 Raymond Cloyd |
Biological control or the use of natural enemies such as parasitoids, predatory mites, predatory bugs and/or beneficial nematodes is an alternative strategy to managing greenhouse arthropod (insect and mite) pests. Many greenhouse operations have and continue to implement successful biological control programs. However, sole reliance on biological control may not always be sufficient to suppress plant-feeding insect or mite populations.
Research has been conducted to investigate the possibility of using insecticides, miticides and fungicides in conjunction with biological control agents or natural enemies. The studies looked to determine “compatibility” when both management strategies are implemented together. This has primarily involved evaluating alternative pesticides such as insect growth regulators, selective feeding blockers, insecticidal soaps, horticultural oils, beneficial bacteria and fungi and other micro-organisms (e.g., spinosad).
Use of pesticide mixtures
Pesticide mixtures involve combining two or more pesticides (insecticides and/or miticides) into a single spray solution and exposing individuals in an arthropod pest population to each pesticide simultaneously. Pesticide mixtures are routinely used in ornamental production to deal with the multitude of arthropod pests commonly encountered in greenhouses including aphids, thrips, whiteflies, spider mites, mealybugs and leafminers.
The primary reason for the using these mixtures is fewer broad-spectrum activity pesticides are registered or available. Today, more selective products with narrow ranges of arthropod pest activity, are being registered.
Growers combine pesticides mainly due to convenience. It is less time consuming, less costly and less labor intensive to mix together two or more pesticides into a single solution to make one application rather than performing multiple applications.
Negative mixture effects
The use of pesticide mixtures with a broad-spectrum of arthropod pest activity and multiple modes of action may negatively impact natural enemies more than separate pesticide applications. Pesticide mixtures may negatively affect natural enemies through direct effects, host elimination, residual activity and/or indirect effects.
Direct effects involve spray applications of pesticide mixtures that directly kill natural enemies, or, in the case of parasitoids, kill the immature or adult stage while developing inside the host (prey).
Host elimination occurs when application of a pesticide mixture kills a majority (greater than 90 percent) of the hosts leading to natural enemies either dying or leaving the area because they cannot find additional hosts to sustain their populations.
Residual activity is when the pesticide mixture does not directly kill natural enemies, but any residues may be a repellent thus negatively impacting the ability of parasitoids or predators to locate a food source.
Indirect effects refer to the application of a pesticide mixture not directly killing natural enemies but affecting reproduction by sterilizing females, reducing the ability of females to lay eggs and/or impacting the sex ratio (females:males). In addition, foraging efficiency may be inhibited, which could influence the ability of a parasitoid or predator to find a host. Parasitoids such as the greenhouse whitefly parasitoid Encarsia formosa may inadvertently consume residues on hosts after the application of a pesticide mixture thus making the host unacceptable.
Evaluating pesticide mixtures
There are only a few studies that have evaluated the direct and indirect effects of pesticide mixtures on natural enemies. These primarily involved predatory mites.
In some of studies at Kansas State University, we found that the nymphs of the predatory mite Neoseiulus cucumeris were more sensitive to certain pesticide mixtures involving spinosad (Conserve), abamectin (Avid), fenhexamid (Decree) and/or thiophanate-methyl (Cleary’s 3336) than the adults. In general, predatory mite susceptibility associated with the pesticide mixtures was not noticeably different than when the pesticides were applied separately.
In other studies, it has been reported that the predatory mite, Typhlodromus pyri is negatively affected when exposed to mixtures of the fungicides mancozeb (Dithane) or thiophanate-methyl (Cleary’s 3336) with the insecticide chlorpyrifos (DuraGuard) compared to when the pesticides are applied by themselves. It has also been demonstrated that the fungicides maneb (not registered for use in greenhouses) and mancozeb (Dithane) are moderately toxic to T. pyri when applied separately. These fungicides are less harmful to the predatory mite when they are mixed together.
Reproduction of the predatory mite, Phytoseiulus persimilis is not negatively affected after exposure to the miticide hexythiazox (Hexygon) and the fungicide triadimefon (Strike) either as separate applications or when mixed together.
A study that evaluated pesticide mixtures on the parasitoid, Trichogramma chilonis, determined that combinations of spinosad (Conserve) with the microbial Bacillus thuringiensis are toxic to adults (based on percent mortality) and inhibit adult emergence.
Combining controls
Pesticide mixtures will continue to be an integral component of pest management programs. For this reason, it is important to understand how pesticide mixtures not only affect target arthropod pests, but any natural enemies that have been released. You can’t just look at one side of the coin.
Pesticide mixtures do not have to be directly lethal to disrupt the efficacy of natural enemies. In fact, pesticide mixtures may decrease natural enemy populations or negatively impact population growth resulting in an increase in arthropod pest populations. It would be “beneficial” to know how pesticide mixtures can be used in conjunction with natural enemies to avoid compromising the success of biological control programs.
Raymond A. Cloyd is associate professor/extension specialist in ornamental entomology IPM, K-State Research and Extension, (785) 532-4750; rcloyd@ksu.edu.
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