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Friday, September 6, 2013

Late summer grape insect update

The Muscadine Field Day last month afforded me an opportunity to highlight some of the late summer concerns in grapes. Spittlebugs have prompted questions from both bunch grape and muscadine grape growers during the last three weeks.  Spittlebugs, commonly referred to as froghoppers, using their piercing-sucking mouth parts to feed on plant phloem. One of the most common species of spittlebugs in North Carolina is the two-lined spittlebug, which as an adult, is a distinctive black with orange/red stripes. Froghopper nymphs cannot fly, and one of the ways they protect themselves from predation is by producing masses of sticky bubbles to cover their bodies. This "spittle" is usually what prompts calls by growers, who are often concerned that their plants have been sickened by a disease.

Spittlebug feeding on muscadine grape vine. Photo: HJB
Spittlebugs rarely cause economic damage in grapes in the southeast. They feed on vegetative tissue, not fruit, and are generally present in low numbers.  Spittlebugs can be washed off plants with water (growers may want to use a high pressure sprayer), but unless they are present in large number or found directly on fruit that will be harvested, treatment is usually not necessary.

Prior to the Muscadine Field DayBill Cline captured a pair of mating grape root borer moths in the research vineyard at the Horticultural Crops Research Station.  Male grape root borers (GRB) are smaller than females, and both males and females are slow fliers, making them easy to spot and capture.

Female (front) and male (back) grape root borer moths. This pair was collected while mating by Bill Cline at the Horticultural Crop Research Station, Castle Hayne, NC. Photo: HJB

I have been getting reports of GRBs captures in monitoring traps in grower fields throughout August, which is similar to the activity periods that have been observed by our Grape Root Borer Volunteer Monitoring Network (GRB*VMN). Peak GRB activity appears during harvest in many vineyards.  Because this also appears to be the case in at the Hort. Crops Research Station, I left them a package of Isomate GRB pheromone dispensers (twist ties) to deploy before next July 1st (before flight activity begins).  This dispensers release pheromones that disrupt the ability of male moths to find "calling" females with which to mate.  This tactic is called mating disruption, and is a better fit, timing-wise, than pesticide applications for GRB management in North Carolina.

Female grape root borer moth. Photo: HJB

Finally, both Bill Cline and I have received calls in the past several years about fruit drop in muscadine grapes.  When these fruit are cut open, the seeds are brown and shriveled, unlike healthy fruit, which have white, firm seeds when unripe.

Damaged muscadine grape fruit which had dropped from the vine. Growers suspect stink bug damage, but I am not so sure. Photo: HJB

Growers have asked if this damage may be due to stink bugs, which have been common in vineyards in recent summers.  I am not convinced that stink bugs are to blame, however.  Stink bug feeding typically results in a tissue response at the feeding site.  For example, when stink bugs feed on the midrib of tobacco leaves, the leaf wilts down. Stink bug feeding on fruit, such as apples, pears, or tomatoes, produces a corky mass surrounding the feeding area, and feeding on cotton bolls produces internal "warts".  The grapes that have dropped from vines do not have any apparent external damage leading to the discolored seeds, suggesting that the cause was not feeding from the outside.

I suspect, regardless of the cause, that muscadine grapes are likely resilient to early season fruit drop and may compensate by making the fruit that remains larger.  Yield on plants with and without fruit drop has not been measured, however.

More information
Do it yourself - Grape root borer monitoring - NC Small Fruit & Specialty Crop IPM