Dr. Bugs: Can plants influence susceptibility to insecticides? By Raymond A. Cloyd

Question: I have read that plants fed upon by insect pests can affect their susceptibility to insecticides. Can you explain?

Answer: You ask a great question. The primary mechanism by which plants can affect insect pests susceptibility to insecticides, or influence the toxicity of insecticides to insect pests, is by inducing a high level of activity of metabolic detoxifying enzymes in insect pests because of exposure to plant allelochemicals when feeding.

Plant allelochemicals or secondary metabolites are considered nonessential to plant growth but are responsible for producing plant defensive compounds to avert insect pest feeding. However, insect pests have evolved the ability to metabolize and/or detoxify plant allelochemicals through the induction or activity of certain metabolic detoxifying enzymes.

The variability associated with insect pest susceptibility to insecticides on certain plants is related to different levels of metabolic detoxifying enzymes, which can be induced by the plant. There are a number of enzymes involved in detoxification or metabolism that are active on a diverse array of substances, including naturally occurring plant allelochemicals and insecticides. Insect pest responses to plants may result in enhanced metabolism of insecticides because the mechanisms responsible for the detoxification of plant allelochemicals by insect pests may also be effective in detoxifying insecticides.

Furthermore, the selection pressure affiliated with exposure to plant allelochemicals when insects feed on plants may promote or induce the activity of metabolic detoxifying enzymes. The primary metabolic detoxifying enzymes are cytochrome P450 monooxygenases, glutathione S-transferases and esterases. Cytochrome P450 monooxygenases, glutathione S-transferases and esterases are common enzymes that metabolically detoxify insecticides. These enzymes convert plant allelochemicals and insecticides into non-toxic metabolites. The enzymes remove lipophilic (substances that dissolve in lipids) compounds that may be toxic to insect pests by converting these compounds into metabolites that are excreted or removed from the insect pest body.

In general, esterases detoxify insecticides in the chemical classes, organophosphate and pyrethroid, whereas glutathione S-transferases detoxify organophosphate insecticides. Insect pests that do not possess these metabolic detoxifying enzymes are more susceptible to insecticides. Expression of these metabolic detoxifying enzymes may be induced when insect pests are exposed to plant allelochemicals when feeding on plants, which can decrease the toxicity of insecticides, resulting in fewer individuals being killed.

Insecticide applications may not be effective against insect pests previously exposed to plant allelochemicals. Photos by Raymond Cloyd.

Insect pests exposed to high concentrations of plant allelochemicals develop resistance to insecticides more rapidly. Furthermore, resistance to insecticides may occur faster or in a shorter time period than resistance to plant allelochemicals. Insect pests with piercingsucking mouthparts — such as aphids — may already have low levels of metabolic detoxifying enzymes due to feeding on a wide range of different horticultural plants. This could increase the exposure of aphids to different plant allelochemicals, thus leading to an increase in insecticide resistance. A consequence of insect pests with inherent metabolic detoxifying enzymes is that higher rates of insecticides may be required to manage populations. In addition, the plant fed upon by insect pests may influence the effectiveness of insecticides. For example, sucking insect pests feed within the vascular tissues [i.e., phloem (food-conducting tissues) and/or xylem (water-conducting tissues)], which may have lower concentrations of plant allelochemicals compared to the leaf tissue (leaf parenchyma cells).

Therefore, sucking insect pests develop resistance at a slower rate than chewing insect pests because of exposure to lower concentrations of plant allelochemicals.

Aphids may already possess metabolic detoxifying enzymes because they feed on many different horticultural plants.

Plants produce a multitude of allelochemicals, which serve as defensive compounds to mitigate attacks from insect pests. Consequently, insect pests exposed to different plant allelochemicals may be less susceptible to insecticides because the metabolic detoxifying enzymes can detoxify or metabolize both the plant allelochemicals and insecticides. However, the selection pressure associated with plant allelochemicals may differ from that of insecticides based on the feeding duration of the insect pest. Generalist insect pests have evolved mechanisms to detoxify a diverse array of plant allelochemicals. As such, generalist insect pests may have a greater propensity than specialist insect pests to detoxify plant allelochemicals due to the presence of multiple metabolic detoxifying enzymes. These same enzymes also allow generalist insect pests to develop resistance to a multitude of insecticides, even those with different modes of action. Therefore, the metabolic detoxifying enzymes that allow insect pests to overcome plant allelochemicals may predispose insect pests to develop insecticide resistance.

Moreover, generalist insect pests feeding on many different plant types may develop resistance to a wide range and classes of insecticides than specialist insect pests due to having a more diverse array of metabolic detoxifying enzymes. Finally, insect pests that feed on a wide range of plants are more likely to develop resistance to a broader spectrum of insecticides due to greater exposure to insecticide applications.

Raymond A. Cloyd is professor and extension specialist in horticultural entomology/plant protection at Kansas State University. He can be reached at rcloyd@ksu.edu.

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