Tuesday, October 7, 2014

Asian Chestnut Gall Wasp

As I planned for next year's planting and grafting season I found myself reminded of the Asian Chestnut Gall Wasp.

I live in Michigan and it is illegal to import chestnut trees or scionwood from states known to harbor Asian Chestnut Gall Wasps.

Light green is the natural range of the American Chestnut before the Chestnut blight showed up.
A list of states with documented populations of ACGW include:
  • Georgia, 
  • Alabama, 
  • North Carolina
  • Virginia
  • Maryland
  • Pennsylvania
  • Maryland
  • Kentucky
  • Tennessee
  • Ohio
  • Connecticu
  • Ontario, Canada 



The first line of defense is to not import it.  If in doubt, don't.

The second line of defense is sanitation. If it looks like it has galls, cut and burn the infected material.  Call your extension agent if you are in a state that is not documented as a ACGW positive state for positive identification.

The third line of defense is a cluster of parasitic wasps: Torymus sinensis, T. tubicola, Megastigmus

Native parasitic wasps are being identified as well.  These native species have not been effective in suppressing populations because they are not synchronized with peak ACGW vulnerability.  This situation is similar to what is seen with the Emerald Ash Borer.  Given time it is likely that a genetic shift will occur so native species can exploit this bonanza of potential hosts. 

One thing that growers can do is to plant prime nectar sources in or near their trees to hit the end-of-May to middle-of-July time window so the parasitic wasps can maximize their time spent hunting the target, ACGW.  More research needs to be done to identify prime nectar sources. Potential plants include Mints, common white clover, sweet alyssum, Aegopodium podagraria,  and possibly Eupatorium perfoliatum if you can find an early blooming race.  These nectar plants are suggestions.

Composites (Dandelion?) and Umbelliferae are highly regarded nectar sources but they may be more difficult to integrate them into your orchard management plan as easily as white clover and sweet alyssum.  Beekeepers tell me that common garden raspberries and rhubarb attract vast swarms of native insects.  The rhubarb is a bit early for our purposes but the raspberries fall within the early part of the sweet spot.  No information is available regarding best cultivars although one would expect more berries to be correlated with more blossoms.

To reiterate, there may be relatively few native parasitic wasps that have populations peaks that coincide with when the ACGW is most vulnerable.  Literature suggests that optimum feeding of parasitic wasps can increase their life span by a factor of three-to-five times.  That increases the breeding efficiency of those races/species of wasps life cycles that do align with the ACGW, will hasten the speed of the genetic shift that will result in synchronization with the ACGW and by slowing down parasitic wasp mortality will increase the number of parasitic wasps per unit of land during periods of peak ACGW vulnerability.

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