Spiders in your garden and Pesticides

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 are a popular choice for spiders, especially the nursery web spider, looking for a home. Plectranthus and agapanthus hardly ever shows 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.

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. 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.

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 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, mercapthothion, 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 authorised 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.

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 colour from very bright yellow, orange or green to darker colours 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 paralyse 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).

Enjoy the hot summer months and remember this is the time to safeguard our many biodiversities found in gardens. Information taken from "The Garden Guardians's guide to environmentally-responsible garden care".

Source: Gardening Eden