A five-year population explosion of caterpillars in Edmonton's river valley could be ending, says the city's pest management department.

Mike Jenkins, a biological sciences technician with the city, said the insects known as "larger boxelder leafroller" have now infected trees all the way from Devon to Fort Saskatchewan.

They attack only Manitoba maple trees (boxelders), and strip them of their leaves.

There is no permanent damage, however, and the trees do recover.

Jenkins said the infestation started in the Rossdale area around 2005.

He said counts show the population may have peaked and this year numbers could drop thanks to growth in the insect's natural enemies, he said.

"So there's a number of parasitic wasps and parasitic flies that actually come and insert their larvae into the caterpillars and that ends up killing the caterpillar and it's part of … natural population control," Jenkins said.

"And last year we found as many as 70 per cent of the caterpillars in a sort of central area were parasitized."

Jenkins said he expects monitoring this year will find even more of the caterpillars were killed by predators.

He believes the number of caterpillars will decrease from year to year.