An emerging orchard pest in northern climates in the apple leafcurling midge (ALCM), an invasive insect from Europe. It was first detected in North America in the 1980s, primarily in eastern Ontario, but it has spread since then to both the Finger Lakes region in New York and Michigan.

Kristy Grigg-McGuffin, horticulture IPM specialist with the Ontario Ministry of Agriculture, Food & Rural Affairs, recently led a Cornell Cooperative Extension webinar about ALCM. She noted it was originally a nursery and young planting pest but has become an increasing problem in the past 10 years and in established orchards for a few theorized reasons: because of a change in broad-spectrum insecticides; because of different training systems, especially high-density plantings, as young, succulent trees are more attractive to them; because of the extended length of activity, with warmer springs and autumns; and possibly because of warmer soils and more overwintering success.

“But it’s a matter of when it’s going to show up, not if,” Grigg-McGuffin said.

ALCM has little impact on older trees except those with higher densities (which can see greater than 60% of leaf area damaged). The damage the pest causes can reduce a tree’s total carbon acquisition, its structure and its winter hardiness. In younger trees, it results in stunting – but it doesn’t impact the fruits, just the trees themselves.

A gall-forming midge, ALCM triggers a response in plants when they hatch. Their larvae grow within the curled leaves, making them well-sheltered from insecticides. Grigg-McGuffin explained they have five instar stages, and when a grower is monitoring for them, they should look for color changes in the larvae.

Early stages are cream/off-white and about the size of a grain of rice. There are usually 20 – 30 larvae inside each leaf. As they mature, they become more orange and grow larger. When they’re about to pupate, ALCM will be a vibrant, neon orange. They will drop down to pupate in the soil. Their drop down is triggered by heavy moisture and rainfall.

The adult midges are small, only 1.5 – 2.5 mm in length, with long legs, beaded antennae and, notably, red hemolymph (a fluid equivalent to blood in insects) found on sticky traps. Grigg-McGuffin said that tell-tale red drop is a good way to identify them. Their eggs are shiny and orange and inserted into new, pale green leaf and bud terminals – making them fairly easy to see in young growth.

Apple leafcurling midge eggs. Photo courtesy of OMAFRA

Growers will notice the margins of infested leaves rolled in toward the mid-vein in really tight curls. Leaves will become purple/red and brittle before dropping from the tree. ALCM is different from the leafroller moth (which is evident from its webbing) and the rosy apple aphid (which curls leaves downward).

Once established, there are no eradication options for ALCM, but they can be controlled with yearly management, according to Grigg-McGuffin. Apple varieties with more vigorous growth tend to be more susceptible – those with soft leaves vs. waxy leaves.

There are some biological controls that show promise. The parasitoids Inostemma contariniae and Platygaster marchali, introduced in New Brunswick orchards from 1981-85, resulted in 40% – 90% reduction of ALCM. P. demades was introduced into Nova Scotia in 1993-94 with similar results. Grigg-McGuffin said they also saw positive impacts from Lyrcus nigroaeneus in Ontario and Synopeas myles in British Columbia.

“Biocontrols are not to be relied on solely but are good to incorporate in IPM,” she added.

Growers can also see ALCM predators inside curled leaves – specifically, the Mullein bug and the minute pirate bug, who are voracious eaters of the pest. Grigg-McGuffin suggested trying to create an environment welcoming to these predators.

Researchers are also looking into studying the efficacy of entomopathogenic fungi or nematodes in the soil to control ALCM while it’s in the soil.

For chemical control, currently only pyrethroids and spirotetramat are registered for control of ALCM, although other insecticides have efficacy. Trials in the past have looked at sulfoxaflor, spinetoram, cyantraniliprole and mineral oil. Spirotetramat (IRAC 23) has proven to be very effective.

“Key timing is hitting that first generation in early spring,” Grigg-McGuffin said of insecticide use. “If you don’t hit them then, you can see three generations throughout a season as warmer temperatures persist.”

If growers are scouting and see galls, it may be too late to stop that first generation from gaining a foothold. If they can spot the leaves just as they’re starting to curl, they still have time to apply insecticide. It’s critical to take a proactive approach to monitoring by looking for any signs of egg laying, especially in regions where ALCM is creeping in.

To learn more about Grigg-McGuffin’s work, follow her on Twitter @appleofmyIPM.

by Courtney Llewellyn