Cabbage White Butterfly
From Allotments4All
- Pest type: Insect
- Order: Lepidoptera
- Scientific name: Pieris brassica
- English name: Large white cabbage butterfly
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Signs of infestation: Small eggs that have been laid on the underside of leaves of brassicas, such as cabbages and broccoli will be visible when the leaf is turned over. Larvae will be visible on the leaves,
Symptoms of infestation: Holes chewed in leaves by larvae, particularly seen initially between veins on the leaves, causing significant visible damage.
Biology, reproduction and spread: The lifecycle is complete, with usually two generations in a year. Pupae over-winter in fissures in bark, on fences, and debris. Adults emerge in April/May. The adults do not attack plants and have no chewing mouthparts. Males and females mate, and the females lay eggs on the underside of brassica leaves, and then die. The eggs hatch after around two weeks, and the larvae emerge. They feed on the brassica leaves from the underside, eventually making holes in the leaves with their chewing mouthparts, and live for around four weeks, then crawl away from the plant to find a place to pupate. The second generation of adults emerge after around four weeks in July/August, and the cycle continues, with the final batch of pupae remaining at the pupa stage for around six months over the winter period. The insect can spread over a wide area, as it is mobile during its reproductive stage - having wings it can fly from one area to another. The female will lay many eggs, and although not all will survive through to the adult stage, those that do, because of their mobility, will mate and reproduce over a wide area.
Prevention and control treatments and methods.
Physical and cultural: Check plants regularly for signs and symptoms of infestation. Pick off larvae by hand and destroying them. Erect a crop cover of fine mesh, such as ‘Enviromesh’ when planting, to prevent the adult reaching the crop and laying eggs. Encourage natural predators such as birds, by developing natural wildlife areas near to the crop area that will provide food and nesting sites.
Biological – together with benefits and limitations: Apply an insecticidal bacterium such as Bacillus thuringiensis. This is purchased as a powder and then mixed with water and the suspension sprayed onto the crop. When the larva eats the leaf, the bacterium enters the gut and the larva stops feeding and dies within one to two days. The benefit of using this method is that the bacterium is only toxic to certain creatures with a specific type of gut. It is harmless to humans, so crops can be eaten safely. The effectiveness is dependent upon the larva ingesting sufficient bacterium, and so the spray coverage must be adequate and the larva must be actively feeding, but if, for example, the larva is feeding inside the head of cabbage, the bacterium may not be present on the inner leaves.
Chemical: Apply a stomach poison or contact insecticide containing pyrethrin (pyrethrum)
Legislative: None
How the pest affects the health and vitality of the host plant: Leaves are seriously damaged and in some cases totally destroyed, so the plant cannot photosynthesise. Growth will be seriously stunted, and the plant may die.
