Parasitism and Development of Degree Day Model for Apple Leafcurling Midge

Shoot of an apple tree infested with leafcurling midge. Terminal leaves are rolled inwards towards midvein and pink in colour.
Figure 1. Typical damage caused by apple leafcurling midge

In the last few years, apple leafcurling midge (ALCM, Dasineura mali) has become relatively well established in many orchards across the province. Injury from this pest can now be found in all growing regions, with pressure ranging from minimal (<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 (Fig. 1). The galls interfere with or stunt normal growth and development of trees, especially in nursery or young plantings. In most cases, impact is minimal on older trees except when pest pressure is high (>60% leaf area damaged). Reduction of photosynthetic leaf area can adversely affect total carbon acquisition, fruit size and bud formation (Allison et al., 1995).

Monitoring

Several studies from 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 et al., 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.

A white delta trap with a white sticky liner covered in thousands of small, dark adult midges.
Figure 2. Daily trap catch of apple leafcurling midge can be in the ‘000s in high pressure orchard blocks.

In 5 Norfolk County orchards monitored in 2020, the average trap catch at petal fall was upwards of 1,500 – 2,000 ALCM / day (Fig. 2), which equals the potential for 210,000 – 280,000 galls / ha / day for a single generation alone.

Commercial pheromone lures are available for use. While price per lure has decreased significantly in recent years, they are still rather costly. However, trapping helps to monitor onset of activity in the orchard and determine population pressure. The proposed economic threshold of 9 ALCM/trap/day (Cross & Hall, 2009) may work for low pressure sites, but those orchards with moderate to high pressure can see populations quickly exceed this threshold in only a matter of a few days.

Current monitoring includes:

  • Trapping – Deployed at tight cluster.
  • Scouting – Presence of a) females laying eggs in new terminals (Fig. 3a), b) presence of orange eggs in unfurled leaves of terminal (Fig. 3b), c) early onset of leaf curling (Fig. 3c) or d) change of larva colour from cream to orange prior to pupating (Fig. 3d).
  • For more detailed description of life stages, see a previous Orchard Network Newsletter article, Apple Leafcurling Midge: What to Look For and When

Biology

Since 2014, OMAFRA has been involved in national projects looking at the biology and management of this pest. One of these projects was just recently published in The Canadian Entomologist journal (Cossentine et al., 2020).

Based on trap captures, it has been determined ALCM has 3 distinct generations with flight beginning as early as tight cluster or pink in some regions (Fig. 7). Depending on the season, activity of the third generation can extend well into the fall. For example, in 2020, adults were still caught in traps and larva was present in late season terminal growth or root suckers until late October.

Two graphs showing number of leaf curling midge males from April to October 2016 in Nova Scotia (A) and Ontario (B) where three distinct peaks in male captures are shown.
Figure 7. Pheromone trap captures of apple leafcurling midge in 2016 in A) Nova Scotia and B) Ontario orchards showing 3 distinct generations: mid-May to mid-June, mid-July to early August and late August to September or later. (Cossentine et al., 2020)

In most orchards monitored in Nova Scotia, Ontario and British Columbia, pest pressure and terminal damage increased as the season progressed (Fig. 8). However, in Ontario, the earlier generations have been shown to cause extensive damage from the onset of activity which stresses the importance of early intervention if this pest has been a problem in previous years.

Bar graph showing mean percent growing tips with leafcurling midge eggs for 1st, 2nd and 3rd generations from populations collected in Nova Scotia, Eastern Ontario, Southwestern Ontario and British Columbia from 2014-2016. In Nova Scotia and British Columbia, growing tips with eggs increased with generation in a year. In Ontario, 1st and 2nd generation generally had more eggs on growing tip than 3rd generation.
Figure 8. Mean percent terminal apple growing shoots infested with apple leafcurling midge eggs in each of the three generations, 2014-2016 (Cossentine et al., 2020).

An interesting finding to come out of the 2014-2016 national study was while populations from Nova Scotia, Ontario and British Columbia were morphologically similar, molecular analysis showed two genetically distinct groups: one from Nova Scotia and the other from Ontario and British Columbia. This could suggest either a high level of intraspecific genetic variation, especially if there were multiple invasions of ALCM in Canada, or that these two populations are cryptic, or separate species. In either case, genetic variability can play a role in how a species responds to control strategies; what works for one region, may not work for another.

Biological Control

In Europe, establishing and encouraging natural enemy populations is an important part of ALCM management. Several classical biological control introductions of parasitoids, Inostemma contariniae, Platygaster marchali and Platygaster demades were made in New Brunswick and Nova Scotia in the 1980’s and 1990’s, respectively. However, establishment, spread and impact of these introductions on ALCM populations have not been documented.

During the 2014-2016 national study, infested terminals were collected from all monitored orchards in Nova Scotia, Ontario and British Columbia. Leafcurling midge populations were reared at the Agriculture and Agri-Food Canada laboratory in Ottawa to identify potential parasitoid species.

The level of parasitism ranged from 2-40% of infested galls, with orchards using minimal spray or reduced-risk insecticide programs having greater incidences of parasitism. The parasitoid species complex differed among provinces, with P. demades being most abundant in Nova Scotia, Lyrcus nigroaeneus in Ontario and Synopeas myles in British Columbia (Table 1). While L. nigroaeneus is native to Ontario, the other two species associated with the Ontario ALCM populations are exotic to the region: P. tuberosula was an introduced biological control of wheat midge and S. myles is a known parasitoid of the invasive swede midge. This study is the first known global record of S. myles parasitizing leafcurling midge.  

Table 1. Parasitoid species associated with apple leafcurling midge infested orchards in Nova Scotia, Ontario and British Columbia, 2014-2016.

Province
Parasitoid species
Number of specimens
Nova Scotia
Platygaster demades (Walker)
391
Platygaster tuberosula Kieffer
3
Synopeas myles (Walker)
1
Ontario
Lyrcus nigroaeneus (Ashmead)
615
Platygaster tuberosula Kieffer
30
Synopeas myles (Walker)
1
British Columbia
Synopeas myles (Walker)
3,026
Platygaster demades (Walker)
5
Platygaster tuberosula Kieffer
1
Adapted from Cossentine et al. (2020)

Apart from this parasitism study, over the years of monitoring for ALCM in Ontario, it is clear that natural enemies play an important role. Mullein bug (Fig. 9a), minute pirate bug (Fig. 9b) and lady beetles are voracious feeders of ALCM larva and eggs. Be sure to note the presence of natural enemies while monitoring for ALCM and consider these populations when selecting control product. Refer to Table 3-6. Toxicity of Pesticides to Honeybees and Mite/Aphid Predators in the 2020-2021 Publication 360A, Crop Protection Guide for Apples.

Development of Degree Day Model

While the biology of ALCM is becoming well understood, management still remains a problem. Even with a systemic insecticide such as Movento, application timing is critical to target the appropriate life stages. Typically, growers don’t notice there is a problem until galls appear which is often too little, too late. A degree-day model could help provide information necessary to know when appropriate measures should be applied.

Since 2018, seasonal trap catch and weather data from 38 sites across Canada, including 16 sites from Ontario, have been used to determine 5, 50 and 95% adult emergence for each generation. This model development is being led by the Bioclimatology and Modelling Research Team with Agriculture & Agri-Food Canada in Saint-Jean-sur-Richelieu, QC.

Regional models are being developed due to the variation in Canadian climate from wet/warm (lower Fraser Valley, BC) to wet/cool (Maritimes) to temperate/dry (Quebec and Ontario) as well as varied winter temperatures and snow cover. All models are using a biofix of March 1st and base temperature of 9˚C.

In Ontario, we are currently validating the models based on trap catch from orchards in Niagara, Simcoe and Harrow. Figure 10 shows a comparison of the preliminary model predictions to the actual ALCM activity from 5 Norfolk County orchards. This evaluation will continue in 2021 with an updated model containing 3 years of trap data. 

Graph showing number of adult male midge caught per day. Graph depicts three large spikes associated with three generations (May, end of June/early July, and August) progressively getting smaller and more spread out over each week.
Figure 10. Number of apple leafcurling midge adult males trapped per day from 5 orchards in Norfolk County, ON from April to November 2020. Red lines represent predicted period of activity for first, second and third generations (Gen 1, Gen 2 and Gen 3, respectively) based on a regional degree day model using 2018 data only.

Acknowledgements

These projects were generously funded through the Agriculture and Agri-Food Canada (2014-2016) and Canadian Agri-Science Cluster for Horticulture 3, in cooperation with Agriculture and Agri-Food Canada’s AgriScience Program, a Canadian Agricultural Partnership initiative, the Canadian Horticultural Council, and industry contributors (2018-present).

References

Allison, P.A., Meekings, J.S., Tomkins, A.R., and Wilson, D.J. 1995. Effects of leaf damage by apple leafcurling midge (Dasyneura mali) on photosynthesis of apple leaves. Proceedings of the 48th New Zealand Plant Protection Conference, 121–124.

Cossentine, J., Brauner, A., Franklin, J., Robertson, M., Buhl, P., Blatt, S., Gariepy, T., Fraser, H., Appleby, A., Grigg-McGuffin, K. and Mason, P. 2020. Parasitism and phenology of Dasineura mali (Diptera: Cecidomyiidae) in Canadian apple (Rosaceae) orchards. The Canadian Entomologist, 152(3): 355-373.

Cross, J. V., & Hall, D. R. 2009. Exploitation of the sex pheromone of apple leaf midge Dasineura mali Kieffer (Diptera: Cecidomyiidae) for pest monitoring: Part 1. Development of lure and trap. Crop Prot. 28(2):139-144.

Cross, J. V., Hall, D. R., Shaw, P., & 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 Prot. 28(2):128-133.


Kristy Grigg-McGuffin
Kristy Grigg-McGuffin

Horticulture IPM Specialist, OMAFRA