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Caterpillars have stripped my annuals of foliage... PDF Print E-mail

Vigilance is important for controlling infestations of caterpillars - some species can significantly harm plants in a few days.

Amaryllis Caterpillar Control

Control can be by hand picking or spot spraying.

Squashing eggs

Itís possible to identify the egg masses of pest species of moth and butterfly in your garden, squashing them before they hatch. Start by watching for common species, like the naturalised cabbage white butterfly, Pieris rapae, laying eggs on crops which include broccoli, kale, cauliflower and cabbage and flowers like stock and wallflower. This is a technique worth practicing.

Confusing pests

A traditional technique for controlling cabbage white butterflies is to cut business card-sized (4cm x 6cm) pieces of white cardboard (or plastic), folding them in half to resemble butterflies. These are then strung on thread and suspended in lines over crops, like cabbages. Passing egg laying adults see these and move on in search for other food plants that havenít already had eggs laid on them.

Hand picking by day

Caterpillars of the heliothis moths attack a wide range of crops and ornamentals. The native budworm, Helicoverpa punctigera and the cotton bollworm or corn earworm, H. Armigera are widespread. The cotton bollworm has evolved resistance to a wide range of toxic chemicals as a result of poor farming practices including widespread, long term, repetitive pesticide use. Look for caterpillars burrowing into ears of corn or holes appearing in ripening pea pods. Drown caterpillars in a bucket of soapy water.Caution: people with sensitive skin should wear gloves when handling hairy caterpillars to prevent skin irritation and rashes.

Hand picking by torchlight

Many caterpillars are day active but some are night active, spending daylight hours hidden under foliage or at the base of plants. These can be controlled by hand picking at night by torchlight. This may seem a strange practice, but it can be more effective than spraying, even with organic remedies. Drown caterpillars in a bucket of soapy water.Caution: people with sensitive skin should wear gloves when handling hairy caterpillars to prevent skin irritation and rashes.

Spot spraying with horticultural soap

Sprays of horticultural soap are ideal for almost instant control of non-hairy caterpillars, such as the amaryllis caterpillar, Spodoptera picta. These moth larvae can strip Crinum, Hymenocallis, Hippeastrum and Clivea of foliage and hollow out their bulbs. Recovery is very slow. Damage by amaryllis caterpillar is increasingly frequent as a consequence of the popularisation of cliveas and they can breed anytime between late winter and late autumn.

How it works

Sprays of horticultural soap smothers soft-bodied pests, which die by suffocation. It can sometimes be difficult to get sufficient horticultural soap spray to penetrate the hairs of really hairy caterpillars.

Spot spraying with Bt

Sprays of Bt or Bacillus thuringiensis are permitted in organic gardens. It is sold as a powdered concentrate.

Some organic gardeners shy away from using Bt because itís used inappropriately by the GM industry. But thereís a big difference between spot spraying with Bt to control one off, serious outbreaks on the one hand and growing crops genetically-modified to produce Bt on the other. These GM plants are toxic throughout their lives and, being mass-planted worldwide, pests have an ideal opportunity to mutate and develop resistance to this currently useful control. Spot spraying in organic gardens has a short-lived, localised effect and the risk of pests becoming resistant by this approach is minimal. Unfortunately inappropriate use on farms is likely to render Bt ineffective in the future.

How it works

Bacillus thuringiensis occurs naturally in soil and on plants. Different varieties of this bacterium produce a crystal protein that is toxic to certain insect groups. Itís currently undergoing trials for controlling curl grubs that feed on the roots of ornamentals, vegetables and turf.

Bt is sprayed so as to coat the foliage. Itís important to wet both sides of leaves because if the bacterium isnít eaten with food by the pest, it wonít harm them.

Technically Bt can be applied to crops almost until the day of harvesting food, but for best results allow sprays two to four days to be fully effective.

Bt, while dormant, quickly die when stored above 21C and especially if exposed to strong sunlight, so itís best to buy and use it fresh. After being consumed during feeding the bacterium gradually kills the pests over two to four days. This can be too slow for controlling ravenous amaryllis caterpillars, which should be controlled by hand or using horticultural soap sprays.

Supporting strategies

Diversity in plantings spreads risks. The more plant species you grow the less likely it is that pests will attack everything. Each plant produces its own unique odour -  a signature smell - and this is what helps attract pests like butterflies and moths. The more diverse your garden becomes the more mixed the odours it releases and the more confusing - or less appealing - it becomes to egg-laying adult moths and butterflies.

Hand watering encourages you to spend time in the garden making it easier - or more likely - youíll observe pest activity like egg laying eggs and early signs of damage.

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