Aphids are the ladybird's favourite food but by mid-summer the typical ladybird is usually plump and sated by its springtime diet, and the new- born grubs are reaching the pupa stage where they do not eat. As a result, aphids thrive during what is effectively a "close season".
By slowing down the reproductive process of the ladybirds, scientists hope that ladybirds will find their appetite in summer and help reduce the numbers of aphids at a time when their infestation is at its worst for hop farmers. The green method of pest control is finding favour with farmers at a time when aphids are showing increasing resistance to chemical insecticides.
The experiment is being carried out by scientists at the Rosemaund Research Station of the Agriculture Development Advisory Service near Hereford. They collected thousands of ladybirds and put them in refrigerators to cool their sexual ardour. As a result, their mating, egg laying and grub hatching have been delayed until the final weeks of August.
The experiment is being carried out on hop fields around the research station in Hereford. The ladybirds are effectively put into hibernation for three months until being woken in August.
Roger Umpelby, senior entomologist at the research station, said the experiment appeared to be working. "It's simplistic to say we altered their lifestyle by just putting them into a fridge but basically that is what happened," he said. "A whole lot of other things had to be taken into consideration including their general fitness, the amount of food and water they were consuming and other factors.
"It was a very complex experiment but it seems to be working. The grubs are extremely active now and are munching up the aphids furiously. It also means the adult ladybirds are at a stage where they have keen appetites, so they too are keeping down the aphid population."
Although the voracious appetite of the aphid makes any crop or garden flower a potential snack, hop fields are a particularly prized delicacy.
This causes problems for farmers since aphids have developed resistance to insecticides and are able to munch away at the cone of the hop, which is the important part of the crop for brewers. The theory is that a hungry ladybird and its larvae will relentlessly pursue the aphids down into the cones. Each ladybird eats about 5,000 aphids during its short life, using the protein for nine-hour mating sessions. However, ladybirds have been known to develop a penchant for human flesh if they manage to scoff all the available aphids. Mr Umpelby said: "We have environmentally manipulated the ladybirds to breed at a time that suits the hop industry. If they were left to nature it would be too early, or, with their second egg laying which happens in the autumn, too late."
A consultant to the hop industry, Jonathan Blackman, who is based at Rosemaund, said: "Aphids have always been a serious problem for hop growers by reducing the yield. But the ladybirds seem to have been tricked into altering their breeding habits and it seems to be working well and to be an effective method of control."
The idea of using insects rather than chemicals to control pests is gaining strength across the UK as farmers seek greener methods of protecting crops.
Another creature at the vanguard of the predatory insects is the rove beetle, Aleochara bilineata, which has an insatiable appetite for cabbage root fly maggots, which cause huge losses among crops. Farmers spend about pounds 9m a year on chemicals to attack this one pest alone.
Using predatory insects is already commonplace in commercial glasshouses. Most tomatoes grown under glass in Britain are protected in this way, rather than by chemical sprays.