Insect I.D.: Oct. 2000, Controlling Pests, The Target Principle

Materials directed at controlling damage from soil-inhabiting insects must reach the primary feeding and/or activity zone (target zone) of the target insect(s) to be effective. Focusing on this objective when applying materials or employing other control strategies is to apply the Target Principle.

THE TARGET ZONE. The Target Zone for control of most soil-inhabiting insects is the first one to two inches of soil. However, in both cool- and warm-season turfs, the soil may be covered by a layer of thatch through which the control materials must pass before reaching the target zone. Thatch serves as a binding site for insecticides and a difficult barrier for some biological agents to pass through. The addition of soaps or other w4etting agents appears to have little effect on reducing the binding potential of soil or organic matter. The control material must reach the target zone in the proper concentration to have the desired affect. The degree to which this is achieved is directly related to the degree of control achievable.

Mobility is more readily achieved when thatch is thin, loose or not present. However, in long established turf sites, the constant dying of plant roots, stolons, rhizomes and crowns results in an accumulation of organic matter in the first two inches of soil (commonly 10 percent or more) which also serves as a binding site. While the organic matter content of thatch (often greater than 30 percent) and soil impede mobility, it also provides a protective filter to slow to stop more extensive mobility past the target zone.

Even when there is no thatch, through management or natural decomposition, the first two inches of soil in established turf commonly contains at least 10 percent organic matter due to the constant dying of plant parts. The binding of insecticides to this zone or thatch is a mixed blessing. While having a major influence on reducing the potential for ground water contamination, binding in this zone provides a reservoir of insecticide residues for pests such as grubs or mole crickets to contact and consume as they feed.

FEEDING HABITS. Knowing the feeding habits of soil-inhabiting insects is essential to understanding how and why control is or is not achieved. The primary means by which control agents enter the body of the target insect is through the natural openings (mouth, anus, spiracles) or ingestion. Contact with a treated surface also occurs, however, with some exceptions, is generally secondary to the impact of ingestion.

What do grubs eat? The standard answer often is, "turf roots." This is an incomplete and actually incorrect answer. A more accurate answer would be, "whatever is in front of them." Grubs are incapable of feeding only on roots. Instead, they ingest the entire medium – roots, soil, organic matter – that occurs in their zone of habitation. Generally, this zone is the upper two inches of soil when no thatch is present and the upper one inch of soil when thatch is present. Soil inhabiting insects, such as mole crickets, consume plant and animal materials as well as soil particles. Turf inhabiting ants are general scavengers and predators, but they do not consume plant parts.

Applied control agents must reach the feeding-activity zone (the Target Zone) of a soil-inhabiting pest to achieve control. The agent is adsorbed to varying degrees and distributed at and through the soil and thatch above the target zone. The target insect ingests thatch and/or soil organic matter containing the agent, which is then absorbed by the insect as the food passes through the digestive system. Living biological agents, such as insect parasitic nematodes, must wriggle through the thatch and/or soil and reach the target zone in sufficient numbers to find and infect the target pest.

IRRIGATION – RAIN. Control materials vary widely in water solubility and capacity for adsorption to organic matter. Water, as rain or irrigation, does not completely circumvent adsorption, but it does accomplish as much movement in the target zone as possible. Generally, control materials for soil inhabiting insects should not be applied to very dry thatch or soil. Greater mobility is achieved when both thatch and soil are first moistened. In order to minimize ultraviolet (UV) degradation, hasten mobility and obtain a maximum effect, liquid materials should be irrigated in immediately after application.

Generally, granular materials should be applied when grass blades are dry so the particles bounce off the grass blades and sift deeply into the turf. While the urgency to irrigate is not as immediate as for liquid materials, it should be done as soon as possible.

With regular irrigation or rain, soil-inhabiting insects such as grubs usually remain in the target zone. However, if the surface soil dries, these insects may move deeper into the soil profile. Timely pre- and posttreatment irrigation often stimulates these insects to remain in the Target Zone.