Over the past few days, I have received several calls from Extension Agents about prevalence of soft fruit, vertical fruit split, small berry size and some reports of worms in the fruit. Although it is difficult to determine the exact reasons, the unusual weather patterns we have experienced this year including warm winters, low night-temperatures lingering late in the season, and excessive rain events over the past few weeks may have contributed to these issues. A number rain events during the harvest above all increase the risk of fly infestations in blueberries. Of course, spotted-wing drosophila (SWD) is our Suspect No. 1 and the most common fly pest that can infest berries. Although not as common, blueberry maggot (BBM), is another fly which is a quarantine pest and can infest berries during similar weather conditions. It is also quite possible that both SWD and BBM infestations may be present in the same field. In that case, correct identification is critical before implementing management strategies.
The adults of SWD and BBM flies are relatively easier to identify because: 1) SWD males have dark spots on their wing margins (Fig. 1) and SWD females have hardened (sclerotized) saw-like (serrated) ovipositor (Fig. 2); and BBM flies have distinctive wing pattern (Fig. 3). But it is very difficult to correctly identify both fly pests at the larval stage by looking at the infested berries. However, correct identification is critical to implementing effective management strategies. Here I am including a schematic diagram of the larval (maggot) stage of both SWD and BBM (Fig. 4) to help you identify them correctly.
Although late for this season, it is still a good idea to put some traps in the field to capture adults to confirm identification and help make management decision for this year and what to expect at your farm next year. Blueberry maggot flies can be best monitored using yellow sticky cards baited with ammonium acetate (or ammonium carbonate) as a food attractant. These traps should be placed on a stake or hung on an upper branch of a blueberry bush in a perimeter row (south facing side of bushes) in such a way that leaves don’t stick to the trap. Hang traps with the colored side down in a V-orientation (Fig. 5). Ideally, traps should be deployed before first anticipated flight (April-May). However, if a resident fly population is suspected from previous infestation, place a trap inside the field to detect internal infestations. Traps should be checked twice a week starting just before first fruit coloring until the first fly is caught. Similarly, traps for SWD (Fig. 6) baited with one of the commercially available lures or apple cider vinegar or yeast:sugar:water slurry should be place in the field before fruit starts to change color. Initiate the management programs at the first fly capture.
A number of broad-spectrum insecticides including Imidan, Malathion, Danitol, and Mustang Max are effective against both BBM and SWD. These are primarily contact insecticides and kill the flies at first contact before they are able to lay eggs inside the fruit. Being broad-spectrum, these insecticides are toxic to beneficial insects, so keep that in mind when applying these insecticides and try to adjust spray timings to minimize impact on beneficial insects as much as possible. The GF-120 NF Naturalyte Fruit Fly Bait can be used to control BBM in blueberry maggot. Some reduced risk insecticides such as Entrust (approved for certified organic systems) Delegate and Exirel can also be used for SWD control. The neonicotinoid insecticides such as Assail and Admire Pro, and other systemic insecticide such Sivanto and Movento have shown efficacy against BBM but not SWD. If you suspect infestation of both SWD and BBM at your farm, contact your local Extension personnel for more specific recommendations to control.