Well summer is here and if you grow any greens in your garden you cannot relax as the scourge of the Cabbage White is here. I must try to remember that it is often called the summer snowflake (a lovely name) the next time I am vainly trying to chase them off my vegetable plot.
The female is distinguished from the male by the presence of 2 black spots, together with a black dash, on the forewing upperside.
In fact there are 2 different species – namely the large white butterfly, Pieris brassicae, and the Small White butterfly Pieris rapae so even though they look similar you will probably be fighting a foreign invader as well. The small white is a native species in the UK whilst the large white usually migrates from southern Europe. Large white butterflies have been seen crossing the English channel in huge swarms.
The reason for the migration is that only the Small White can pupate and survive the British winter.
After mating each female lays a few hundred eggs, on the leaves of cabbages and related plants. You may think that you are better having the Small White because the Large White’s eggs are often laid in batches of ten to 20, whereas the Small White’s eggs are laid singly. The eggs can be laid at a rate of 4 per minute so if you turn you back for a very short time they will beat you!. Each egg is laid directly on the foodplant (rather than on top of other eggs) and also abuts other eggs, resulting in an organised egg mass. An individual female may lay up to 600 eggs in total and these hatch in a week or two, depending on temperature. Even after hatching you may still think the Small White is better because the Large White eggs hatch into yellow and black slightly hairy caterpillars in about two weeks and after feeding for a month or more and can reach 50mm in length before they turn into pupae. On the other hand the small white’s eggs hatch into bright velvety-green caterpillars which remain smaller. Beware though, as unlike the large white, these caterpillars often burrow into the hearts of cabbages to feed so you could have an unpleasant surprise with your lunch.
Cabbage white caterpillars leave their food plants when ready to pupate and usually attach themselves to a vertical surface such as a fence or wall with a silk girdle before their final skin shed. It is therefore worth a look around later in the season to try and cut down on the over wintering pupae, but to be honest it will probably be butterflies from someone else’s garden which cause you problems.
The numbers of cabbage white vary considerably from year to year due to weather, immigration and diseases.
Prevention is best if you can do it.
Covering plants with insect-proof mesh or fleece can help a lot but if there is a gap and one or two butterflies get in the damage might happen without you noticing. Also using fleece can lead to overheating during the warmer summer days. On the other hand if you can stop the butterflies getting in it is the best way. Do not forget that any leaves pressed against a net are fair game for the butterflies
Do not remove any parasitised caterpillars but move them to plants where new caterpillar infestations are expected as the parasites may kill new infestations. You can recognise these as the old large dead (sometimes dry looking) caterpillars.
Encouraging insectivorous birds within the garden by using bird-feeders in winter and nest boxes in spring is claimed to help but in our garden they do not seem at all interested in this bumper food source. It may be because the larvae accumulate poisonous oils in their bodies which deter birds etc.
You can buy a naturally occurring bacterium, Bacillus thuringiensisto spray on which kills only caterpillars and not the predatory insects, and is claimed to be very effective.
We are often advised to plant colourful, high nectar plants to help bees and other pollinators but do not plant them near the cabbage patch as these will encourage adult butterflies into the garden.
These are both effective against the butterflies but ask for advice at the garden centre and read the label carefully.