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Tuesday, July 13, 2010

Grape root borer captures in NC vineyards

Mating pair of grape root borer moths (female, left; male, right).  Photo: H C Ellis, University of Georgia,
Two North Carolina vineyards have reported grape root borer (Vitacea polistiformis) trap captures in the last week.  Both vineyards are in the Yadkin Valley, where many of our wine grape growers are located.  If these places are capturing moths, they are likely active in other vineyards in the area.  Many wine grape growers do not monitor for grape root borer, so we do not have a good sense of how wide spread they are NC, but they are capable of infesting grapes throughout the state (both bunch/wine grapes and muscadines) and occur throughout the eastern US, from Florida through Ohio.

Grape root borer injury.  Photo: James Solomon, USDA Forest Service,
Grape root borer larva.  Photo: Clemson University - USDA Cooperative Extension Slide Series,

Grape root borers are potential devastating pests.  As their name suggests, the larvae of grape root borers feed on the roots and crowns of vines.  Like related species (ie. raspberry crown borer), larvae may live for 1-2 years, longer in northern latitudes, and shorter in southern.  According to Tennessee entomologists, larvae there take 22 months to develop on average.  This same Tennessee factsheet has a great overview of grape root borer biology.  Briefly, egg laying begins about 8 days after emergence, and the eggs take 14 days to hatch.  Eggs are laid at the soil surface.  These newly hatched larvae are the most easily controlled life stage, and chemical treatments (if used) are targeted to them.  Once the larvae enter the roots, they are very difficult to control.  Adult control is also impractical, because the moths do not feed on grapes and therefore would not contact insecticides applied the crop.  Adult moths emerge and lay eggs over a relatively long period of time as well, further complicating management. 

Much of this difficulty arises from the materials registered against grape root borer.  Only one conventional insecticide, Lorsban (chlorpyrifos) is registered.  Lorsban has a long pre harvest interval in grapes (35 days in bunch grapes).  Because moths may continue to emerge until fall, this application timing may be too early in some cases.  Ideally, a material which could be timed to moth trap captures would be more useful.  Identifying an alternative to Lorsban with a shorter PHI is a high priority IR-4 project, but the long life cycle of this pest slows this process.  Non chemical management tools are also available.  Entomopathogenic nematodes and pheromone-based mating disruption (essentially confusing male moths) have shown promise experimentally, but neither have been brought up to commerical scale.  Cultural control by soil mounding (in early July or the start of moth emergence, whichever comes first), can also reduce moth numbers.  Soil mounded around the base of vines creates a longer distance for emerging moths to travel, and they die before reaching the surface.  See the Southern Regional Small Fruit Consortium's Bunch Grape IPM Guide for specifics on treatment options.

Pheromone lures attractive to male moths can be used to detect adults, and these are traps growers have caught moths in this year.  Several trap types can be used in combination with these lures (typically rubber septa impregnated with pheromones), but the easiest to use is probably the bucket trap.  These traps are available from several suppliers (some are listed below).
Grape root borer monitoring trap.  Traps are baited with a pheromone lure attractive to Photo: Insect pests of grapes in Florida

Although the grape root borer pheromone is relatively specific, one related species can also be attracted to the traps.  The squash vine borer (Melittia cucurbitae, below) is easily distinguished from grape root borer.

Squash vine borer adult are orange rather than yellow/brown like grape root borers and have more bristles on their rear legs.  See here for another useful image.  Photo: Purdue University Cooperative Extension

In addition to traps, which attract male moths, growers should also scout for pupal cases under the vines.  This will provide a better sense of the total moths present in the vineyard, as traps may pull males in from nearby plantings or wild grapes.  Other states have used pupal case counts to develop treatment thresholds.  Kentucky entomologists recommend treating when 5% of vines have pupal cases present.  Thresholds are an important component of an IPM program, because there are consequences (economic, environmental, etc.) to each management decision, and unnecessary treatments should not be made.
Trap sources

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