September 2025 Plant Health, Production

Dr. Bugs: What are sublethal effects? By Raymond A. Cloyd

Answer: The topic of sublethal effects is complex, so I will provide an overview. Insecticides are used to manage or maintain insect pest populations below plant-damaging levels. Insecticides can have direct toxic effects, which may be acute or chronic. Acute toxicity, in general, results in death of pests within 72 hours, whereas chronic toxicity can take longer (days). 

However, there are instances where pest populations are exposed to sublethal concentrations, that is concentrations below the actual LD50 (lethal dose that kills 50% of the population), thus resulting in sublethal effects. Sublethal effects include changes in reproduction, lifespan, development, population growth and feeding behavior. Sublethal effects typically occur after exposure to sublethal concentrations of insecticides. 

Furthermore, exposure to sublethal concentrations of an insecticide may lead to sublethal effects related to the production of enzymes that detoxify insecticides, thus resulting in the development of resistance in pests. Sublethal effects influence insect biology and the behavior of individuals or populations that survive exposure from an insecticide application at lethal or sublethal concentrations. Sublethal concentrations are those that individuals in a pest population are exposed to during an insecticide application, but insects are not killed. This exposure may be due to a sublethal concentration of insecticide because of an improper application (e.g., poor coverage and improper timing) or degradation of an insecticide by sunlight or irrigation, which reduces residual activity. 

Sublethal concentrations of insecticides may affect insect biology and physiology by reducing survival, reproduction, longevity (e.g., lifespan of adults) and increasing development time. In addition, foraging/searching time and mobility (movement) may be affected. Although exposure may result in sublethal effects that negatively affect pests, there are cases where exposure to sublethal concentrations of insecticides can positively affect pests by increasing fitness, enhancing survival, decreasing development time and increasing reproduction, which is known as hormesis or hormoligosis. 

OTHER ASSOCIATIONS 

Sublethal effects are not only associated with pests. BCAs, such as parasitoids and predators, can also experience sublethal effects when exposed to sublethal concentrations of insecticides. Sublethal effects affiliated with BCAs are divided into two categories: physiological and behavioral. Physiological effects include changes in reproduction, development time and adult longevity. Behavioral effects include changes in searching or foraging time, mobility (movement), prey consumption, prey handling time, prey acceptance and attack rate. 

Exposure to sublethal concentrations of an insecticide can enhance foraging time, increase prey handling time, reduce predation rates and decrease the number of attacks by parasitoids, thus influencing the ability of BCAs to manage or maintain pest populations below plant damaging levels. It’s important to understand that multiple physiological and/ or behavioral parameters can be influenced by sublethal effects, not only for BCAs but also for pests. 

When exposed to sublethal concentrations of an insecticide, BCAs may experience sublethal effects, including reduction in reproduction, increased development time, decreased survival, reduction in prey consumption and a change in behavior, such as an increase in walking speed. The ability of predators to detect volatiles emitted by pests or plants may be altered, which could diminish the capacity of a predator to find pests. However, exposure to sublethal concentrations can enhance the reproduction of some BCAs. For example, exposure to sublethal concentrations of pyriproxyfen, an insect growth regulator known as Distance and Fulcrum in the greenhouse industry, increases reproduction of the female parasitoid, Eupelmus vuilleti. 

The sublethal effects of insecticides on pests and BCAs is complex and varies depending on factors, such as insect pest or biological control agent species, age, sex, life stage (egg, larva, nymph, pupa and adult) exposed, duration of exposure, insecticide active ingredient, mode of action of insecticide, insecticide formulation, rate applied, timing of application and residual activity (persistence) of insecticide.

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.