Natural Insecticides May Be Hiding in Plain Sight

Targeted plants may provide what’s needed to fend off attackers

Spotted Wing Drosophila (SWD) is a worldwide scourge of fruit-bearing plants, and it’s quite possible it may develop immunity from some insecticides. How can farmers fight it?

SWD, a highly invasive vinegar fly that is native to Asia, has moved around the world feasting on soft-skinned fruit. Since 2008, it’s become a major pest of U.S. fruit crops, especially berries, cherries, and some grape varieties. These crops have a combined annual value of over $5.8 billion in the United States and losses due to SWD exceed $718 million annually.

In an effort to control SWD, Blair Sampson, an entomologist at the Agricultural Research Service’s (ARS) Southern Horticultural Research Unit in Poplarville, MS, is looking to a plant’s own "pharmaceutical factory" to fight the predatory pest. In doing so, he hopes to eschew popular chemical insecticides and provide an alternative that’s less environmentally harmful.

"[SWD] is worldwide and has [required] substantial insecticidal input almost weekly," Sampson said. "Given that this fly can complete 13 generations per year, and chemicals are used against it virtually year-round, this species has high potential to develop resistance to some of the most potent insecticides."

To counter the threat, Sampson is investigating natural plant compounds that may thwart chemical pesticide resistance, manage fly populations, and yet remain benign to the environment.

"Plants produce a pharmacopeia of chemical compounds, many of which do not directly benefit plant growth or development," Sampson said. "Instead, these ‘secondary’ compounds are largely responsible for erecting a formidable chemical defense against herbivores, especially leaf-feeding insects."

Spotted wing drosophila laying an egg inside a blueberry. (Photo courtesy of Blair Sampson).

These natural substances can appear anywhere inside the plant – roots, stems, leaves, flowers, fruits, and nuts – and are stored in the form of oil, sap, or latex within plant tissues. People have commonly used these oils as medicines, perfumes, and candle scents, as well as culinary herbs and spices. Now, researchers are testing them for their intended purpose in plants: as pesticides and feeding deterrents against plant pests.

Two of the essential oils that Sampson and colleagues from the University of Hawaii have worked with have a 50-100% effectiveness rate against adult and larval SWD flies.

"We have focused our testing on two chemical components found in many essential oils," Sampson said. "They are ‘pulegone,’ which smells like peppermint, and ‘linalool,’ which has a subtle citrus aroma, much like some fruit-flavored cereals.

"Many essential oils have proven highly effective for killing small bugs almost instantly," Sampson said. "In fact, some commercial insecticide/miticide products labelled for residential use or on vegetables, fruits, field crops, herbs, spices, and nut crops rely on essential oils as the sole source of active ingredients."

For instance, products based on cinnamon oiland compounds from rosemary, geranium, and peppermint are particularly effective. These and other such products offer effective control of small-bodied pests like aphids, leafhoppers, mealybugs, whiteflies, plant bugs, thrips, mites, leafrollers, and other caterpillars.

According to Sampson, there are about 3,500 known compounds produced by plants to protect themselves from insects, and he and his team have tested hundreds of them. While studying their efficacy on SWD, the scientists must ensure the compounds are safe for non-targeted insects and consumers as well.

"As we develop the insecticidal formulation, we want to be sure that concentrations that are effective against a pest pose no risk to beneficial insect species," Sampson said, "especially bees, which are vital for pollinating numerous fruit and vegetable crops."

While the essential oils are proving to be effective, they are not intended to be a stand-alone solution. According to Sampson, these natural products should be included with chemical pesticides in the chemical component of an integrated pest management program. As natural products, they are seen as alternatives to synthetic pesticides, and many qualify for organic certification. – by Scott Elliott, ARS Office of Communications