July 2015 - The Cabbage White Butterfly
Large White Butterfly Caterpillar
It is officially summer and Bawburgh is alive with buzzing insects, singing birds, and the sweet smell of nectar plants. I have even heard the cuckoo still 'cuckooing' over towards Marlingford.
This year has been a good year in our garden for Honesty, that wonderful spring plant with its purple flowers, scent and moon-like seed pods. Honesty belongs to a genus of flowering plants, Lunaria, in the Brassicaceae family (Lunaria - means 'moon-like' and refers to the seed pods). This family includes many vegetable plants such as cabbages, turnips, broccoli and radishes. Cabbage White butterflies, which include the Small Whites as well as the Large Whites, may decimate crops of these plants and are the bane of gardeners and farmers alike. This week, commencing 15th June, I noticed several Large White butterfly caterpillars feeding on our Honesty, but interestingly several that looked decidedly listless. At first I thought they could be about to shed their old skins having outgrown the old ones (these caterpillars will shed their skins 5 times before pupating), but as they remained unhappy for several days, the answer is probably that they are parasitized.
Cabbage White caterpillars are very susceptible to several parasitic wasp species, including the ichneumon. The plants that are being eaten by the caterpillars produce blends of volatile chemicals that attract these wasps, which then lay their eggs inside the caterpillars. When the eggs hatch, the caterpillar hosts are eaten from the inside. Once mature the parasitic wasp larvae break out of the skin of the caterpillars and pupate, killing the caterpillars in the process. This is a good example of biological control.
Many of the Brassicaceae family contain mustard oils, and the Cabbage White caterpillars have the ability to concentrate this oil thus becoming distasteful to birds.
In August 1911 a cloud of Large White butterflies estimated to number six million was seen at Sutton Fen in Norfolk
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