Fluctuating winter weather in Vancouver, which saw warmer January temperatures turn cold in February, may have helped kill an invasive beetle with a ferocious appetite for plants, according to a University of British Columbia scientist.

Juli Carrillo, who studies plant-insect ecology and evolution at UBC, says the grubs of the Japanese beetle, which spend the winter in the soil, may have been lured to the surface during January's warm spell, but then froze as below normal cold temperatures set in.

"Similar to the early flowers we saw in January, that bloomed and then died from the cold," Carrillo wrote in an email to CBC News.

Read more about the Japanese beetle, an invasive, regulated pest that has been found in Vancouver and what we are doing about it: <a href="https://t.co/HbLBhlu4nc">https://t.co/HbLBhlu4nc</a> <a href="https://t.co/61bZnVSQmy">pic.twitter.com/61bZnVSQmy</a> —@CityofVancouver

In 2019, Vancouver experienced the coldest February since Environment Canada began recording weather data in 1937.

"Sudden and prolonged freezing temperatures that cause those upper soil layers to freeze could potentially limit the beetle's population growth this year," wrote Carrillo.

The Japanese beetle was first discovered in B.C. in Vancouver's False Creek in 2017. The beetle, Popillia japonica, is a invasive pest that feeds on the roots of grass and the foliage of more than 300 plant species.

Plants into skeletons

It's unclear how the pest came to B.C., but most likely it arrived on plants transported from other regions.

The province says the beetle poses a serious risk to plants, trees and agricultural crops. The pests turn healthy plants into skeletons by eating their soft green parts, ultimately causing the plants to die.

The beetle eats away at leaves leaving just the skeleton. (Enzo Zanatta/CBC)

Adults have an oval outline and are approximately 10 millimetres long and 6 millimetres wide, with metallic green abdomens and heads, copper-brown wing coverings and white tufts of hair along their sides and rear.

Since the beetle was discovered, the City of Vancouver along with the province and the federal government have worked together on measures to prevent its spread.

They've sprayed a larvacide to kill the grubs, which live in soil, and also trapped adults. They have also restricted the movement of plants and soil and have asked people to report sightings.

A map from the Canadian Food Inspection Agency show an area where moving plant and soil materials is restricted to try and prevent Japanese beetles from spreading to other areas of the city. (Canadian Food Inspection Agency)

There are no adults present in the wintertime, but grubs, which emerge from the soil in late June or early July, spend the winter underground eating on the roots of grass.

However, the province is not so certain the cold weather made a dent in Japanese beetle numbers.

The Ministry of Agriculture says grubs bury deep into the soil during winter and are also insulated by snow on the ground. While the cold may have killed some grubs, the most the cold weather will do, it says, is delay the time when grubs emerge from the soil and develop into adult beetles.

"We do not expect the temperatures were severe enough to significantly impact the overall larvae population," said the ministry in a statement.