The Seasonality of Mite Infestations

This article is the second article in-part series on mites. The previous article published in Indiana Nursery & Landscape News, Volume 82, Issue 4, “There Mite Be A Problem”, discussed general information on how to monitor and identify mites. This article will provide you with additional information on identification by having a better understanding of when certain mite species may be active in the environment, host specificity and the types of damage that you may observe.

Mites can be very serious pests in landscape and greenhouse environments. Understanding the seasonality of mites and their preferred hosts can help you better monitor for early stages of mite outbreaks. Mite outbreaks are often associated with hot dry weather. Frequent heavy rains tend to wash mites off plants and reduce feeding injury. High relative humidity levels are more suitable to beneficial predatory mite species that can help keep pest mite species under control. Most plant feeding mites prefer dry conditions with low relative humidity. However not all mites prefer hot weather. Some mites actually thrive in cool weather, especially if the relative humidity level is low. Mites in the family Eriophyidae (eriophyid mites) prefer cool temperatures as do certain species of Tetranychid mites such as the spruce spider mite (Oligonychus ununguis).

Typically, plant injury from warm season mites appears as fine stippling (white dots), bronzing, leaf yellowing, webbing and in severe cases, leaf drop. Mite feeding injury on coniferous and broad leaf evergreens is more problematic than the damage that occurs on deciduous trees because the damage is more permanent. Once it is there, it can be seen for many years. Deciduous trees are able to grow new leaves in the following growing season with little damage carried over from previous years. Large mite populations can also cause serious damage to herbaceous annuals resulting in plant decline and plant death. Unlike herbaceous perennials, if the leaves are severely damaged there is little time or food reserves for plant recovery. Eriophyid mite feeding injury appears as galls, bronzing foliage or erenium (fuzzy areas on a leaf).

Cool season mites are typically active primarily in spring and if conditions are suitable a resurgence can occur in fall. They can be difficult to detect because early stages of feeding activity often go unnoticed due to cool temperatures. Damage from these species may not become apparent until temperatures increase and damaged tissue desiccates. Being knowledgeable about the life cycles of these species may help you better monitor populations and prevent plant damage from occurring.

Cool Season Mite Species

Spruce spider mite (Oligonychus ununguis) is probably the most economically significant and damaging cool season mite species you will encounter. Again, this is not an Eriophyid mite, but a true spider mite belonging to the family Tetranychidae. This mite has a broad host range and causes damage to many types of evergreens with both needles and scale like growth. Feeding activity usually occurs in mid to late spring, especially in May. However, damage may not be seen until temperatures warm enough for damaged tissue desiccation to occur. Damage from mite feeding that occurred in May is sometimes not seen until June when mites are no longer actively feeding.

The hemlock rust mite (Nalapella tsugafolia) is an Eriophyid mite that can be found in conditions that are even cooler than those of the spruce spider mite. Typically, it is active in late winter or early spring. This species causes bronzing on previous year's growth. It will feed on hemlock as the name indicates, but I have also observed this species on yews and spruce, as early as late February during warm winters. I do not frequently encounter this pest in the nursery industry.

Typically, we do not often think of hollies and azaleas as being prone to mite infestations. However, there is a cool season mite called the southern red mite (Oligonychus illicis) which can cause problems in early to mid-spring. This mite is in the same genus as spruce spider mite. It infests broad leaf evergreens such as holly, azalea, cherry laurel and Pieris. The damage caused by this species appears as fading or greyish leaves which is a result of very fine stippling caused by the mite feeding. Infestations are usually seen in spring and fall. Look for white shed skins, red eggs and adults on the underside of infested leaves. I occasionally encounter this pest on nursery stock that is shipped from southern states.

There are some species of mites that cause leaf galls. These frequently occur on maples in Indiana. Two of the more commonly encountered species that occur in Indiana are Vasetes quadripedes (Maple Bladder Gall and Vasetes aceris-crumina (Maple Spindle Gall)). The maple bladder gall is primarily found on silver maples and occasionally red maples, especially those red maples that are hybrids and have silver maple parentage. The maple spindle gall produces finger like galls on sugar maple. I have not encountered this pest as frequently as maple bladder gall. Finally, the pear blister mite (Eriophyes pyri) produces small, raised bumps on the leaves. This is generally not an issue on ornamental pears but this species can cause damage to pear fruit since feeding occurs as the new buds expand. Most galls caused by Eriophyids are not usually harmful and don’t require chemical control. However, they may be objectionable to customers. If control is desired, pesticides must be applied at bud break.

Warm Season Mites

These are the mite species that you may encounter more frequently during hot dry weather. Infrequent rains allow for mite colonies to remain undisturbed and thrive while drought stressed plants are more likely to show damage symptoms. One of the most common spider mites you will encounter in Indiana is the twospotted spider mite (Tetranychus urticae). Twospotted spider mite has a broad host range and is found throughout the world causing a large economic impact on field crops, vegetable crops, cotton as well as ornamental crops in greenhouses, nursery and landscape environ(Continued on next page)

Mite Infestations Continued

ments. It can be identified by two dark feeding spots that are located on its sides. This species predominately infests herbaceous and deciduous plants and is typically not found on conifers. Large populations of this species can produce webbing, stippling, leaf yellowing and leaf drop and eventually plant death. In the landscape this mite is often found on English ivy, burning bush, roses and butterfly bush. In greenhouse environments it will feed on almost anything.

feeding injury. However, as feeding continues the damaged areas coalescence into fading and bronzing of foliage. Boxwoods are an evergreen plant, thus feeding injury will be visible on boxwoods for years after infestations have occurred. It is a good idea to treat this pest in its early stages to prevent undesirable plant damage.

Twospotted spider mite is the most common mite you will encounter in Indiana, however there are a number of other warm season mites that can also cause injury to ornamentals. Some are host specific while others have a broad host range. The honey locust spider mite (Platytetranychus multidigituli) is host specific to honey locust. Infestations can start early in the season and are active throughout the growing season. Multiple generations can cause leaf yellowing and eventually early defoliation. Typical mite feeding injury is more of a cosmetic issue to large established deciduous hosts because the plants can simply produce new leaves in the following growing season. However, if other pests are present on tree at the same time it can add another stress factor in the overall health of the tree.

The European red mite (Panonychus ulmi) is a warm season mite that infests plants in the rose family including fruit trees, hawthorn, mountain ash, crabapple and serviceberry. This mite is red in color and has 6 white dots on its back. There can be multiple generations during the summer, and it can cause noticeable feeding injury on infested plants. Although this pest is present in Indiana, I do not often encounter it.

The Boxwood mite (Eurotetranyhcus buxi) can be found feeding on boxwood in late spring through summer. This species of mite prefers Buxus sempervirens, but I have also seen it feeding on B. microphylla in Indiana. The early-stage damage of this mite species is unique because it is scratchy in appearance, similar to thrips

Mites on oaks are quite common in Indiana. Several mite species can infest oaks and damage is primarily seen in late summer. These species of mites do not have official common names, but all should be treated in the same manner when it comes to control tactics. Oligonychus bicolor occurs on the top side of leaves while. Oligonychus propetes and Eotetranychus hickoriae occur on the underside of leaves. The damage from mite feeding can look similar to lace bug feeding injury. Stippling from mite feeding is finer than that caused by lace bug feeding. Also, with lace bug feeding you will find fecal spots in addition to stippling damage. Beware, both lace bugs and mites can feed on oak leaves at the same time.

Mites frequently infest maples during the warm part of the growing season. Interestingly, I did not encounter mite feeding injury on maples until I started working in the Midwest. Oligonychus aceris mainly infests red and silver maple, but I have observed it on sugar maple. Acer x freemani ‘Autumn Blaze’ is very susceptible to this mite. This is an interesting species that belongs to the same genus as spruce spider mite which is a cool season mite. Maple mites seem to get an early start in cool weather and then continue to expand in numbers even during hot dry weather which typically results in diapause of other species in this genus. Even though they can be found early in the season, they tend not to cause damage until later in the season. Growers that are achieving control are using both ovicides and adulticides and are getting applications on plant material early in the season. Also reducing the amount of nitrogen will help reduce mite populations.

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