There have lately been questions about trapping for spotted wing drosophila, Drosophila suzukii, in strawberry.
While there have been few reports of severe infestation of spotted wing drosophila in California fresh market strawberries, nevertheless many growers want to know how to effectively detect this pest. The trapping system previously described for raspberries is designed for a tall plant with plenty of cover and a trellis to hang it out on. Strawberries, by contrast, are short in stature and do not provide much in the way of shade, in addition to having no trellis to support a trap.
The photos below are of a system of trapping for spotted wing drosophila used successfully in British Columbia. The trap holding the same yeast-sugar-water medium as previously described for raspberries is a flattish 750 ml container with holes approximately of ½ inch in diameter in the lid. Since the trap is on the ground, smaller size holes in the lid may be more effective in excluding carabid and other ground beetles. It is partially dug into the soil to keep it from overturning as well as to maintain the bait at a lower temperature. Note also the placement of the container well into the shade of the strawberry plant.
Thank you to Entomologist Tracy Hueppelsheuser from British Columbia Ministry of Agriculture and Lands (BCMAL) for sharing this information.
What does this mean for growers? Simply put, while the goal of the CDFA and USDA may have gone from eradication to containment, the threshold for regulatory action is still the positive identification of one light brown apple moth larva in a production field. Therefore it is imperative for growers in the area under quarantine DO NOT waver in their efforts to control leafrollers in their crops. Any sign of leafroller activity in their crops is still a signal to act.
In some of the first literature written in Japan in 1939 (Kanzawa, T.) about spotted wing drosophila, Drosophila suzukii, (SWD), experiments were made regarding the sensitivity of the egg and larval stages of spotted wing drosophila to periods of temperatures above and below freezing (32o F).
As is noted in the two graphs below, at constant temperatures of up to 35o F, 96 hours or more of cooling resulted in total mortality of spotted wing drosophila eggs and larvae. This was also anecdotally confirmed in tests conducted in 2009 in California.
While temperatures below freezing are not useful to fruit shippers, temperatures in the area of 35o F are. However, it is important to note that for success the constancy of the temperature is critical. So, while in an ideal situation constant temperatures of 35o F or a little below are effective in SWD egg and larvae suppression when extended for periods longer than 96 hours, the reality can vary significantly from the ideal. Shipped fruit ordinarily do not experience lengthy regimes of constant temperature as they are moved from place to place. Temperatures of a refrigerator truck can vary by location inside and placement of the produce (ie on the side, towards the bottom etc.), and certainly the temperatures at the point of sale can vary from the ideal to room temperature to even warmer.
Additionally, while initial damage from SWD on raspberries, blackberries and strawberries can be difficult to detect, this is not the case for other fruits such as cherries or blueberries, where the activity of SWD will leave an unsightly blemish.
The take home message from this information is that while extended cooling can be suppressive of SWD, growers should not rely on cooling alone. It will still be important to manage SWD in field.
Thanks to Shinji Kawai for making the information from the 1939 Kanzawa paper available.