Brown Widow Spiders: New Oil-Based Pesticides Most Effective At Limiting Populations, Researchers Say

In an attempt to manage brown widow spider populations, researchers from the University of California, Riverside, have come up with an oil-based pesticide that is more effective than water based pesticides at killing the arachnids' egg sacs.

"The brown widow spider egg sac is constructed with highly hydrophobic silks, so the water-based pesticide sprays were not very effective in penetrating the egg sac and impacting the eggs inside," Dr. Dong-Hwan Choe, one of the study's co-authors, said in a news release. "In contrast, the oil-based aerosols were highly effective in penetrating the egg sac silk, providing the complete prevention of the spider emergence."

Researchers were interested in developing such a pesticide to comabt adult spiders who are prolific egglayers, produce many egg sacs, each of which can contain hundreds of eggs.

"Field-collected brown widow egg sacs in southern California average around 135 eggs per sac with a range of 23 to 282 eggs," Choe added.

Brown widow spiders have been on the pest controllers radar since the species established themselves in Southern California in 2000 and then rapidly expanded their range northward into Central California.

Brown widow spiders are generally gray to brown in color with white and black markings. Their legs also have dark bands and they have yellow to orange markings on the underside of their abdomen. Mature female brown widow spiders can grow to be 1.5 inches long and live for up to two years. Unlike their close relatives, black widows, brown widows are not poisonous.

The key to limiting these non-native pests from spreading further is to control populations at the source: egg sacs. That's where the newly designed oil-based pesticides come into play.

"The labels on pesticides may say that they kill spiders, but the fact that it may be ineffective against egg sac contents is a novel concept for control of spiders around homes. The public should realize that the pesticides that they purchase from the home improvement centers may not be doing the job that they want them to do. This is not because the pesticides are ineffective. They do kill the contents if the pesticide can contact the eggs or spiderlings. The trick is to get it past the silk layer of the egg sac, which some of the oil-based pesticides do," Rick Vetter, co-author of the study, explained in a statement. "If you want to control spiders, you have to control egg sacs too, and if you want to kill spider egg sac contents, you need to use an oil-based pesticide."

The findings were recently published in the Journal of Economic Entomology. 

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