IPM-Recommended Insecticides Harm Beneficial Insects through Contaminated Honeydew
Many hemipterans (true bugs) feed on plant sap, including phloem which contains plant sugars. Hemipterans convert the sugars in the phloem to honeydew, which is then excreted. A very sweet substance, honeydew is the most abundant carbohydrate source in most agroecosystems and is utilized by beneficial insects including bees, ants, predators, and parasitic wasps important in natural pest control. Hemipterans and other plant feeding insects are increasingly controlled through the use of systemic insecticides transported in the phloem.
This study demonstrated that honeydew-producing insects, exposed to systemic insecticides in plant phloem, excrete honeydew contaminated with insecticide. Furthermore, this honeydew can cause lethal effects in beneficial predatory and parasitoid insects feeding on the honeydew.
The researchers applied systemic insecticides on citrus tree leaves, under both controlled (greenhouse) and field conditions. Leaves were treated with either flonicamid or pymetrozine using manufacturer-recommended doses. When treated, the trees were already hosting populations of the honeydew-excreting species, mealybug (Planococcus citri). Another honeydew-excreting species, whitefly (Aleurothixus floccosus), was also present on the field trees.
Both pymetrozine and flonicamid were present in honeydew excreted by the mealybug. Only pymetrozine was detected in honeydew excreted by the whitefly.
Honeydew excreted by mealybugs feeding on trees treated three days earlier was fed to two different species of beneficial insects: hoverfly (Sphaerophoria rueppellii) and parasitic wasp (Anagyrus vladimiri).
Hoverflies were more sensitive to Honeydew contaminated with flonicamid (56% mortality after three days) than honeydew contaminated with pymetrozine (22% mortality). No control hoverflies died.
In contrast, there were no significant differences in mortality of the parasitic wasp between the treatments and their control.
The two insecticides, flonicamid and pymetrizone, are both commonly used in Integrated Pest Management programs, but this study illustrates the potential for these insecticides to be toxic via honeydew excretion to natural enemies of common hemipteran pests.
