Ladybirds; they don't make a meal of aphidsThe widely-held theory that ladybirds can control aphids may not be as simple as it appears. Scientists in the School of Biological Sciences at the UEA, part of the Norwich Research Park, have challenged this idea for many years, and have come up with a convincing alternative explanation. It was in 1888 that ladybirds were first reported as being effective in the biological control of insect pests. And ever since, ladybirds have often been released on to aphid-infested crops in the hope that they would reduce the aphid population. In many cases, there does appear to be fewer aphids once the ladybirds have been introduced. But as the UEA scientists have discovered, it’s not entirely due to the ladybirds’ appetite for aphids. During an aphid infestation, massive numbers of young aphids are produced. The population increases very quickly to a peak, then falls, as winged adults fly away and colonize other plants. After a month or so, the infestation is over. So, can ladybirds really make any difference to this aphid population explosion? Well yes, to some extent. The females lay their eggs very early in the aphid infestation, and crucially, don’t lay any more. This is because ladybird larvae are cannibalistic and will eat any eggs that are laid later. Female ladybirds have developed a clever way of knowing when to stop laying eggs. The UEA scientists have discovered that ladybird larvae leave chemical “footprints” behind as they crawl over the plant. Females can detect these tracks, and know that any more eggs would probably be eaten, so stop laying. But as well as having larvae that will eat their unborn siblings, another factor is against the ladybirds. Aphids develop much more quickly than ladybirds. Their faster development means that they can produce three aphid generations to just one ladybird generation. And as ladybirds only lay eggs in the early stages of an infestation, they are actually pretty inefficient at controlling aphid populations. So how can the early observations of 1888 be explained? Easily – the ladybirds weren’t eating aphids at all. They were actually preying on mealy bugs, closely related to aphids. Mealy bugs don’t develop as fast as aphids, so the ladybirds have a much better chance of being able to control them. The (wrong) conclusion at the time was that if ladybirds could control mealy bug infestations, they were bound to be able to do the same for aphids. But ladybirds just don't develop fast enough to be able to control them. On a more positive note, ladybirds can help against aphids if their larvae are introduced onto an infested plant very early – before the aphid population has started to really take hold. The larvae can’t leave the plant, so if there are enough of them, they can make a difference. But as the UEA researchers point out, there is little to be gained from releasing adult ladybirds onto plants that are already heavily aphid infested, as most simply fly away. The UEA researchers have met resistance to their conclusions, mainly from those who remain convinced that ladybirds are a good way to control aphids. This shows how important it is to keep an open mind about new ideas, and to be willing to let go of long-held beliefs if a more reasonable explanation comes along. © Dr Belinda Clarke 2002 This Article originally appeared as part of the "Science on your Doorstep" series, published in the Eastern Daily Press 1st June 2002
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