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Study Finds Oil-Based Pesticides Most Effective at Killing Contents of Brown Widow Egg Sacs

By Josh Lancette

While going through pre-marital counseling earlier this year, my then-fiance (now wife) and I were talking about expectations we had for each other. Of course, the typical things like honesty, teamwork, and faithfulness were mentioned. But high on my wife’s list of her expectations for me was something I didn’t quite expect.

Josh Lancette

She said that I had to be the one to remove spiders from the house.

And she made sure to emphasize that this expectation was just as important as any other we had discussed. It was non-negotiable.

In fact, I know a lot of people who feel the same way about spiders. While they are great to have around for the control of insect pests, not many people enjoy having spiders inside their homes, and they are looking for effective ways to control them.

Luckily for those people, researchers at the University of California, Riverside might have found a breakthrough in the spider-control field. In a paper published in the Journal of Economic Entomology, the researchers wrote that oil-based pesticides are more effective than water-based pesticides at killing the contents brown widow spider 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,” said Dr. Dong-Hwan Choe, one of the researchers. “In contrast, the oil-based aerosols were highly effective in penetrating the egg sac silk, providing the complete prevention of the spider emergence.”

This finding is important because trying to control adult spiders without controlling their eggs is like plucking dandelions without taking out their roots.

“Spider egg sacs are important because each female spider can produce many egg sacs, and each egg sac has many eggs in it,” said Choe. “A female brown widow spider can produce 22 egg sacs on average throughout her lifetime and can produce an egg sac every four days during the early portion of her reproductive life. Field-collected brown widow egg sacs in southern California average around 135 eggs per sac with a range of 23 to 282 eggs.”

With the new knowledge that oil-based pesticides are more effective than water-based pesticides at killing the contents of an egg sac, the pest control industry and everyday consumers should have better insight into how to control spiders in the home.

Rick Vetter, a co-author of the paper, said most pesticides used around structures in the pest-control industry are water-based, and therefore might not be effective.

“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,” said Vetter. “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.”

“If you want to control spiders,” Vetter continued, “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.”

Read more at:

Efficacy of Several Pesticide Products on Brown Widow Spider (Araneae: Theridiidae) Egg Sacs and Their Penetration Through the Egg Sac Silk

Josh Lancette is Manager of Publications at the Entomological Society of America.

1 Comment »

  1. My wife has a similar feeling about spider removal from the house although if they are not that large or fast moving she will occasionally move them outside. In her defense she has experienced (at different times) a brown recluse and a black widow in her clothing, and was not bitten either time.

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