A feared outbreak of gypsy moth caterpillars in Hamilton's wooded areas appears to have been held in check -- and not just by this year's aerial spraying.

Ben Porchuk Ben Porchuk, head of conservation at the Royal Botanical Gardens, said early indications are that the leaf-devouring caterpillars have also succumbed to natural diseases.

Add in plenty of wet weather, and the prognosis for stressed-out trees is looking bright even before the RBG determines how many of the caterpillars make it to the moth stage, he said.

Like the city and Hamilton Conservation Authority, the RBG took part in last month's spraying of the natural pesticide .

"For the most part, things are looking great. There are very few caterpillars," Mr. Porchuk said.

"In some of the trees that were missed or that were in areas that were planned not to be sprayed, we did have quite a few caterpillars that emerged from the egg cases and then they died of a virus and a fungal infection that are naturally occurring in the insect population," he said.

"It was unexpected but we knew it was a possibility."

The story is similar at the conservation authority.

Ecologist Lisa Jennings said although formal egg mass surveys won't be conducted until this fall, there appears to have been less defoliation of the Dundas Valley's forest canopy than last summer.

Rainy weather has not only given trees "more of a fighting chance" than during last year's dry conditions, she said, but encouraged the growth of the caterpillar-killing fungus and virus.

"It has helped a lot," Ms. Jennings said. "It's been so humid, we've seen spots as well where there's been a high indication of the fungus and the virus killing them off."

But with the gypsy moths just reaching their adult stage, the real test will come this fall, she added.

"They pretty much lay live eggs for two weeks and then they die, and that's when the egg masses start to show up, so we usually start up (egg surveys) in September, just to be safe."

Mike McNamara, the city's manager of forestry and horticulture, said he expects a consultant to report in the next week or so on the success of the $450,000 municipal effort.

"We know from what we've been able to gather ourselves, from talking with people, that it has been effective," he said.

"It has reduced the gypsy-moth infestation substantially in areas that we've sprayed."

Mr. Porchuk said the Royal Botanical Gardens will also be conducting a more detailed analysis, including on 11 test plots measuring 100 by 100 metres.

Eight were sprayed and three were not, "so that we can see what's happening with the spray directly,' he said.

Mr. Porchuk said the RGB has also started a long-term study on forest decline to gauge the impact of other threats, like climate change, drought, the growing numbers of deer and problems created by having "stamp-sized" forests.

"You have a greater perimetre area compared to the interior," he said.

"With that you allow a lot of the trees to have a greater exposure to pathogens and pests. We have all sorts of other introduced species that are really impacting the forest and the trees themselves, a lot of bacteria and fungal infections that are hurting the roots."

To counter such problems, the RBG is in the process of returning areas to their more native vegetation, including through controlled burns.