to survive and grow, insects are less likely to develop resistance to these materials.

Attack the insect at different levels by using different chemical classes--nervous system toxins, stomach poisons or growth regulators. However, it also is important to take advantage of different routes of entry. If you have been relying on contact insecticides, consider products that work through ingestion. Similarly, while systemic insecticides have many attractive features, avoid the temptation to rely on them alone. Be sure to use contact products, as well.

Use an application technique suitable for the material you are applying. For example, ensure root-absorbed insecticides make it well into the root zone when you apply them. You usually accomplish this by watering the treated area immediately after application or by using sub-surface application technology.

Finally, remember that insecticides usually are most effective against certain stages of the target insect. Typically, these are the small immature stages. Sometimes, however, the adult stage is the target. Read the product label for this information and ensure that the proper stage is present when you make the application.

Keeping these points in mind will help turf and landscape managers obtain the best possible performance from insecticides.

Dr. Patricia J. Vittum is associate professor of entomology at the University of Massachusetts--Amherst

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