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Wednesday, January 30, 2013

Prionus larvae found on blueberry

In October 2010 and again in January 2013 growers reported 'Duke' highbush blueberry fields with severe damage to roots and crowns caused by grubs feeding on the underground parts of the plants.  The grubs appear to be the larval stage of a species of Prionus beetle.

Larvae removed from the crown of a dying blueberry bush

Prionus feeding damage to the crown of a blueberry bush
The larvae reportedly take three to five years to mature, and may be more likely to occur on stressed bushes.  Above-ground symptoms are hard to distinguish from drought stress or nutrient deficiency.  Affected bushes are weakened progressively as the larvae grow and inflect more damage.

The only way to confirm the presence of these insects is to dig bushes and examine the roots and crown for damage or larvae.  As pictured, the larvae are larger than most other grubs found in the root zone,  are widest at the head end, with strong jaws, and with bodies that are elongated rather than the typical "C-shaped" grubs of other beetles.

Update, 2 February 2013
As Bill indicates, these larvae have only been observed on 'Duke' bushes.  We aren't sure what this means, but we suspect that 'Duke' may be stressed in some way that predisposes it to infestation.  It's also important to note that management action taken against larvae this size will likely be ineffective because they are so entrenched within plant tissue.  If Prionus is a more wide spread issue than we currently believe, we will need to determine when adults are active and egg laying occurs, since this would likely be the most effective time to initiate management.  

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