‘The secret lives of butterflies’, a talk by Dr Richard Fox
Richard began his talk by saying that a more appropriate title might be “Sex and Violence in the Garden”. Much has been learned recently about some of our common butterflies and in this talk he reports on several recent studies.
Sex and death amongst the cabbages: It has been known for some time that the caterpillar of the large white butterfly is parasitized by the parasitic wasp Cotesia glomerata. However, much more is going on. While mating the male large white passes an anti-aphrodisiac pheromone to the female which makes her smell less attractive to other males. This pheromone also attracts the minute Trichogramma wasp which locates the female and hitches a ride on her. When she settles to lay her eggs the little wasp hops off and parasitizes some of them. The eggs which were not parasitized hatch and the caterpillars start to munch the cabbages. This causes the plant to produce herbivore-induced plant volatiles which spread through the air. It is these volatiles which are picked up by Cotesia glomerata which homes in and lays its eggs on the caterpillars. The eggs hatch and the larvae feed on the caterpillars and eventually burst out as little yellow grubs. The saliva of caterpillars parasitized by the grubs changes and the cabbages which they munch give off a different smell and this attracts hyperparasitoid wasps which lay their eggs on the Cotesia grubs. Similar chains of events involving many other species have been discovered. Great tits can smell the difference between oak trees with and without winter moth caterpillars.
Sex and resources: Butterflies are usually short-lived as they feed on sugar (honeydew and nectar) rather than protein. Egg production requires a lot of protein so females usually mate just once soon after they emerge, lay their eggs and soon die. However female green-veined whites usually mate with several males. This is possible because the spermatophore which the male passes to the female contains sperm and also nutrients from his body. She uses this for egg production and to repair her own body tissue thus increasing the number of eggs which she is able to produce by some 60%. It also enables her to live five or six days longer than the average female.
Self-defence: Butterflies have evolved various strategies to avoid being eaten. They:
- produce distasteful chemicals
- mimic a distasteful species
- use camouflage eg the comma looking like a dead leaf
- have eyespots which attract a predator away from the head.
Richard showed us a video of a peacock butterfly, which with its wings closed looks like a dead leaf. However when a blue tit came too close it flicked its wings showing the eyespots and made a rustling sound. The blue tit was clearly startled. In an experiment all the peacocks survived their encounters with blue tits. Eyed hawk moths appear to be less successful in repelling predators. Small tortoiseshell butterflies also flick their wings but they lack “eyes” and are much less likely to survive an encounter with a bird. Surprisingly chickens are afraid of a wing-flapping peacock butterfly, using the same alarm call that they use for a fox. Do they think that the eye spots are the eyes of a large predator? In an experiment chickens were not alarmed when a peacock butterfly with the eyes painted over flicked its wings. The visual display clearly gives peacocks protection from birds but they hibernate over winter and mice may kill as many as two thirds of them. However by rubbing the forewing and hindwing against each other they make a hissing sound and when part of the forewing membrane buckles it produces ultra-sonic clicks. In an experiment the sound caused a mouse to flinch. In the complete dark a border terrier barked and growled at a peacock butterfly.
The lady vanishes: Painted ladies are long-distance migrants but until recently there was uncertainty about what happened at the end of the season. Red admirals were seen feeding up and heading off south, straight out to sea but painted ladies were not seen leaving the country. In 2009 there was a great painted lady invasion and high-tech Vertical Radar at Rothamstead recorded the butterflies flying through a beam up to 1km above ground, way out of sight. The butterflies overwinter in North Africa where they breed. This first generation migrates north to the Mediterranean area, where the butterflies lay their eggs and die. The second generation continues north, reaching Britain and Central Europe where they lay their eggs and die. The third generation caterpillars emerge in late July and as butterflies they migrate south. However some butterflies from northern Europe reach Britain on their way south. These may breed though by September/October the chances of survival are much reduced. Painted ladies can fly at 50km an hour with a tail wind. Most fly at about 100 meters where they are visible but in late summer and autumn they fly at about 400 metres, out of sight. Some painted ladies are found south of the Sahara in winter and isotope studies have shown that these lived in Europe as caterpillars. Likewise, some found in Europe in spring were caterpillars in sub-Saharan Africa. They cover a distance of some 7,500 miles and each leg is undertaken by a different generation with no experienced butterflies to follow.
We thank Richard for a superb talk. The work that he told us about was probably new to all of us and it was absolutely fascinating. It was clear from the questions and comments that his talk was much appreciated.
Helen Baker
