Adult tipworms are approximately one tenth the size of an adult mosquito. Note the blurry finger in the background
When two insects, such as the cranberry tipworm and the blueberry gall midge, look so alike that they can’t be distinguished visually, but are actually different insects, they are called cryptic species.
This dry distinction is very important for British Columbia’s cranberry growers, who are suffering the onslaught of the cranberry tip worm.
As cranberry plantings took off in this province during the ‘90s cranberry farmers believed the infestation on their plants came from nearby blueberry farms where the gall midge had been detected a decade earlier.
Given that the insects are identical in appearance it is easy to understand why farmers made the obvious link. Even Dr. Sheila Fitzpatrick, an entomologist at the federal government’s Agassiz Research Centre, says that was her first thought.
It has taken years of testing to prove the two species are distinct and cannot interbreed. Results from Fitzpatrick’s latest study show that neither male nor female tipworms will mate with gall midges of either sex.
That means the infestation of tipworm was imported in with cranberry vines when the plantings were expanding. It was, she says, a politically touchy conclusion.
Vine growers out of the United States, where the vines were purchased, didn’t want to accept responsibility and undoubtedly, farmers putting in the new vines didn’t want to be told their plantings were the ones that brought the problem to British Columbia.
At this point, where the tip worm originated is academic.
“They’re here to stay,” she says. “But cranberry farmers don’t need to worry about what is happening on a neighbouring blueberry farm.”
As the gall midge is not nearly as destructive, research is now focusing on the tipworm, starting with a look at some wasps that are parasites on the tipworm, but Fitzpatrick says they have a natural incursion rate of only about one in five (18 per cent). That helps, but it won’t be enough on its own.
Tipworm populations expand most quickly during early growth of the plants. To encourage faster growth of young vines some growers will apply large amounts of fertilizer, but this fast, succulent growth provides the perfect feeding and breeding ground for the tipworm. Careful management of nitrogen application, suggests Fitzpatrick, is needed to balance growth and breeding opportunities for the pest.
The two pesticides available are only licensed for use before berry production so once the plants are bearing fruit the pesticides cannot be used. Further, tipworm larvae reside inside the plant buds. These two pesticides are contact pesticides so the plant itself shields the insects.
A new pesticide, known by its trade name of Movento (Spirotetramat), is undergoing studies, but won’t be available for use until 2013.
Meanwhile Fitzpatrick is focusing her work on finding a relatively easy way for farmers to determine how large an infestation might be. Her past work identified four pheromones that attract the tipworm, making it easier to get good counts. Since the most effective pheromone probably can’t be manufactured at a reasonable cost she is working on developing a cost effective combination of the four pheromones. ■