Two-spotted spider mite damaging Ontario hops

It's a good news/bad news situation in hops right now. The good news is, in Simcoe at least, leafhopper activity seems to be dying down a little bit. The bad news is that, amongst all the hopper damage, we are seeing signs that mites are becoming more active (Figure 1). The two-spotted spider mite, Tetranychus urticae, is a well known pest of hops and many other crops, and often becomes problematic when conditions are hot and dry.

Figure 1. Hop leaf with both hopper burn (A) and mite damage (B). Hopper burn tends to begin along the leaf margin, while bronzing caused by mites can occur anywhere on the leaf.

Figure 1. Hop leaf with both hopper burn (A) and mite damage (B). Hopper burn tends to begin along the leaf margin, while bronzing caused by mites can occur anywhere on the leaf.

Figure 2. Two spotted spider mite and eggs on a hops leaf. Note the different colour of the eggs as compared to the oil droplets secreted by the leaf.

Figure 2. Two spotted spider mite and eggs on a hops leaf. Note the different colour of the eggs as compared to the oil droplets secreted by the leaf.

Spider mites are extremely small (0.5-1 mm) relatives of spiders. They have eight legs, with the exception of the first instar, which has only 6. Mites have two body segments (head and abdomen) which are a translucent yellow colour, with a distinct, dark reddish-brown spot on each side of the abdomen (Figure 2). Mite eggs are spherical and a translucent yellow in colour (Figure 2). They are often suspended in a fine network of silk webbing (Figure 3), with the mites themselves located on the lower leaf surface. On hops, the eggs are similar in size and shape to the yellow oil droplets excreted by the plant's trichomes, however the oil is not located on webbing and is a brighter yellow in colour than the mite eggs.

Spider mites use sucking mouth parts to remove the contents of individual plant cells, causing a stippled bronzing on the surface of leaves (Figure 4). Severe infestations can result in leaf necrosis and defoliation. Spider mites can also damage hop cones, causing a dry, brittle, reddish discolouration. Economic damage from this pest results from reduction in both yield and quality of the cones.

Two-spotted spider mites overwinter as adult females in crop residue (e.g. hop crowns), in cracks in poles or in other sheltered areas in or adjacent to hop yards. In early spring, females emerge and lay eggs on young hop shoots, or on grassy weeds, in fence rows or wheat fields. Each female can lay up to 100 eggs. Mites pass through three immature stages (or instars) before becoming adults. Development from egg to adult can take three weeks to as few as 6 days (under hot dry conditions), and consequently there are numerous mite generations per year.

Because they may overwinter in hop yards, mites may be present from the beginning of the growing season. However additional mites can move onto hop plants as wheat fields and other grasses begin to dry down, and population development is favoured by hot, dry and dusty conditions. Consequently we often see mite populations begin to increase in hops yards from late June through July.

Figure 3. Spider mite webbing on hops leaf.

Figure 3. Spider mite webbing on hops leaf.

Figure 4. Bronzing of hop foliage caused by spider mite feeding.

Figure 4. Bronzing of hop foliage caused by spider mite feeding.

To monitor for mites on hops, sample hop leaves beginning in late May and continuing throughout the season. Collect several leaves from 10 to thirty plants, depending on the size of the hops yard, and ensure that you are collecting leaves from different heights on the plant (a pole pruner may be required to collect leaves near trellis wires). Inspect the leaves for bronzing and stippling, and the leaf undersides for the presence of mites, eggs and webbing. Spider mites and their eggs are very difficult to see with the naked eye, and webbing may not be immediately visible until infestations are severe. A dissecting microscope or hand lens with a magnification of 25-40X may be required to properly see them, and distinguish them from predatory mites.

A formal economic threshold has not been established for two-spotted spider mites on hops. Low numbers of mites on hop foliage can be tolerated if conditions are mild, natural enemies are present in the yard and cones are not infested. However if conditions become hot, dry and dusty populations can build rapidly.

Spider mite populations on hops are often kept in check naturally by a complex of natural enemies, including predatory mites, lady beetles, predatory midges and others. Avoiding removal of basal foliage can help preserve natural enemies, but this would have to be balanced by the impact of retaining foliage on other hops pests. There have been some suggestions that introducing natural enemies such as predatory mites or lady beetle larvae early in the season may help limit mite population build up in hop yards, however this would have to be further studied, as these natural enemies often do not stay where they are released. A better strategy may be to plant ground cover or other plants which provide habitat to promote natural enemy development. Use well managed cover crops between rows to limit dusty conditions that favour a build up of pest mites. If ground cover dries, pest mites may move into crops.

Spider mite development in hops is favoured by both dust and excessive nitrogen, so minimizing dust in and around hop yards (e.g. by covering or watering dirt roads, planting ground covers, etc.) and ensuring sufficient, but not excessive, soil fertility may help in reducing mite populations. Neusodan commercial and Opal Insecticidal Soap are possible organic solutions that are registered on hops and may have some impact on mite populations. Check with you certifying body on acceptability of these products.


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