Neonicotinoids, A Brief History IV: Systemic Issues

Corn Rootworm Larva

The management of insect pests of agricultural crops through the use of pesticides has undergone a transformation. Many agricultural crops have been genetically engineered to deter, kill or otherwise tolerate the most damaging insect pests. Insect control is built on the use of bacterial genes (BT) that are engineered into the plants. These genes cause the plant to produce proteins that are highly specific in their effects on a limited number of insect species, with little to no effect on other animals. The engineered seeds are more expensive than non-engineered seeds, but there is substantial energy savings compared to spraying pesticide on crops during the growing season. Pest control with BT is typically specific, for example, the BT crop may only prevent damage from a major pest but still be susceptible to minor pests. The seed themselves may be vulnerable to attack by soil pests. Thus, seed treatments (coating the seeds with a systemic insecticide) have significant advantages. This has led in part to increasing use of seed treatments (dressings) that contain neonicotinoids.

Neonicotinoids are the insecticide of choice because:
1) they are systemic, that is they are taken up by the plant roots and translocated throughout the plant and
2) they are effective at concentrations that are present in the plants.
Can neonicotinoid seed treatments be overused? For some crops, the majority of seed are treated with insecticide. Large contiguous areas of crops may all contain systemic insecticide from the seed treatment. Are exposures to non-target insects such as pollinators that visit multiple crops (as discussed in previous posts) and the development of insect pest populations that are resistant to the insecticide (no longer controlled) problems?

How does resistance to an insecticide develop in a pest population? An insecticide will kill the most susceptible individuals in the pest population; the most resistant individuals will survive and reproduce. When the basis of resistance is genetic, the offspring of the resistant individuals will contain resistance genes bequeathed them by their parents. If this “resistance selection” process occurs generation after generation, a pest population may rapidly become resistant, that is a substantial portion of the pest populations carries “resistance genes” and in the worst resistance cases, can no longer be controlled by the insecticide.

Ideal conditions for the development of insecticide resistance are:
1. Exposure of most of the pest population to the insecticide.
2. Exposure of multiple generations.
3. Few “refuge” populations remain that are not exposed to the insecticide.

There is growing concern that neonicotinoids are overused. In some areas, most of the crops contain neonicotinoids, and most of a pest population is exposed. Neonicotinoids are systemic and can persist in a plant throughout much of a growing season. For pests that have multiple generations, every generation of the pest will be exposed to the systemic insecticide. Those who are concerned about resistance development are in favor of better management practices and less widespread use of a single insecticide.

Will resistance develop to seed treatments? Time will tell.
Is the current use pattern of neonicotinoids sustainable? It is worth discussion.