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Thursday, March 8, 2012

What to watch for: A preemptive question about mites and hops

Hop yard in western NC, July 2010 Hop Tour. Photo: HJB
With the news that Sierra Nevada Brewing Company is opening a facility in Mills River, NC, interest in North Carolina hop production has again surged.  I think we still have a lot to learn about the feasibility of growing hops in North Carolina, but several, mostly small scale producers are already active.  One of these growers emailed me today asking about their main arthropod pest, spider mites, and what action she might take to prevent large populations developing this year.  Last year, she released two predatory mite species, Neoseiulus fallacis and Phytoseiulus persimilis, and she was wondering if those populations could overwinter and be relied to provide control this year. 

Hops leaves respond to relatively low mite densities.  The yellow stippled area on this leaf had a population of spider mites feeding on the opposite side. Photo: HJB
 My reply: "We have native populations of N. fallacis in the western NC mountains, so it is reasonable to assume that they should be able to overwinter (although lab reared mites may be less adapted to winter conditions).  We have had success overwintering P. persimilis under high tunnels on strawberries, but I can't say how well it will do out in the open.

The only way to know if your mites from last year have stuck around is to look for them. Sample 10 leaves per variety weekly and observe with a minimum 10x hand lens.  It will take some practice, but you can see and count mites with a hand lens.  You can distinguish predatory mites from pest mites by size (they are smaller), shape (most are tear drop shaped or oval), and color (P. persilimis) is orange.  Predatory mites also move much faster than pest mites. See here for images of some of the predatory mites commonly used for biological control. Start sampling when you have new leaves and continue weekly.  When and if you release additional predatory mites will depend on when spider mite pests appear in your planting."

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