Refer to page 11 of the “ Pesticides Registered for Use on Hops in Michigan 2015” for more information about pesticide toxicity and predatory mites. Growers needing to treat for mites at this time are encouraged to avoid applying pyrethroids insecticides which are hard on beneficial mites and have been shown to flare pest mite populations in hop. Note the sand and cast skins collecting on the webbing associated with the mites, giving the leaves a dusty appearance. Twospotted spider mites as seen with the naked eye and under magnification. It is unclear at this time how applicable these thresholds are in Michigan.
By mid-July, the threshold increases to five to 10 mites per leaf. Thresholds developed in the Pacific Northwest have established that more than two adult mites per leaf in June indicates the need to implement a pest management strategy.
Use a hand lens to evaluate two leaves from 20 plants per yard. Some miticides are better positioned early in the season when mite levels are low, others are more effective in situations with high mite populations. Michigan State University Extension recommends scouting for mites season-long and making applications only when needed and as recommended by the manufacturer. Predatory mites play an important role in balancing the TSSM population and should be protected when possible. Predatory mites are often translucent, larger than TSSM and move at a faster speed across the leaf surface. When you are observing the underside of leaves, keep an eye out for beneficial, predatory mites that actually feed on the TSSM. The larvae themselves are small, translucent versions of the adults which begin the season with a distinctly orange hue that changes over to translucent, yellow or green as they feed. The eggs look like tiny, clear spheres and are most commonly found in close proximity to adults, webbing, cast skins and larvae. They may be hard to see even using a hand lens growers can look for movement to help them locate the mites. TSSM are very small, but can be observed on the underside of leaves using a hand lens. Conversely, cold and wet weather is not conducive to development. TSSM thrive under hot conditions, with the pace of development increasing until an upper threshold around 100 degrees Fahrenheit is reached. TSSM damage on young hop leaves from nursery plants Additionally, the mites themselves act as a contaminant issue for brewers.Īdvanced symptoms of TSSM damage. Infested cones develop a reddish discoloration, do not hold up to the drying process and commonly have lower alpha levels and shorter storage potential. Intense infestations weaken the plant and reduce yield and quality. Leaves take on a bronzed and white appearance and can defoliate under high pressure conditions. TSSM feed on the liquid in plant cells eventually causing visible symptoms.
TSSM feeding decreases the photosynthetic ability of the leaves and causes direct mechanical damage to the hop cones. Twospotted spider mites are a significant pest of hops in Michigan and can cause complete economic crop loss when high numbers occur. Good control at this point in the season is critical as cones are present and vulnerable to infestation. Mites are most effectively managed while still at low population levels, so growers are encouraged to scout carefully for mites season-long, starting before damage becomes visible. Dry conditions and hot temperatures have provided ideal conditions for outbreaks of twospotted spider mites (TSSM) in Michigan Hopyards.
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