Turns out those beautiful painted lady butterflies that enamoured Islanders this summer have a nasty side.

Before it becomes a butterfly, in its larval stage, it's the thistle caterpillar — and some P.E.I. fields were full of them this summer.

"The first call came in the Rocky Point area about seeing this caterpillar in a field and it wasn't anything I had ever seen before," said Alicia Newman, an agronomist with Cavendish Agri Services in Charlottetown. 

"The first question growers had was how much damage is this pest going to do to the crop and originally we didn't think it was going to be very much."

Painted-lady butterfly

Lots of Prince Edward Islanders shared photos of painted lady butterflies this summer. (Submitted by Bill Perks)

In late August, the P.E.I. Department of Agriculture sent out an advisory about the thistle caterpillar.

They were mostly along the south shore, Newman said, but they also caused some damage in the east and north.

No way to spray

There were similar reports in other parts of Canada, including soybean fields in Saskatchewan.

Spraying wasn't an option, Newman said, because of the lack of registered pesticides specifically for the caterpillar.

"So we just basically had to let it take its course and hope that it doesn't happen again."

Thistle caterpillar 1

The P.E.I. Department of Agriculture and Fisheries sent out an alert about the thistle caterpillar in August, calling it a "soybean pest." (Submitted by P.E.I. Department of Agriculture and Fisheries)

Newman visited several fields, mainly in the Cornwall area, to see the damage for herself.

"If you walked into a field — it didn't have to be beans — you would just get all these butterflies flying around and it was very eerie."

Photos on social media

She describes the damage as being site specific, including one field in Meadowbank.

"There's a field on one side of the road that had very, very heavy damage," she said.

"Literally, on the other side of the dirt road, another soybean field with very minimal damage."

'If you walked into a field — it didn't have to be beans — you would just get all these butterflies flying around and it was very eerie.' - Alicia Newman

Newman said it was frustrating to see photos on social media of Islanders celebrating the beautiful butterflies.

"I just wanted to comment on all of the pictures and say, 'it's not as nice as you think, it's destroying some fields.'"

It will still take some time to assess how much damage has been done.

"At the stage that the caterpillar attacked, the leaves were obviously very crucial for gathering energy to fill the pods," she said. 

"So any of those plants that did get defoliated would have very minimal pods and many pods on them wouldn't be full."

John Klymko, a zoologist at the Atlantic Canada Conservation Data Centre, started noticing in August higher than average numbers of painted lady butterflies around the Maritimes.

May not be around next year

"Their preferred food is thistle," Klymko said. "In years when there are a lot around, they can feed on quite a variety of host plants and can be pests on things like soy."

The last large migration was in 2012, he said.

"It is certainly part of a pattern where you see very large numbers of painted ladies once in a while. So the very high number of painted ladies seen this year does not mean that there will be lots of them around next year."