Know Your Friends


Spiders are an appropriate topic for our Halloween issue. Spiders, mites, scorpions, and several related groups are not insects, but belong to the arthropod class Arachnida. The spiders (order Araneida) and mites (order Acari) are the two largest groups of arachnids. All spiders are predaceous, mostly on insects. There are about 50 families of spiders in the United States. About 15 families of spiders are frequently encountered in crops, where they provide benefit as natural control agents.
There are three main ways in which spiders capture their prey. The largest group constructs a web of some sort that is used to capture the prey. Common families of web spinners include the orb weavers (family Araneidae), the sheet web spiders (family Linyphiidae), the comb-footed spiders (family Therediidae) that construct a very haphazard type of web, and the funnel web spiders (family Agelenidae). The second most common method of capturing prey is that exhibited by the hunting spiders. These do not construct a web to capture their prey, although a silken refuge may be constructed. These spiders are very active and often run down their prey to capture it. Examples of hunting spiders include the wolf spiders (family Lycosidae), the jumping spiders (family Salticidae), the lynx spiders (family Oxyopidae), and the two-clawed hunting spiders (family Clubionidae). One large family, the crab spiders (Thomisidae) exemplifies the ambush method of prey capture. These spiders are common on flowers and vegetation and sit motionless until their prey comes within easy grasp.

Virtually any active stage of an insect's life cycle can fall prey to a spider. Some spiders will even eat insect eggs or pupae. However, each spider species is more likely to catch a certain prey type, based upon the method of prey capture. For example, orb weavers are more likely to capture adult flying insects than crawling insects.

The importance of spiders as natural enemies in pest control is somewhat questionable. Several studies have indicated that spiders can help regulate pest populations. However, spiders have several attributes that may reduce their overall impact as biological control agents. Spiders are generalist predators and do not discriminate as to type of prey. A given species is likely to eat anything that is of the appropriate size, occurs in the spider's habitat, and can be captured by the specific prey-capture method of that species. If a particular pest species that fits these characteristics becomes more abundant, then that spider species is likely to prey on it more. But if some other non-pest insect is more abundant, it will more likely be killed than the pest. Also, spiders tend to have a single generation per year, and therefore are unable to rapidly increase their numbers in response to the buildup of a multi-generational pest. Therefore, instead of being specialized predators of specific pests, spiders are considered to be part of the overall natural enemy complex that helps add stability and a buffering capacity to keep pest populations from rapidly expanding. No single general predator species can exert this type of buffering and a complex of spiders and predatory insects is necessary. It is generally recognized that the diversity of spiders is greater in undisturbed natural environments than in agriculture. In comparing crop types, spider diversity tends to be greater in less disturbed perennial crops (forests, orchards, vineyards) than in annual crops. When considering annual crops, field size may also be a consideration; some spiders are less able to recolonize large fields than small fields that are surrounded by relatively undisturbed vegetation.

Many broad spectrum insecticides are as damaging to spider populations as to beneficial insects. Insecticides not only directly kill spiders, but also kill many non-pest insects that the spiders use for food during periods of low pest numbers. Therefore, insecticides should be used only when needed and the most selective materials possible should be used.

Many cultural crop practices also disrupt spider activity. Harvesting and tillage in particular are damaging, and, when conducted in late summer or fall, these practices can destroy much of the overwintering spider population.

No spiders have been successfully used in natural enemy importation programs (classical biological control), and none are commercially available for augmentative releases. However, there are a few examples of human manipulation of spider populations for improved natural control. In China bundles of straw are placed in fields with high spider populations. The straw provides refuges, and the bundles can be moved to relocate the spiders to other areas. This certainly qualifies as a form of natural enemy augmentation.

- Dan Mahr, University of Wisconsin - Madison

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