Spotted wing Drosophila catches are picking up: Maintain monitoring if fields are still ripening

Spotted wing Drosophilas in Michigan are increasing as 30 more were trapped over the past week. Continue to scout fields and check your traps.

Widespread monitoring in commercial fruit farms across Michigan by Michigan State University research and Extension staff has so far detected spotted wing Drosophila (SWD) at only a small proportion of the hundreds of sites where traps are deployed. The pattern that we are seeing from this monitoring has three main components.

1) SWD is active in the regions of the state that had the highest catches last year (detected so far in Van Buren, Ottawa and Allegan counties in 2011) and again at some of the farms with high counts in 2010.

2) It is being suppressed by many of the typical insect management programs being applied for blueberry maggots and Japanese beetles.

3) SWD becomes most active in mid-late summer. The current weather conditions of daily high temperatures in the 70s are ideal for reproduction and growth of this pest, and so we expect trap catches to climb in the coming weeks.

Catches of SWD are increasing at a few of the sites we are monitoring and the patterns of detection are providing insight into where SWD are most likely to be found. The majority of sites where SWD has been detected are in traps in wild areas close to crop fields, indicating a risk to fields from the outside by this new pest. The catches over the past week indicate that activity of SWD is increasing; the average number of flies trapped has gone up, and the number of sites with catches has increased. This is clearly seen by finding 30 SWD (11 females, 19 males) in monitoring traps over the past week, whereas we had only trapped 15 flies in the previous weeks this summer.

The flies trapped this week have been found in yeast traps and not in the standard apple cider vinegar baited traps. These captures have mostly been at sites where the fruit are ripe, suggesting that the yeast can compete better with the ripening fruit than the apple cider vinegar. However, for this season we still recommend that scouts and consultants use the apple cider vinegar traps because these have been reliable for trapping this pest in previous studies in Michigan and in other regions. If you are interested in trying the yeast traps, the recipe we are using is:

4 Tbsp. sugar: 1 Tbsp. Red Star active dry yeast: 12 oz. water, with about an inch depth of this mixture per trap.

The yeast traps catch many more “other” species, creating a challenge for sorting through the other insects to look for SWD. Accurate identification is important and we have found native fly species this season that have some similar characteristics to SWD. Our approach has been to look for the spot on the wing AND the dark combs on the foreleg to identify male SWD, and for female SWD it is still critical to look at the ovipositor. This can be challenging without a microscope, so we still encourage scouts and consultants to send samples to MSU Diagnostics Services if they have a question. Images of the main features for identification of SWD can be found at MSU IPM Program’s Spotted Wing Drosophila website.

In our monitoring of alternate hosts for SWD in Michigan fruit farms, this week we have also detected SWD reared out of wild blackberries. These fruit were collected in the adjacent habitat and in weeds at a non-managed crop field. This highlights the importance of wild hosts for this insect’s ability to reproduce, as well as the importance of controlling perennial fruiting weeds inside crop fields. As the fall approaches, make plans to control weeds such as wild blackberry, wild raspberry, Virginia creeper, wild grape and other plants that will provide alternative sites for SWD egglaying.

Our research team is actively engaged in testing insecticides for SWD control in Michigan blueberries this summer. We are in the middle of some trials where insecticides are applied in the field and then the treated shoots are brought back to a laboratory and exposed to SWD flies at different times after the application. Based on the results of our most recent field assays, we have found that one-day-old residues of Delegate (6 oz.), Imidan (1.33 lb.) and Malathion (32 oz.) have the highest activity against SWD. Lannate (1 lb.) and Entrust (2 oz.) had intermediate levels of activity, whereas Mustang Max (4 oz.) and Pyganic (64 oz.) had low activity. Growers should use this information plus the pre-harvest interval (PHI) restrictions, and any potential MRL considerations, when making decisions regarding which insecticide to use to protect fruit against SWD. For example, Delegate and Imidan have three-day PHIs in blueberry, whereas Malathion has a one-day PHI.

Dr. Isaacs’s work is funded in part by MSU‘s AgBioResearch.

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