Our Friends The Garden Spiders

Garden spiders are one of the more beneficial creatures to have in the yard or garden, although in the eyes of many they can be rather frightening. Some of us simply don’t care for creepy crawly things, particularly on us, and spiders have two more legs to creep and crawl with than do insects. For the most part, we fear walking through a spider web, while the spider happens to be in residence. This actually seldom will happen during the daylight hours. Most garden spiders spin their webs at night, and stay out of sight, though near their webs, during the daylight hours. When we do see the garden spiders in their webs, it is most apt to be in the early morning hours, or immediately after an insect has become caught up in the web.

A Harmless Spider

Garden spiders are much more afraid of us than we need to be of them. They do have fangs and could bite given the opportunity, but generally would try to get away. Spiders have very soft bodies and are easily crushed or injured, so they will normally not approach something much larger than they are. There may be exceptions to this general rule, but garden spiders are not a part of that particular group. All spiders are venomous, but with very few exceptions, the venom is not toxic enough to harm, or even irritate humans, and most spiders do not have jaws strong enough to puncture human skin. One of the most venomous spiders we are apt to run across is the daddy-long-legs. These spiders have such a small mouth, such as small fangs, plus such a limited amount of venom, that they can do us no harm. When the spider does kill its prey it does so by injecting venom through its fangs into its prey. It then uses digestive enzymes to liquefy the prey before feasting on it. Garden spiders do not eat solid food.

The Web Is A Thing Of Beauty

Even if the sight of a garden spider can make some of us uncomfortable, nearly everyone has an appreciation for their web. The garden spider’s web is a thing of architectural skill and aesthetic beauty, especially in the early morning when the dew has settled onto the strands. A garden spider’s web is circular, and typically around a foot in diameter, although a web two feet in diameter isn’t all that uncommon. A little known fact regarding the garden spider’s web is that the inner portion of it is almost always made of recycled material. The spider eats this inner portion daily and then rebuilds it, using the eaten web to replenish some of the silk-making materials in its body. Another interesting fact about the garden spider’s web is, while the silk could conceivably have some commercial uses, one would have to harvest the silk from a single spider at a time. Put several spiders in a “silk production” container, and they would kill one another, with the survivor perhaps then constructing its web.

Males Live A Life Of Danger

Male garden spiders are smaller than the females, usually about half the size of the female. While the typical male may have a strong mating instinct and actively seek out a female for companionship, it also must have a strong survival instinct. It is quite common for a female spider to kill and eat the male after mating. Those who study garden spiders have noted that the male garden spider goes about the mating process very carefully, and in a manner designed to ensure its survival. Where it learns this no one knows, it must simply be programmed into the spider.

When you see garden spiders around your house, or in your yard, leave them alone. They are doing a great deal of good, can’t bother you, and probably wouldn’t even if they could.

Spiders and Pesticides

First of all, I would like to wish you all a wonderful 2007 with minimum pests and maximum beneficial organisms in your gardens! Due to the fact that I have overcome my fear for spiders by learning more about them I thought it to be appropriate to start 2007 as the year to protect them in gardens. There is only one real “spider-woman” in this country and that is Dr. Ansie Dippenaar and if you really want to learn everything about them she is the one to be contacted at ARC Rietondale Pretoria.

Spiders prey on many insects, including various moths, gnats, flies, mosquitoes, and ants. Popular plants for attracting spiders in the garden are dill, cosmos, daisies, marigold, spearmint, and caraway. I have found that plectranthus planted as a groundcover and agapanthus as fillers below and around trees is a popular choice for spiders, especially the nursery web spider, looking for a home. Plectranthus and agapanthus hardly ever show evidence of destructive insects and therefore needs no spraying. Snails and slugs sheltering underneath can be controlled by hand or with baits. Once plectranthus and agapanthus have covered a planted area, you will hardly ever need to walk there and the spiders can be left undisturbed to silently prey on insects. Unfortunately, however, those insects are not always unwanted insect pests, but also beneficial insects.

Nursery web spider nesting on an agapanthus

There are quite a few poisonous spider species in South Africa but they all play an important role, especially in the garden and natural habitats. All spiders have eight legs, two body parts (cephalothorax and abdomen) and fangs (chelicerae). All spiders undergo an incomplete metamorphosis: they hatch from eggs and look like tiny adults. They shed their skin as they grow. Spiders are generally excellent predators of various insects. Some might wait patiently on flowers for their prey, others chase after their prey and many build special webs that do the catching for them. Crab spiders, jumping spiders, nursery web and fishing spiders, funnel weavers and grass spiders, orb-weaver and cobweb spiders and wolf spiders are some of the species to look out for, and protect, in your garden.

Jumping spider with prey

When the lawn is covered in early-morning dew, I often find lace-like networks of spider webbing reflecting in the sun. What would happen to these spiders if the lawn was sprayed with a broad-spectrum insecticide to kill off lawn caterpillar? Without a doubt, both the lawn caterpillar and the spider populations would be destroyed. Biological insecticides containing the bacterium Bacillus thuringiensis var. kurstaki can be used to control young active feeding larvae of lawn caterpillar with no harm to spiders, birdlife or any other beneficial organisms.

Avoid the use of most chemical insecticides containing active ingredients such as chlorpyrifos, mercaptothion, fenitrothion, carbaryl, deltamethrin, cypermethrin, bifenthrin, permethrin, d-phenothrin, tetramethrin, gamma-BHC, diazinon, alpha-cypermethrin, cyfluthrin, propoxur, to name a few. Organic insecticides containing natural pyrethrins, nicotine, potassium salts of fatty acids and canola oil should be avoided where direct contact of spiders is possible. Canola oil and fatty acids will only harm very small-bodied spider species on contact. Spider species are unlikely to be repelled by insect repellents such as garlic juice extract formulations used to dissuade unwanted insects from landing on plants under protection in the flower, herb or vegetable garden. Biological control formulations containing Bacillus thuringiensis var. kurstaki and B. thuringiensis subsp. israelensis are completely harmless to spiders. Chemical insecticides containing pyriproxyfen should also not harm spiders. Most chemical and organic fungicides available in the home garden will not adversely affect spiders unless the label indicates toxicity. This includes fungicides containing mancozeb, chlorothalonil, copper ammonium acetate, plant organic acids and procymidone microbial formulations containing Trichoderma.

Commercially available insecticides authorized for use in organic farming and which are unlikely to cause harm to spiders include Margaret Roberts Biological Caterpillar Insecticide, Margaret Roberts Biological Mosquito Insecticide, Dipel DF and Vectobac WG. Insecticides which should have minimal or no effect on large spider species, at recommended dosage rates for small-bodied insects such as aphids, include Vegol, Margaret Roberts Organic Insecticide, Neudosan, and ECO Insect Control. Fungicides include Margaret Roberts Organic Fungicide, Copper Soap, and other approved copper and microbial formulations.

Bont flower crab spider

African mask crab spider with prey

Masked flower crab spider

Spotted crab spider

To learn more about spiders we start by having a closer look at crab spiders, a very important group of spiders found in the home garden environment which should be protected.

CRAB SPIDERS

The crab spider’s first two pairs of legs are very long compared to its back two pairs. Crab spiders are flat-bodied compared to other spiders and get their name from their habit of holding up their front legs to resemble a crab with claws. They vary in color from very bright yellow, orange or green to darker colors with grey and brown patterns. Most crab spiders do not live for more than a year. Hundreds of eggs are laid by the female in autumn and the offspring hatch in spring.

Most crab spiders ambush their prey after sitting patiently and motionless on flowers, leaves, fruit or other spots frequently visited by flying insects such as flies and bees. Many are well-camouflaged and blend in with their surroundings. Crab spiders can walk in all directions with ease!

Even though crab spiders prey on beneficial insects such as bees, they also feed on various fly species, adult mosquitoes, moths and many other insect pests. Even though research has found that the venom of crab spiders is more potent than many other spiders due to its ability to paralyze insects very quickly, they are not considered to be dangerous to humans. However, some larger species can bite humans and it is, therefore, best to leave them undisturbed and rather allow them to carry on hunting for their next meal.

The most common crab spiders found in the garden are the flower spider (Thomisus sp.), common green flower crab spider, two-spotted crab spider, and the ‘bont’ flower crab spider (Thomisus citrinellis).





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