Pest Alert: Drosophila Damage Observed in South Carolina Strawberries
Last week, South Carolina cooperative extension personnel issued a pest alert for drosophila damage in strawberries. It is impossible to visually distinguish drosophila larvae between species, but if fruit appear otherwise undamaged (meaning they have no holes, rotted areas, or disease damage) except for a soft spot with larvae underneath, it is very likely that those larvae are spotted wing drosophila (SWD).
In recent years, SWD damage has not been common in spring fruit strawberries, although they have been more common later in the season and in day neutral plantings. Our warm winter and spring weather has likely shifted infestation risk to earlier this growing season. Given the greater likelihood of SWD damage in spring fruiting berries, what should strawberry and early blueberry growers do to prepare?
When infestation risk is low (such as during recent spring strawberry seasons), growers should monitor for adult flies and begin insecticide treatments when they are detected. However, risk this year, appears higher, and I would encourage growers to consider taking preventative management action in addition to monitoring for adult flies. Preventive management consists of: Weekly applications of registered pesticides, reapplied in the event of rain. Rotate between at least two modes of action to reduce the likelihood of resistance development. See the Southern Region Small Fruit Integrated Pest Management Guides for recommended insecticides.
Even if fruit are being managed preventatively, growers should still sample fruit to determine if it is infested. Sample fruit at each harvest via salt water extraction or by cutting fruit open to observe larvae. Salt water will cause larvae to exit fruit as illustrated in the video below. Sample at least 30 berries per field. See here for more larval sampling information.
If growers find damaged fruit, what should they do? I put together these suggestions a few years ago, including:
1. Remove ripe fruit and ripening fruit starting to change color and destroy it (by freezing, “baking” in the sun inside a clear plastic bag for a few days, or removing from the site). Eggs or larvae may be present in otherwise sound appearing fruit. Leaving this fruit means that potentially infested fruit is present and could be harvested.
2. Begin an aggressive spray program as described above.
3. Practice excellent sanitation. Thoroughly harvest all fruit and sell or destroy it. Destroy culled fruit. Unpicked fruit is a reservoir for SWD larvae. Clean up after rain, and send a clean up crew after you-pickers.
4. Sample fruit each harvest as described above.
5. Consider trapping flies using yeast and sugar baits or commercially available lures. Traps tell you SWD presence or absence and are not perfect, but they can be helpful if damage has not already occurred in fruit. Place traps in strawberry rows, near fruit for the best chance of catching flies. Currently available traps are not helpful in making management decisions once damage has already occurred.
I will post more information as the North Carolina strawberry harvest begins and we have a sense of what our actually damage scenario looks like.
Update, 28 March 2016
It is also important to note that many broad spectrum insecticides, such as those recommended for SWD, can flare spider mites! See here for considerations when making these applications.