Adults are smaller than house flies with an average length less than 5 mm. Cherry fruit flies have a light yellow dot in the middle of their thorax, whereas apple maggots typically have a white dot. The black cherry fruit fly’s body is black, whereas cherry fruit flies have white bands on their abdomen (four on females, three on males). As is evident in the photos, each species' wing patterns are noticeably different.
In Saskatchewan, adult cherry fruit flies and apple maggots emerge and are present in orchards in general correspondence with the chart below (expressed as a percentage of adult fly emergence over the course of the growing season).
The maggots overwinter as grain-sized puparia buried roughly two to 15 cm deep in the soil. Emergence of adults typically begins from mid-May to mid-June for the cherry fruit fly, but does not start until early to mid-July for the apple maggot.
Peak emergence corresponds with the early stages of fruit maturation and ripening. In general, the cherry fruit flies emerge earlier, perhaps because their native fruit hosts (pin cherries) would mature at a relatively early date. Apple maggots have evolved in correspondence with apple crops to such an extent that they no longer preferentially oviposit in native hawthorns (which had been their natural host before apples were brought to North America), and the closely related fruit flies that infect hawthorns are now considered a distinct species. In any case, apple maggots commonly infect saskatoons, apples and dwarf sour cherries, so potential oviposit timing is extensive. The timing of emergence is also somewhat fluid, depending upon the climatic conditions in any given year. For example, most apple maggot emergence was late in 2017 and only very small maggots were found in late-maturing apple varieties. This likely occurred because conditions were exceptionally dry in many regions of southern Saskatchewan.
Following emergence, the adults need between a week and 10 days to feed and mature before they begin mating, after which the females insert eggs under the skin of the host fruit. Normally, only one female’s egg is inserted per fruit because pheromones warn away other females from ovipositing on that fruit.
Eggs hatch in five to 10 days, and then the maggots feed for 18 to 30 days. They pass through three growth-stages (instars) during the feeding period.
After the larvae mature through their three instars, they exit the fruit, creating visible exit holes, drop to ground level and burrow into the soil.
Dwarf sour cherry fruit infection surveys conducted from 2004 to 2006 found 60 per cent of the reared maggots were black cherry fruit flies and 40 per cent were cherry fruit flies. It appears the black cherry fruit fly has a greater preference for dwarf sour cherry than the cherry fruit fly has, but the greater percentage of infestation may perhaps be due to the adult’s earlier emergence.
Both of the cherry fruit fly species infect native prairie pincherries, chokecherries and other stone fruit. In contrast, apple maggots appear to have a preference for pome fruit like apples, pears, saskatoons and hawthorn, as well as mountain ash and cotoneaster. They may also infect chokecherries (Prunus virginiana), and other Prunus species. Cherry fruit flies are far less likely to infect apples and other pome fruit.
Recommended monitoring procedures evaluated by the University of Saskatchewan’s Fruit Program are similar to those in other jurisdictions. Yellow sticky traps (which can be purchased at farm and garden centres) in combination with a small bag of ammonium carbonate (or ammonium acetate) serve as very effective lures for monitoring purposes.
Red-coloured traps were less effective, and traps without ammonium carbonate (or acetate) were also far less effective.
In Saskatchewan, insecticide application should not be made prior to mid-June for cherry fruit flies or July 10 for apple maggots. The first application should be made seven to 10 days after the first adult flies are captured on the yellow card traps. A number of insecticides are registered for control of cherry fruit flies and apple maggots on apples, crab apples and dwarf sour cherry, but there are not insecticides registered for use on saskatoons. Fortunately none should be required since the eggs are laid too late in the season to cause significant economic damage to the fruit. Growers must refer to product labels for specific directions, rates and other limitations.
Registered control products and application scheduling can be found in OMAFRA’s Guide to Fruit Production.