Saving Fruit from Insect Pest
Matthew Gullickson, University of Minnesota Graduate Research Assistant, Mary Rogers, University of Minnesota Assistant Professor, Steve Poppe, WCROC Horticulture Scientist and Esther Jordan, WCROC Communication Specialist
In late summer, just as fall-bearing raspberries are starting to ripen, researchers from the University of Minnesota West Central Research and Outreach Center (WCROC) are attempting to protect the berry crop from destruction. Their main concern is to protect the fruit from a recently established invasive insect pest, spotted-wing drosophila (SWD), which feeds on many kinds of soft fruit, such as raspberries.
SWD began appearing in Minnesota in 2012. Since its arrival, this pest has been responsible for significant damage to berries with major economic losses in Minnesota and nationwide. Adult SWD lay eggs in ripening fruit, which later hatch and feed on the flesh of the fruit, which results in unmarketable, mushy fruit with unwanted maggots inside at the time of harvest. SWD populations grow quickly; it takes just over a week to develop from egg to adult, and there can be more than 10 generations in a single growing season. In response to this threat, some Minnesota growers have simply stopped harvesting their berry crops or removed acreage altogether.
Current SWD management strategies are limited to regular applications of only a handful of broad spectrum insecticides. Weekly sprays can have harmful ecological impacts to beneficial, non-target species including pollinators. Additionally, frequent spraying fails to adequately protect fruit from infestation. Weekly conventional insecticide rotations in fall-bearing raspberries can still result in 60% of fruit infested with maggots, likely due to high SWD populations, arrival of new individuals, and/or inadequate spray penetration through the dense plant canopy. Since the arrival of SWD, fruit producers have had to spend more time and money applying insecticides while still suffering from a loss of revenue. Additionally, there are environmental concerns of balancing pest and pollinator management goals. Researchers at the University of Minnesota are investigating the efficacy of newly developed insecticides and the possibility of using high tunnels, unheated greenhouses with plastic stretched over a frame, fitted with mesh netting to prevent SWD from coming in contact with raspberry fruit.
There is increasing interest in using high tunnel production as a pest management tool, particularly since several growers in Minnesota already use high tunnels for season extension and improved fruit quality. The tunnel environment might prevent SWD development due to higher air temperatures compared to open field plots and could be a viable management strategy for SWD.
Researchers conducted experiments at the WCROC to investigate how plastic covered, enclosed tunnels affect raspberry fruit yield and quality, as well as SWD infestation. They collected data on yield, proportion of marketable compared to unmarketable fruit, proportion of infested fruit, and the temperature inside of the plots. The objective of these experiments are to improve insecticide practices and develop sustainable management strategies for controlling SWD.
After the first harvest season of data collection, SWD infestation of fruit was higher in the high tunnels than in the open field with organic insecticide application plots. Although this result was unexpected, this could be due to the high tunnels actually protecting SWD from environmental conditions such as wind. When tested in other parts of Minnesota, however, high tunnels were successful at preventing widespread infestation. When we analyzed fruit yield at the WCROC, high tunnels had more than double the marketable yield compared to open plots, suggesting that even if high tunnels do not prevent SWD from laying eggs, high tunnels can still increase fruit yield substantially. Funding for this research was provided by the Minnesota Invasive Terrestrial Plants and Pests Center through the Minnesota Environment and Natural Resources Trust Fund.