ramtopsrac's picture

Life cycle of the 6 Spot Burnet

I pose some questions here http://ramtopsrac.wordpress.com/2011/06/29/up-close-and-personal-with-go... the life cycle of the 6 Spot Burnet, particularly as to what happens over the winter - do the eggs survive, or do the caterpillers hide, or what?

Also, we've observed numbers late and down this year - is this a general observation or just our local area? (NE Hants)

Would love some feedback, many thanks



Martin Harvey's picture

life cycle

Interesting questions! Burnet moths, including the six-spot, spend most of their life as larvae (caterpillars), and spend the winter in the larval stage. The summer adults lay eggs which hatch after about 11 days, and the larva then feeds up and changes skin so that there are three larval instars in fairly quick succession.

It then enters a fourth instar which does not feed, and is specialised to overwinter, usually in the base of the vegetation that forms its habitat. It spins a silken pad on which to remain securely attached, and can survive most things the weather throws at it, including flooding.

The burnets are unusual in that their larvae sometimes go on to produce cocoons the following spring and adults soon after, but sometimes the larval stage lasts for another whole year or even two years, producing cocoons and adults in the second or third year after the eggs were laid. There are various theories as to how and why this happens, and it may be that this is a way of not having all the population's eggs in one basket - if the weather is bad during the flight period in one year there is a chance that the larvae that have waited until the following year will do better, or perhaps this strategy helps them avoid the parasitic insects that can take a heavy toll in some years.

The above information is from the great expert on burnet moths, W.G. Tremewan, from his chapter in volume 2 of The Moths and Butterflies of Great Britain and Ireland.

As to how well they are doing this year it's a bit early for all the data to be in, but I certainly haven't seen large numbers of burnets myself this year (in Buckinghamshire). Colonies of burnets are known to experience quite large variations in numbers from one year to the next, no doubt partly due to the effects of weather but it is also believed that parasitic insects have a relationship with them that causes the population size to 'bounce' up and down, similar to that seen in species such as the Holly Blue butterfly. The burnet moths do well for a few years, building up large numbers, which allows the parasitoids to build up their numbers, which causes the burnet numbers to drop, which causes the parasitoids to drop, which allows the burnets to expand again, and so on and so on.

Entomologist and biological recorder

miked's picture

"haven't seen large numbers

"haven't seen large numbers of burnets myself this year" well you obviously have not been across the corridor in my office!! There were a few crawling around the place after having hatched out from cocoons that must have been on some of the canes we had been using for botanical surveys in the meadows and the canes had been brought into the office when we'd finished.