Biological control is primarily a preventative plant protection strategy that involves releasing parasitoids or predators (Figs. 1 and 2) among greenhouse-grown horticultural crops to regulate insect or mite pest populations and keep pest numbers below damaging levels. However, there are cases where biological control agents or natural enemies of pests are not effective, not commercially available, or too expensive. Therefore, a general interest has developed among greenhouse producers in integrating or combining natural enemies with pesticides to suppress insect and mite pest populations.
Why combine strategies?
The justification for combining both plant protection strategies may be associated with issues regarding 1) pesticide resistance among insect and mite pest populations, 2) increased costs and regulations affiliated with registering new pesticides, 3) potential impact of neonicotinoid systemic insecticides on pollinators, and 4) concerns related to pesticide residues on plants. The integration of natural enemies and pesticides may provide long-term suppression of pest populations compared to either strategy used alone. However, the effects of pesticide applications on the life cycle and efficacy of natural enemies are complex. Furthermore, there may be instances where pesticides and biologicals cannot be used together.
The current terminology regarding using natural enemies and pesticides simultaneously may be somewhat confusing. For example, the term “compatibility” is often used in scientific publications (even my own initially), but “compatibility” does not define what impact pesticides may have on natural enemies from a quantitative assessment. For example, what level of mortality (based on percent) to natural enemies, when exposed to a pesticide, would constitute “compatibility”? Even mortality levels of less than 50 percent can delay a parasitoid population for one generation and impact regulation of pest populations. In addition, mortality levels of greater than 50 percent are a concern in the establishment and sufficient regulation of pest populations. Furthermore, some pesticide labels state that products are “safe” or are “soft” on beneficials. However, what does this mean in terms of mortality percentage or survival of natural enemies? Below are examples of label information associated with the effects of pesticides on natural enemies:
- “Does not adversely affect populations of beneficial insects or mites”
- “Limited to no effect on beneficial insects”
- “Does not significantly impact most predaceous arthropod complexes”
- “Has low toxicity to beneficial insect (including honeybees and bumblebees) and mite populations.”
Direct vs. indirect effects
Again, the terminology and verbiage being used has no meaningful quantitative connotation. When determining the impact or effect of pesticides on natural enemies, the terms “direct” and “indirect” may be more appropriate, allowing for a more quantitative assessment to determine the possibility of integrating pesticides with natural enemies. In reality, greenhouse producers want to integrate or combine natural enemies with pesticides in order to regulate insect or mite pest populations without directly or indirectly affecting life history parameters (e.g. longevity or survival, reproduction, sex ratio [males:females], and foraging behavior) or the population dynamics (interplay between populations and environmental factors) of natural enemies; thus optimizing regulation of pests. Both direct and indirect effects can influence the success of integrating natural enemies with pesticides by negatively affecting life history parameters and/or population dynamics.
Direct effects are associated with acute mortality or survival of the active life stages (larva, nymph, or adult) of natural enemies over a given time period (e.g. 24 to 96 hours). Indirect effects, which are sometimes referred to as “sublethal,” “cumulative,” or “chronic” effects, are affiliated with negative effects on physiology or behavior of natural enemies by means of inhibiting reproduction, longevity, mobility, searching and feeding behavior, and/or prey consumption. Any indirect effects on these factors alone or in combinations may inhibit the ability of natural enemies to effectively regulate pest populations.
Indirect effects of pesticides on natural enemies may be due to differences in exposure based on feeding behavior. For example, predators such as ladybird beetles (Fig. 3) with chewing mouthparts that consume the entire prey (or nearly) may be exposed to pesticide residues that persist on the pest exoskeleton (skin) whereas other predators, including predatory bugs, such as Orius spp., (Fig. 4) that have sucking mouthparts may be less exposed due to their selective consumption of prey tissues.
Parasitoids are known to be significantly more susceptible than predators to certain insecticides (e.g. spinosad/Conserve), which are associated with indirect effects, including reduced longevity and reproduction. However, the direct and indirect effects of insecticides on parasitoids will vary depending on stage of development (egg, larva, pupa, and adult) exposed to insecticide residues.
In conclusion, the integration of natural enemies and pesticides is a plant protection strategy that is complex due to the various interactions between pesticides and natural enemies. For instance, any delay in population growth by a pesticide can impact the ability of natural enemies to regulate pest populations. Moreover, certain pesticides (even fungicides) can disrupt natural enemies by means of direct and/or indirect effects, which may result in increased problems with insect or mite pests.