Cherry fruitworm injury on ripe and unripe blueberry fruit in Bladen County, NC. 7 May 2010. Note entry hole on unripe fruit. The other fruit on this branch appear uninfested. Photo: HJB
These first fruit are a good indicator of the damage to expect, and based on what we saw Friday, 2010 looks like a worse cherry fruitworm year than last year (2009). Cherry fruitworm (Grapholita packardi) and cranberry fruitworm (Acrobasis vaccinii) are the two most common internally feeding insect pests of blueberries in NC and are also important pests in other blueberry growing regions. Since I have been at NC State, we have seen more cherry fruitworm injury than cranberry fruitworm. The biggest difference between the two species with respect to feeding is the number of fruit injured. Cranberry fruitworm larvae feed on several (3-5 berries) to complete their larval life stages, and their frass is clearly visible between injured fruit. On the other hand, cherry fruitworm larvae feed on 1-3 berries as larvae. No cherry fruitworm frass is visible outside the fruit, although injured fruit may stuck to one another where the larvae have tunneled. Fruitworm injury results in early ripening and fruit drop, so the first ripe fruit will typically show the most damage.
Cherry and cranberry fruitworm eggs are laid after petal fall. Upon hatching, the neonate larvae tunnel into the fruit and feed. Because internally feeding larvae cannot be controlled with insecticides, fruitworm larvae are controlled with insecticide treatments timed to petal fall (see the NC Ag Chem Manual for NC recommendations, A note on pesticide recommendations), although the flight timings of both fruitworm moths in NC is not clearly understood. We conducted fruitworm trapping in 2009 and found that cherry fruitworm flights occur early into blueberry bloom. This means earlier fruiting varieties may be at greater risk. The fact that blueberry bloom in 2010 was so concentrated (~2 weeks rather than 3-4 weeks) may also have contributed the high cherry fruitworm populations this spring.
At some locations damage was extensive (nearly every ripe fruit was infested), but no live larvae were found in any of the fruit observed. This means that the damage will likely not persist in the next round of ripe fruit. It also means that fruitworms are unlikely to become contaminants this season. We saw a similar fruitworm infestation pattern in 2008. These two growing seasons are similar in that decent early spring precipitation was followed by prolonged dry weather. This dry spring resulted in fruit remaining on plants that would have normally dropped with normal precipitations. In 2008, however, we had a cooler spring, and slower larval development. This led to a significant number of cherry fruitworm larvae in harvested fruit which rendered some fruit unusable. This year, our unseasonably warm spring has led to quicker cherry fruitworm development, and if our observations from Friday hold true, less risk of larvae being found in harvested fruit.
Even if cherry fruitworm larvae are not present in damaged fruit, these fruit are likely still unmarketable and need to be removed post harvest, representing a yield loss. We need to conduct more research on fruitworm biology in NC. We have a poor understanding of their flight and egg laying timing, nor do we know what potential biological control agents are present and the impact our management strategies may have upon them. There is ongoing cranberry fruitworm populations modeling work in Michigan, but our plant and insect phenology differ greatly.