Friendly invasion by ladybirds

An “incredible” breeding year for aphids has led to an explosion in the number of ladybirds in various parts of the country, with the Norfolk coast one of the areas which has seen particularly large numbers.

An “incredible” breeding year for aphids has led to an explosion in the number of ladybirds in various parts of the country, with the Norfolk coast one of the areas which has seen particularly large numbers.

This photograph, taken at Stiffkey at the weekend, shows just how many have congregated in one place at a time. Scores of crushed ladybirds have also been found on roads and pavements.

Ian Bedford, entomology facility manager at the John Innes Centre in Colney, said there were two key factors affecting the numbers of ladybirds.

One was the “invasion” of the Harlequin ladybird from Europe, which some claim can, at times of lack of other food, be a threat to the native species - although some scientists claim a lack of evidence to support this theory.

The other was a set of conditions earlier in the year suited to aphids.

“Go back a few weeks and the weather for aphids was just right to see them breeding in great numbers,” said Dr Bedford.

“There have been milder winters, which is one factor, but the really important one was that we didn't have the heavy rainfall earlier in the year that we have had in recent years - and rain is lethal to aphids. Add to that the fact there was nice, young, lush growth at just the right time.

“Our seven-spot and two-spot ladybirds feed mainly on aphids, so it follows that large numbers of aphids are very beneficial to them.

“A ladybird will typically lay its eggs on a plant which normally has aphids on it. The ladybird larvae are like little crocodiles and they devour the aphids.”

The difference in survival rates of the larvae could be affected massively by high aphid populations, added Dr Bedford.

“Where a batch of several hundred eggs might produce half a dozen surviving adults, that figure might be as many as 150, so you can see why there are so many ladybirds around.”

It may not all be good news. With the high population of ladybirds, the enemies they have, such as parasites, might build up in numbers and create a high mortality rate.

However it is currently good news for gardeners, as David North of Norfolk Wildlife Trust explained: “The huge majority being seen at the moment are seven-spot ladybirds, one of our commonest of 24 native species of ladybird.

“Being voracious eaters of aphids they are natural allies of gardeners.”

Mr North said that 1976 was one of the most striking years for ladybirds, when there was a “spectacular emergence” of seven spots, but which was “enormously greater than what we have witnessed so far in 2009”.