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Arachnids
Spiders and Mites
Arachnids are not categorised as insects mainly because they have eight legs instead of six (although mites do have a six-legged stage during their development). Most spiders are beneficial in the garden but some mites aren't.
Spiders
As with any beneficial critter, the message is to encourage them.
Don't rip down webs for a start and don't disturb spider egg cases. Egg cases are usually white, fuzzy balls of various sizes, depending on the spider species. Egg cases of Huntsman Spiders, for example, are marble-sized.
Incidentally, Australian Huntsman spiders are a diverse and relatively harmless group of spiders, with 13 genera and 94 described species. They are one of the most beneficial garden spiders, plus it's good luck to have them in your house - at least that's what my mother assures me!
Female Huntsman are protective of their eggsac (shown here) and will stand guard over it, without eating, for about three weeks until the spiderlings emerge. She will then stay with her brood for several weeks.
Huntsman spider bites usually result only in transient local pain and swelling. However, some Badge Huntsman spider bites have caused prolonged pain, inflammation, headache, vomiting and irregular pulse rate.
To encourage spiders minimise the use of insecticides and miticides. Heavily vegetated areas or hollow structures are good habitat for spiders. However, the most important thing gardeners can do to encourage them is not to kill them!
Spiders hatch as miniatures of their parents and grow by gradually molting their exoskeleton.
For home gardeners, avoiding spiders is often their goal. The solution is simple - wear gloves when gardening!
Mites
Mites undergo incomplete metamorphosis. They hatch from eggs into six-legged larvae. They then molt and emerge as eight-legged nymphs, which molt again into an eight-legged adult.
The most familiar pest mite is the red spider mite (also called two-spotted mite) Tetranychus urticae.
It's a serious pest of a wide range of plants, including indoor plants. Both nymphs and adults suck plant sap, mainly on the under surface of leaves but when the infestation is heavy they will be found on the upper surfaces as well. The adults are only just large enough to be seen (the photograph left is magnified)
Damage to plants is seen as rust-like, mottling or speckling of leaves (as shown here). Severe infestation will cause yellowing of leaves and eventually leaves may die and fall off. Damage can be confused with other plant pests though, so correct identification is necessary. Webbing can often be seen on leaves.
Control of Mites
Mites spread by many methods. They can crawl from plant to plant; they can be windblown, transferred by moving pots around, and even on cutting material. High temperatures during dry or humid conditions will increase mite activity.
Pesticide (miticide) resistance can be a problem, so more than one miticide is usually required. And because mites tend to congregate on the under side of leaves, contact sprays are not very effective. Miticides can target specific life stages of mites too. Avoid non-selective pesticides as these will be toxic to useful predators as well.
Sanitation in the home garden is the most effective control. Be vigilant and take action before mites become a severe infestation, by removing and destroying affected plant parts, avoid handling infested material or touching affected plants with clothing. Keep weeds in check as these can harbour mites.
Predatory Mites
These mites actually consume other mites. They prey on eggs, nymphs and adults and their use in the horticultural industry is now common place. Refer to our information sheet on insect predators available for home gardeners.
Information sources:
www.amonline.net.au/factSheets/huntsman_spiders.htm
Carroll, S.B. & Salt, S.D., 2004, Ecology for Gardeners, published by Timber Press.
Cloyd, R.A., Nixon, P.L., & Pataky, N.R., 2004, IPM for Gardeners, published by Timber Press.
Kerruish, R.M. & Unger, P.W., 2003, 3rd edition, Plant Protection 1, published by RootRot Press, ACT.
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You may not use the information for any other purpose without the express written permission of SGA.
