Insect pests to watch for in Ontario hops

While the continued hot, dry weather in many sections of the province has helped with downy  mildew management, weather conditions have been favourable to the development of several insect pests in many hopyards.  Growers should be actively scouting for potato leafhopper, two spotted spider mite and Japanese beetle.

Potato leafhopper – Potato leafhoppers are small, extremely active, wedge-shaped insects with a wide host range.  They have piercing-sucking mouthparts that they use to pierce leaf tissue and remove plant fluids.  As they feed, leafhoppers inject a toxin into the plant which blocks the flow of water and nutrients, leading to a characteristic damage pattern of brittle, brown leaf margins with a yellow edge, known as hopper burn.  While they do not directly damage the cones, they reduce the ability of the hops plant to photosynthesize, which can stunt growth, reduce yield and quality of cones and even cause death to very young or extremely stressed plants.

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“Hopperburn” on hop leaves.  Note the browning of the leaf edges.  With sufficient feeding pressure, the entire leaf can become necrotic and fall off.

Adult with moltskins

Scout for leafhoppers weekly.  In addition to looking for signs of feeding damage, randomly select 3 leaves per plant on approximately 30 plants (at least 1 plant per cultivar), flip over and check the underside for presence of adults or nymphs.  Adults are light green and about 3 mm long and will hop or fly away when disturbed.  Nymphs are about 1. 5 mm and wingless but still very active, and tend to rapidly move sideways or backwards across the leaf underside when disturbed.

There are no established thresholds for potato leafhopper on hops, however in some areas an average of two leafhoppers per leaf is used.  Leafhopper populations have reached this level in some hop yards over the last few weeks. No pest control products are currently registered against leafhoppers on hops in Canada, however some products registered and applied for aphids may provide some control of leafhoppers, including Admire (imidacloprid) and Sivanto Prime (flupyradifurone).  However, these products can be harmful to beneficial insects and repeated use can lead to secondary outbreaks of mites.  Organic control options are limited.  Purespray Green Spray Oil can no longer be applied after burrs have developed.  Kopa Insecticidal Soap is registered on hops for aphid control, and may have some impact on leafhopper nymphs if spray coverage is adequate, however this is unlikely to be sufficient to control high populations.  Check with your CB to confirm acceptability of this and any product applied to organic hops.

Two-spotted spider mite – Spider mite numbers are building in many hop yards and are likely to increase quickly since population growth is favoured by hot, dry conditions.  Spider mites can be a very significant pest of hops because they will feed both on the leaves, reducing photosynthetic capacity, and directly on the cones, reducing their quantity and quality.  Mites feed by removing the contents of individual plant cells, causing a stippled, bronzed appearance to the leaves.

Bronzing mites

Hop leaf bronzing caused by spider mite feeding. 

webbing mites and eggs.png

Webbing, mites and eggs on underside of hop leaf.

Hop growers should be scouting for mites weekly to catch populations before they increase, as it is important to apply miticides to spider mites early, when populations are small and mites have not moved into the cones.  Mites are extremely small and can be difficult to see with the naked eye, but spotting them can be made easier using a high magnification hand lens, and looking for mite movement and webbing.  Mite adults and most nymphs are straw-coloured with 2 dark spots on their back, while eggs are clear to cream and spherical.  They produce dense webbing which can coat the leaf underside.  The webbing, mites and eggs are generally constrained to the leaf underside, while feeding damage (stippling and bronzing) can be viewed on top of leaves and cones.  However, once damage becomes very noticeable, mite populations may be very high and harder to control  To monitor for mites, sample 2-3 leaves per plant (at varying heights in the canopy) from at least 10-30 plants per yard and use a hand lens to record numbers.  In the Pacific Northwest, a threshold of 1-2 mites/leaf is used until mid-July, after which it is increased to 5-10 mites/leaf.

There are three conventional miticides registered for use on two-spotted spider mites in hops, and growers need to be mindful of pre-harvest intervals when selecting products.  Agri-Mek SC (abamectin) has a 28 day pre-harvest interval and must be applied with a surfactant, Envidor 240 SC (spirodiclofen) has a 21 day PHI, and Acramite 50 WS (bifenazate) had a 14 day PHI.  All of these products are effective against adults and nymphs, however only Envidor has activity against mite eggs.  All products have a residual activity of several weeks, and good coverage of the leaf underside is required for effective control. For organic growers, Kopa Insecticidal Soap is registered for control of spider mites.

Japanese beetle –  Japanese beetles are a little later than last year, however populations have arrived and are starting to increase in some yards, particularly in southwestern Ontario.  Japanese beetle are large, green and copper-coloured insects with tufts of white hairs along the wings.  They aggregate in large numbers and skeletonize the leaves of hops and a wide variety of other plants.  While they do not normally feed directly on the cones, sufficient feeding can affect the ability of hops plants to photosynthesize.

Picture1

Japanese Beetle

Because they are so large, scouting for Japanese beetles can be done by visual inspection. However, in many Ontario hop yards, Japanese beetles seem to concentrate much of their feeding on the upper third of the plant. When scouting hops, it’s a good idea to bring a pair binoculars and scan the top wire for signs of feeding.

Feeding top wire

Japanese beetle skeletonization of upper leaves along top wire.

There are no insecticides registered for Japanese beetle on hops in Ontario.  The neonicotinoid insecticde Admire (imidacloprid) is registered for control of hops aphid, and growers applying this products for aphids may see some reduction in populations of Japanese beetles.

 

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