A quick update on what we saw when we scouted hop yards in southwestern Ontario. At the risk of beating this pun to death, things are hopping in Ontario hops right now. Leafhoppers and aphids are present in low numbers, and we are finding isolated hop loopers hatching. Downy mildew is still present and may increase with the recent wet weather. We are also finding Alternaria lesions on isolated leaves. More details on these pests will be posted in coming days, but today we’ll focust on mites.
Although it hasn’t been particularly dry and dusty (conditions that seemed to favour mite increases last summer), we are still finding farily large numbers of eggs, nymphs and adult mites on the lower surface of hops leaves. Largest numbers are low in the plant, particularly where lower leaves have not been stripped, however leaves with patchy mite populations can be found even on leaves at the very top of plants.
Growers should be scouting their yards for presence of these pests. Sample 2-3 leaves per plant from 25-30 plants per yard (fewer plants can be sampled in very small yards). A threshold for mites has not been established for Ontario hops, but in the northeastern US an average of 2-3 adult mites/leaf in June is a recommended threshold for implementing a control strategy. See my previous post for products registered on mites in hops.
Remember that two spotted spider mites are extremely small, and their eggs are smaller still. Early in an infestation, you may not see the characteristic webbing on the leaves. A hand lens with a magnification of 20X or higher is needed to adequately assess mite populations. Below is a photo of a hops leaf that had no obvious webbing or mite activity.
However, on close-up examination of this leaf showed 2 adults, 9 nymphs and 416 eggs. Below is a close up photo of the leaf under the microscope with a dime for size perspective. The yellow spots are hop trichomes and associated oils, the clear circles are unhatched mite eggs.
It’s not all bad news. Beneficial insects are also active in Ontario hops yards. Lacewings are actively laying their eggs – these are oblong and visible with the naked eye, laid on long stalks that protude from the leaf:
Below is an the immature stage of an aphid midge – essentially a maggot that feeds on aphids, mite eggs and other insects. The midge is the maggot-like creature in the bottom corner, moving towards the eggs and webbing on the upper portion of the leaf.
Predatory mites are also active. They can be distinguished from two spotted by the lack of the two distinct spots and their teardrop shaped bodies. They are also often more active on the leaf than their prey. A two spotted spider mite is shown in the right photo for comparison.