JoAnne Skelly: Black widow spiders: Are they on the rise?

A reader, Pat, wrote me: We’ve lived here in Carson City for the past 13 years and this is the first year we’ve experienced an issue with black widow spiders. The fact is we had no idea they were here. But a month or so ago, I found one inside our house. A few days later, I found one in the garage. A day or two later, I found another one inside the house by the doggy door. We sprayed the exterior of the house, sprayed along the baseboards inside the house and set off two of those bug ‘bombs’ inside the garage. When I checked after setting off the bombs in the garage, I found four more dead and or dying black widows. I haven’t seen any more in this past month. Have black widow spiders always been an issue here and have we just been fortunate not to have had to deal with them up until now? I wonder if we need to call an exterminator as a neighbor did?

Black widow spiders are common here. Cold weather or drought can drive them indoors. They are quite shy and avoid the light, staying in undisturbed places, coming out at dusk. Some years numbers are few; yet other years can have large “blooms” of spiders. One female can lay hundreds of eggs, although only one to 12 may survive due to cannibalism by their siblings. Adult females may survive for over a year.

To reduce spider habitat, minimize hiding places in, around or under the house and garage. These include woodpiles, rock walls, meter or irrigation boxes, clutter, plywood, tires, shoes, gloves, things stored under beds or in closets to name a few. To discourage spiders, keep the foundation of the house clear of plants, grasses, shrubs, weeds or woodpiles. Seal all cracks and openings. Vacuum under furniture, in closets, in corners and behind water heaters. Keep basements, crawl spaces and garages clean. Outdoor lights attract insects, which in turn attract spiders. Use sticky traps along baseboards to catch spiders. If you see a really messy web mass — beware — that’s probably a widow’s web.

Although various pesticides are registered for control of spiders, they usually aren’t very effective. They have to contact the spider or its web. They don’t affect egg sacs. Applying to the perimeter of the house doesn’t keep spiders from moving in. There is little residual effect.

For more information: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74149.html.