The apple leafcurling midge (ALCM) has become relatively well established in many orchards across Ontario in the last few years. Injury from this pest can now be found in all growing regions, with pressure ranging from fairly light (<1% of terminals infested) to severe (>80% of terminals infested) in both dwarf and semi-dwarf blocks.

Typical damage caused by this gall-forming midge can cause leaves to form a tight curl (gall) around the insect (Figure 1). The galls interfere with normal growth and development of young trees, delaying or stunting structural development of nursery trees and, in extremely high densities, can even have impacts on growth of older trees. Reduction of photosynthetic leaf area can also adversely affect fruit size and bud formation.

Several studies from the UK and Europe have demonstrated a relationship between the number of ALCM caught on a pheromone trap for a particular generation and the number of galls that developed subsequently (Cross & Hall 2009). It was estimated that a single ALCM male caught corresponds to approximately 140 galls/ha for that generation, provided there are sufficient shoots and new growth. In Ontario orchards monitored in 2016, the average trap catch at petal fall was 1,500 ALCM/day, which equals the potential for 210,000 galls/ha/day for a single generation alone.

Figure 1. Typical damage caused by apple leafcurling midge.

In order to properly manage this pest, an understanding of the various life stages and how to identify them in your orchard is critical.

Adults (Figure 2)

  • Very small, dark flies (1.5 to 2 mm) with long, beaded antennae.
  • Due to their extremely small size, they are often overlooked in an orchard. However, during times of terminal growth, female ALCM (red abdomens) can be seen laying eggs in the newest leaves of shoot tips.
  • In Ontario, adult emergence initially peaks late May to early June followed by subsequent peaks in activity in late June or early July and again early to mid-August. This indicates that there are typically 3 generations of ALCM in Ontario each year with adult emergence beginning as early as pre-bloom. However, overlapping generations often occur as the season progresses, resulting in the presence of adult, egg and larval stages at the same time. As well, in years such as 2016, when the fall has been warmer than average, a potential 4th generation has been observed.
  • Commercial pheromone traps are available. Deploy traps around tight cluster to pink in the lower branch scaffolds and monitor (bi)weekly for adult flight.
  • Recommended spray threshold: 9 ALCM/trap/da

Figure 2. Adult apple leafcurling midge.

Eggs (Figure 3)

  • Shiny orange eggs, elliptical- or bullet-shaped with dark orange spots in the centre are laid in the leaf folds or along margins of the youngest unfurled leaves of shoot tips. Peaks in egg-laying activity have been observed to occur typically early to mid-June, early to mid-July and mid-August.
  • Depending on environmental conditions, these typically hatch in 3-5 days.
  • When eggs are laid in large masses, can be seen with the naked eye. However, the use of a hand lens or microscope is recommended to ensure visual identification.
  • When monitoring for eggs, check unfurled leaves of several shoot tips per tree weekly (10 terminals on 10 trees). During key times (pink, petal fall, first cover), monitoring should be more frequent.

Figure 3. Apple leafcurling midge eggs.

Larva/Pupa (Figure 4)

  • In recent years, peak egg hatch typically occurred mid-June, July and August.
  • Following egg hatch, small (1.5-3 mm long) maggot-like larvae feed on the edges of tightly curled leaves for 2-3 weeks.
  • Feeding on the upper epidermis layer triggers a physiological response in the leaf itself, inducing a gall, or curl. The larvae remain within the leaf roll for all developmental stages.
  • ALCM have 5 instar, or larval growth stages. Early instar larvae are cream- to pale yellow-coloured. As they mature, larvae become a vibrant orange in colour.
  • Pupation can occur inside the rolled leaf; however, most larvae drop to the soil to pupate at approximately the drip line. The exact soil depth that pupation occurs is currently being studied in Ontario. For summer generations, pupation generally takes 1-2 weeks.
  • Emergence from the rolled leaves is largely dependent on moisture. Lack of rain can delay larvae from exiting the leaves, prolonging development by up to 10 days. A rain event following an extended period of dry weather can result in a mass exodus of large numbers of larvae from the leaf rolls.
  • Timing for the subsequent generation can be predicted by monitoring terminal damage throughout the year. Within a roll, the colour of the larvae indicates relative maturity and time before pupation occurs. Between generations, the youngest leaves will not be damaged as eggs are only laid in the newest growth of a terminal.

Figure 4. Apple leafcurling midge larva.

Management

Currently, the pyrethroids, Mako and Up-Cyde, are the only products registered for the control of ALCM. However, there may be other products applied at petal fall that will have efficacy on this pest. Since 2012, OMAFRA has been working with various companies to determine if currently registered products would have a place in an ALCM control program. Results from the 2012, 2013 and 2014 trials were presented in earlier Orchard Network issues: Volume 16(3) September 2012 (archived), Volume 17 (4) December 2013 and Volume 18 (4) December 2014. Label expansion of Movento 240SC is anticipated in Spring 2017 and an update will go out to growers when this is finalized.

In 2016, Exirel (1 L/ha) and Delegate (420 g/ha) were applied at petal fall with a conventional check of Clutch (210 g/ha) in a Norfolk orchard and Movento (365 mL/ha) in a Durham Region orchard. All treatments were re-applied 10-14 days later. Due to high spring feeding caterpillar damage, the initial application of the conventional check in the Norfolk orchard was applied with Delegate (420 g/ha). However, the subsequent application was Clutch alone.

In the Norfolk orchard, ALCM pressure was extremely high, resulting in extensive terminal damage in all treatments (Figure 5). However, 3 weeks after the first application, both Exirel and Delegate+Clutch treatments had lower terminal damage compared to the Delegate block. This trend was also present when comparing the average number of leaf curls per terminal in each treatment (Figure 6). The high pressure observed at this orchard site confirms that the management of ALCM will take multiple seasons once the pest is established and requires control of each generation within a season. A single petal fall spray will not be enough to reduce populations at this pressure level.

Figure 5. Apple leafcurling midge terminal damage following petal fall insecticide application in Norfolk, ON orchard, 2016. (Different letter above bar represents significant difference (p<0.05) compared to other treatments on same date. Bars without letters represent no significant difference.)

Figure 6. Average number of leaf curls caused by apple leafcurling midge per terminal following petal fall insecticide application in Norfolk, ON orchard, 2016. (Different letter above bar represents significant difference (p<0.05) compared to other treatments on same date. Bars without letters represent no significant difference.)

In the Durham orchard, ALCM pressure was much lower overall. Both Exirel and Delegate blocks showed a reduction in terminal damage and provided comparable control to Movento (Figure 7). Statistical analysis was not performed.

Figure 7. Apple leafcurling midge terminal damage following petal fall insecticide application in Durham Region, ON orchard, 2016.

Insecticide applications will have maximal efficacy when timed correctly since the larvae quickly become well protected rolled up inside the leaf. Hot, dry summers affect the phenology as rainfall softens leaves, making it easier for larvae to escape while reduced rainfall delays development by up to 10 days. Forecast models are currently being developed at Durham College (project lead: Margaret Appleby) and Agriculture & Agri-Food Canada, QC (project lead: Dominique Plouffe).

There are several predators and parasitoids that feed on ALCM during the season, including minute pirate bug (Orius sp), mullein bug, and the parasitoid Platygaster demades. Research from Agriculture & Agri-Food Canada continues to determine the presence and abundance of native parasitoids in commercially managed Canadian orchards. Maintaining good ground cover in the row middles will provide habitat for natural enemy populations to build.

Infestations tend to be more severe where trees have abundant vigorous shoot growth. Avoiding excessive growth and/or removing watershoots during summer pruning when eggs and larvae are present may reduce populations, though this has not been demonstrated.

We gratefully acknowledge co-operating growers and Bayer CropScience, Dow AgroSciences and DuPont for product donation.

Reference

Cross, J., Hull, D.R., Shaw, P. and Anfora, G. 2009. Exploitation of the sex pheromone of apple leaf midge Dasineura mali Kieffer (Diptera: Cecidomyiidae): Part 2. Use of sex pheromone traps for pest monitoring. Crop Protection. 28(2): p. 128-133.