A bee on a chaste tree. Photo by Heather Kirk-Ballard/LSU AgCenter

Many gardeners want to protect their vegetables and fruits from insect pests while avoiding synthetic or chemical insecticides. One solution is to use organic pesticides to maintain yields without synthetic products, and the LSU AgCenter strives to share information to bring organic practices to home gardeners.

These organic insecticides are designed to control insect pests, but what happens if beneficial insects are exposed to organic pesticides? Let’s examine some of the effects of popular organic pesticides on beneficial insects such as pollinators and predators of insect pests.

• Bacillus thuringiensis: This pathogen, also known as Bt, attacks the stomach of caterpillars that become moths and butterflies. Bt is lethal to butterflies so gardeners should take care when applying Bt around the preferred host plants of their favorite butterfly. Dennis Ring, a former entomologist with the AgCenter, wrote about the safety of Bt for bees, “Applications of Bt to insects not susceptible will not kill them. It does not kill predators, parasites, or bees. Bt is most effective against young larvae and usually does not kill insects in the adult or other growth stages.”
• Diatomaceous Earth (DE): Joe Willis, a horticulture agent with the AgCenter, wrote, “Diatomaceous earth is not a poison; it does not have to be eaten to be effective. Diatomaceous earth causes insects to dry out and die by absorbing the oils and fats from the cuticle of the insect's exoskeleton and its abrasive, sharp edges speed up the process.” DE is effective on insects that forage on the ground and would have little impact on bees and other flying insects.
• Horticultural Soaps: Ring shared these notes about using horticultural soaps as an organic pesticide, “Insecticidal soaps are effective on small, soft-bodied insects, but they do not kill many beneficial insects or other insects. These products’ modes of action include suffocation of the insect, dehydration, removing waxes from the cuticle of the insect and disrupting cellular membranes.”
• Kaolin Clay: Willis reports that, [kaolin clay] “acts as a broad-spectrum agricultural crop protectant for controlling damage from various insect and disease pests, a growth enhancer, and as a protectant against sunburn and heat stress.” The U.S. Environmental Protection Agency (EPA) published a kaolin fact sheet that said, “EPA finds that kaolin is not harmful to non-target organisms or to the environment. For example, studies with spiders and honeybees indicate that kaolin appears to have no adverse effects on beneficial insects/spiders.”
• Neem Oil: Among organic products, neem oil seems to be very risky and affects pollinators and other beneficial insects. Willis made a couple of observations about neem oil and its products, “Neem oil has several modes of action. The oil forms a coating on the insect body that blocks the breathing openings (spiracles) and suffocates the insects.” He also adds, “Azadirachtin (an active ingredient in neem oil) is a powerful insecticide that affects insects in several ways. It is an antifeedant, insect growth regulator, sterilant, repellant and oviposition inhibitor.” The Master Gardeners of Contra Costa County, California, reported, “The University of California considers neem oil as moderately toxic to bees and recommends application only during late evening, night, or early morning and when plants are not blooming, i.e., when bees are not typically out foraging.”
• Pyrethrum: An EPA evaluation from 1991 succinctly reported, “This study is scientifically sound, and shows pyrethrum extract to be highly toxic to honeybees.”
• Spinosad: Willis reported that Spinosad is “toxic to honeybees and other pollinators but only during the first three hours following application.”
• Horticultural Oils: Daniel Lewis, a retired entomologist at Iowa State University, discusses the advantages of using horticultural oils and said, “Oil sprays are exceptionally safe to humans (the pesticide applicator as well as others). They have little if any effect on wildlife and nontarget insects in the environment (ladybugs, parasitic wasps, honey bees, etc.) Oil sprays are less toxic because of the method by which they kill target pests. Oils are not poisons. Instead, the thin film of oil covers the target insect or mite and plugs the spiracles or pores through which it breathes. The cause of death is primarily suffocation. Large, motile insects and animals that breathe by another method are not affected by these oils.”

If you want to contact Beehive Buzz, please email your questions and pictures to khawkins@agcenter.lsu.edu or call 337-463-7006. Also, you can join the “beemail” email list by sending your request to the email address above.

This work has been supported, in part, by the USDA National Institute of Food and Agriculture, Renewable Resources Extension Act Award, Accession Number 1011417.