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Climate change could upset the balance between insect crop pests and the 'natural enemies' that control their numbers, say researchers.

This might make pest outbreaks more frequent and severe, particularly in the tropics where the climate is naturally more consistent, they warn in a study published this week (11 November) in the Proceedings of the National Academies of Science.

The team looked at how climate affects parasitoids — insects such as wasps and flies that lay their eggs on or inside caterpillars, allowing the hatched larvae to feed on the host.

"There is a complex relationship between parasitoids and their caterpillar hosts," says lead researcher Daniel Janzen of the University of Pennsylvania, Philadelphia, United States. "We wanted to see what would happen when there's a change like we are experiencing with global climate change."

Global warming is expected to make regional climates more varied and unpredictable, which could affect relationships between plant-eating insects and their enemies.

The team assessed the impact of parasitoids on more than 5,000 caterpillar species collected in forests from central Brazil to southern Canada. They found that the caterpillars had significantly fewer parasitoids in years when rainfall was most variable.

This could be because the parasitoids use cues such as changes in local climate to determine the best time for laying their eggs. If so, unpredictable rains might disrupt the parasitoids' ability to 'track' their caterpillar hosts.

"The wasps use the start of the rains as a cue to hatch out of their cocoons and look for a caterpillar to lay their eggs on," said Janzen. "If the rains are late, they will emerge late, and miss the narrow window of time when their host is most vulnerable."

Previous studies of rapid climate change have concentrated on individual species, but Janzen's team looked at relationships between interacting species.

Although the study focused on species in forests, the researchers claim their results are also relevant to agricultural land.

Many of the same parasitoids are also widespread in agricultural areas, says co-author Lee Dyer of Tulane University in New Orleans, Louisiana, United States.

Dyer told SciDev.Net that parasitoids are an important means of natural pest control for many tropical crops and that if that relationship broke down, farmers might need to use more insecticides.

Frank van Veen of Imperial College, United Kingdom, points out, however, that parasitoids are not the only things that can control caterpillar numbers.

"Other agents, such as predators, diseases and fungal pathogens, might be affected differently by climate change," he said.

Link to the full paper in the Proceedings of the National Academies of Science

Reference: Proceedings of the National Academy of Sciences doi 10.1073/pnas.0508839102 (2005)