Land Crabs

Community Contributed

By Glenn I. Teves, County Extension Agent

Crabs and spiders are closely related under a family or phylum called arthropods, and some spiders look like crabs. One in particular is the spiny backed spider, also called the crab spider. One of two species found in Hawaii, the more recent nuisance arrived in Hawaii in 1985 and on Molokai around 1989. This spider, with two distinct white spots on its back, is the one that makes webs all around houses, clothes lines, trees, and even your car if you leave it in one place long enough.

Most spiders are solitary, meaning they work and live alone, but the spiny backed spider form community webs and work in teams in areas with high populations. As a team they can cover very large areas with webs over a very short time. The female spiders hang by their short legs in the center of the webs, while the supporting lines or radii are adorned with wooly tufts of white silk believed to act as lures for insects. The male is much smaller than females and may be found hanging from a single web off the female web.

After mating, females lay egg masses on surfaces other than their webs, such as house walls. The eggs are enclosed in a thick, silky fluorescent green web covering, and turn yellow when the eggs hatch. One way of controlling them is to destroy these egg masses, which can contain over 100 eggs. Destroying egg masses is one way of keeping populations down.

Spiny backed spider populations are very cyclic, with high numbers during rainy humid periods, but will drop off in other times of the year. This insect is not known to be an indoor pest, but people have complained of being bitten or irritated by the web, especially in sensitive areas such as the neck. The bite has caused localized swelling in some individuals. Please seek medical attention if you’re allergic to bites.

Insecticide screening tests conducted by the UH College of Tropical Agriculture and Human Resources have determined that Johnson’s No Roach, Strike Roach Ender, Black Flag Ant and Roach, and Raid House and Garden aerosols killed 90 percent of the females within 24 hours. These chemicals are not cleared for use of plants and trees. As with any pesticide, please read the label before using them, and follow instructions. Again, crushing the egg masses is another way of keeping populations down.