By Forrest Scharf, PAg, Provincial Specialist Fruit Crops, Regina
June 2022
Invasive insect pests have become an increasingly significant threat to Saskatchewan fruit growers. Saskatchewan’s relatively challenging climate traditionally presented a significant establishment barrier to insects whose native habitats are centred in areas with milder and moister climates. Unfortunately, some have had the ability to adapt and perhaps take advantage of various changing conditions to establish themselves here.
Spotted Wing Drosophila (SWD) is a type of fruit fly (Drosophila suzukii) whose centre of origin includes Japan and China, it is adapted to temperate habitats and is believed to have evolved to infest unripe temperate fruit crops (Zachos et al. 2001). Temperate fruit species are cold tolerant but yield fruit seasonally (due to requiring a chilling period to flower). The lack of available overripe fruit limits fruit fly egg laying capacity throughout most of the year. So, SWD have adapted to ensure they could place their eggs (oviposit) into unripe fruit whose skin was relatively tough to penetrate. To meet the need to penetrate unripe fruit, female SWD anatomy evolved to have prominent serrations along the ovipositor that allows them to cut into unripe fruit when they lay their eggs. SWD are also attracted to CO2 and volatiles emitted from ripening fruit, whereas other species are attracted to fermenting or rotting fruit (Burrack, 2020). Visual identification of serrated ovipositors on SWD caught in survey traps is the primary way female SWD are identified, males have distinct spots at the distal end of their wings (but are less prevalent).
In North America, SWD was first documented in California in 2008. By 2014, it was detected in Southern regions of B.C., Ontario and Quebec, North Dakota, Southern Manitoba and Alberta. It was then confirmed in Saskatchewan in 2019 and surveying since that time has consistently found this pest to be widely distributed throughout the province’s grain-belt. Females lay one to three eggs per oviposition site and average 380 eggs in their lifetime (up to one year). Larvae feed on host fruit until they exit to pupate. In the lower mainland region of B.C., SWD can remain active over winter. In regions with harsher winters, adults usually don’t become active until mid-June to early July. In lab studies, it was found that an “adult to adult” generation took 50 days at 12 C but only 7 days at 28 C. In other studies, cold tolerance of adults and some late season larvae were observed to increase as generations progressed into Fall, with between 10-15 percent of winter morphological phenotypes able to supercool and survive temperatures below -20 C (Shearer et al, 2016). So, it is possible SWD have become capable of overwintering in Saskatchewan. On the other hand, based on in-field overwintering survival studies some researchers believe SWD is unlikely to overwinter in cool climates, and that populations are likely blown in from warmer southern states.
The Ministry of Agriculture is supporting a SWD survey in 2022 that will track population dynamics over the production season. As opposed to prior surveys that employed a wide distribution and only two collection times. The 2022 survey is designed to reduce the distribution of traps but increase the number of collections. Despite cool spring conditions, all traps that were deployed in mid-May and collected prior to June 1, have already presented significant numbers of female SWD. If this finding reflects SWD overwintering capability, it is astounding so many were able to survive through this past year’s relatively harsh winter and become active so soon after temperatures slowly started to rise. Michigan and New York State investigations revealed that SWD can sustain themselves feeding on mushrooms and complex bird manures. Given the vast number of migratory birds that pass-through Saskatchewan, it isn’t difficult to discern that an early season food resource is readily available to sustain SWD populations in this province.
Due to the relatively strong population of SWD detected, the use of pest control products is recommended in 2022. Various insecticides are registered for use on a wide variety of fruit crops to control SWD. Fruit growers are encouraged to seek more information about registered pesticides from the Crops and Irrigation Branch Fruit Specialist. The greatest SWD susceptibility to insecticides and level of SWD activity is in the early morning and evening when conditions are cool and moist. SWD become less active and less susceptible to insecticides in the heat of the day. Reduced susceptibility to insecticides in mid-day is not only due to reduced likelihood of exposure but relates to circadian rhythm gene expression that increases genetic resistance to pesticides mid-day.