Nursery, Greenhouse, and Landscape News

Spot Application of Insecticides Reduces Rate of Parasitism on Adjacent, Aphid-infested Plants

Growers practicing biological control in greenhouses, conservatories, and landscapes are sometimes faced with the need to apply insecticides to small areas. Information on pesticide compatibility with natural enemies can help guide growers seeking to minimize the impact of spray applications on natural enemies. Growers seeking to further reduce the impact of these spot applications on natural enemies may consider moving infested plants out of the greenhouse before spraying. This practice eliminates drift of spray volatiles during the application process.

However, recent research with aphid parasitoids suggests that advantages conferred by this practice may be limited when the pesticide is both toxic and repellent to the parasitoid. Aphidius wasps are attracted to their hosts by volatile compounds produced by the aphids and their feeding injury. Sugars in the honeydew provides a handy source of nutrition for foraging adult parasitoids and will often cause them to linger on infested plants.

The question boils down to whether volatiles from dried pesticide residue will interfere with the ability of the parasitoid to follow aphid volatiles that direct it to its prey. Deltamethrin, a pyrethroid insecticide, is known for its toxicity to wasps in general and its ability to repel insects. Pots of wheat were treated with deltamethrin, allowed to dry, and then were placed around untreated pots of infested wheat in a greenhouse. Parasitoids were then released in the greenhouse to see if they could find and search the plants, and if they would parasitize aphids. Results were compared to a similar study where aphid infested plants were surrounded by untreated plants.

Parasitoid visitations and rates of parasitism were about 80% less on plants surrounded by deltamethrin-treated plants than when surrounded by untreated wheat. Similar results were also observed after spot applications on outdoor plantings of wheat. Based on these result, growers should restrict the use of insecticides in spot applications to those with low toxicity and repellency to natural enemies.

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