Watch out for twospotted spider mite on hops

Reports are coming in from hopyards in drier areas of the province that mite populations are increasing.  Mite populations are not as high in areas that have received more rain, however in the Simcoe area we have seen a significant increase in the number of mite eggs on hop leaves, probably brought on by the very high temperatures experienced at the end of July.  All Ontario hops growers should be scouting their plants for mites, if they are not already doing so.

For new hop growers not familiar with this pest, the two-spotted spider mite is a serious pest of a wide range of plants, including hops.  On hops, they feed by removing plant fluids from leaves and cones.  On leaves, this leaves to bronzing, defoliation and a reduction in plant vigor.  Mite feeding on cones causes tissue to become reddish and brittle, reducing yield, affecting acid levels and storage life. and acting as direct contaminants in the cones.

Leaf bronzing

 

Leaf bronzing caused by mite infestation

When scouting for mites, randomly select 2-3 leaves/plant from 10-30 plants and examine them, particularly the leaf underside for the presence of mites.  Mites are extremely small and are not easily seen with the naked eye.  Use of a a high-powered hand lens can be very helpful.  While many of my students can spot mites even with a 10X lens, I have found that my aging eyes cannot detect small populations of mites without at least a 20X lens.

Adult and immature two-spotted spider mites are straw-coloured with 2 dark spots on their back, although very young mites lack spots.  Eggs are clear to cream and round.  As populations build, they produce dense webbing on the leaf underside, in which eggs and molt skins may be embedded.   Note that beneficial mites may also be present – these are generally larger and faster moving than two-spotted spider mites and lack the dark spots. The more beneficial mites present, the more likely the spider mite populations will be kept under economically damaging levels.

Mites on leaf undersideWebbing, mites and eggs on underside of hops leaf

At this time growers should also be checking hops cones for presence of mites and associated feeding damage.

Infested coneMite feeding damage to hop cone

There is no established treatment threshold for mites in northeastern North America.  In the US Pacific Northwest, a threshold of 5-10 mites per leaves is used.  In Ontario, plants may be able to tolerate higher numbers of mites if cones are not infested, weather is mild (not too dry or hot) and/or many natural enemies are present.

A number of products are registered for control of spider mites on hops in Ontario, however selection may be limited at this time for growers nearing harvest due to the long preharvest intervals associated with some of them.  Conventional products registered for mite control are:

  • Agri-mek 1.9EC (abamectin) – pre-harvest interval is 28 days
  • Envidor 240 SC (spirodiclofen) – 21 day pre-harvest interval
  • Acramite 50 WS (bifenazate) – 14 day pre-harvest interval

Products potentially acceptable for organic production registered for control or suppression of mites on hops (always check with your certifying body prior to application to confirm acceptability):

  • Purespray Green Oil (mineral oil) – Suppression only.  Do not apply as of burr development
  • Neusodan Commercial, Opal and Kopa insecticidal soaps (potassium salts of fatty acids) – 0 day pre-harvest interval.

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