Eriophyid mites are insect relatives in the Eriophyidae family. The majority of species of mite in this family are undescribed and many have complex life cycles that are yet to be fully understood. Eriophyid mites are tiny, often "carrot-shaped", and can only be seen with magnification. They typically have only two pairs of legs. Many are associated with plant foliage and live on host plant leaves. In the case of the "rust mites", these are species within the Eriophyidae whose feeding habits cause russeting of the leaves of their host plants. These species are sometimes referred to as "leaf vagrants", since they do not always cause the formation of a gall. Knowing the host plant identification, as well as the type of damage being caused, can help one narrow down the identification of the species of Eriophyid mite. Such species include but are not limited to: the privet rust mite (Aculus ligustri), the hemlock rust mite (Nalepella tsugifoliae), the spruce rust mite (Nalepella halourga), the lilac rust mite (Aculops massalongoi), the orange camellia rust mite (Acaphylla steinwedeni), and the honeylocust rust mite (Aculops gleditsiae).
The privet rust mite (Aculus ligustri) becomes active as soon as new leaves develop on plants, even in the early cool weather of spring. Females overwinter beneath bud scales and will migrate to the host plant leaves in the spring to lay their eggs. This species of rust mite is active from early spring to early summer and many overlapping generations can occur. This species is capable of growing to prolific numbers on a single plant - 2,000 individuals on a single leaf may be found on up to 70% of the foliage. The activity of this species will slow and cease during the hot part of the summer and fall. Activity may last longer if a cool growing season is experienced. The life cycle can vary with climatic conditions depending upon geographic location. The privet rust mite is found in Massachusetts and New York, south to at least Virginia and Tennessee, and was first described from California.
The hemlock rust mite (Nalepella tsugifoliae) is a tiny, wormlike, and slow moving eriophyid that requires magnification to be seen on their host plants. The hemlock rust mite feeds on the needles of its hosts openly by removing fluids from the needles. When mite populations are high, foliage may appear blue-ish in color and turn yellow before dropping from the plant. The hemlock rust mite causes most of its damage in the spring and may be a pest in nurseries. Damage by this eriophyid mite is most noticeable by mid-summer, but by that time the populations typically die off. With 10X magnification, adult mites are yellow and spindle-shaped with four legs. Thin, white colored cast skins may be visible with magnification, as well as the tiny, yellowish-orange eggs. Several generations may occur per year with adults overwintering in the cracks and crevices of bark.
The spruce rust mite (Nalepella halourga) is only known on spruce hosts. This species of rust mite, if viewed with magnification, will appear pale yellow in color during the growing season. By fall, they may appear reddish-purple in color. Females lay the tiny, round, red eggs that will overwinter on the undersides of the needles of their spruce hosts. Eggs hatch very early in the spring, and the spruce rust mite passes through two nymphal instars before maturing into an adult. The spruce rust mite prefers cool growing season conditions, and populations will decline naturally during the heat of the summer.
The details of the life histories and biologies of the lilac rust mite (Aculops massalongoi), the orange camellia rust mite (Acaphylla steinwedeni), and the honeylocust rust mite (Aculops gleditsiae) are also not well understood. The orange camellia rust mite (Acaphylla steinwedeni) injures camellia leaves to the point where leaves are bronzed and rusty in color; however, this species may be more common in warmer climates. Little is known about the biology of this eriophyid. In the case of the honeylocust rust mite (Aculops gleditsiae), this native North American species is yet another example of an insect relative that has been accidentally introduced as a non-native, invasive organism in other parts of the world (ex. in Europe in 1993; Petanović, 1993).
Eriophyid mites known as rust mites are those species which specifically cause leaf bronzing or yellowing. The type of damage caused depends upon the species of rust mite involved, and the species of host plant impacted. For example, with the privet rust mite (Aculus ligustri), Ligustrum ovalifolium is impacted by the new leaves curling and turning brown. On L. amurense, the leaves become severely cupped by the feeding of the privet rust mite, and can drop from the plant while still green in color. When hemlock rust mite (Nalepella tsugifoliae) populations are high, foliage may appear blue-ish in color and turn yellow before dropping from the plant. Both surfaces of host plant needles may be fed upon. The hemlock rust mite often goes undetected until damage is very apparent. Activity begins in the spring, and hemlock rust mites are typically active up to the early summer before populations are reduced. Black and Norway spruce are impacted by the spruce rust mite (Nalepella halourga). The needles of these hosts appear bronzed or russeted in color as a result of spruce rust mite feeding. Typically the upper section of the host plant is impacted, and when populations of spruce rust mites are high, entire trees may appear grayish or yellowish in color. In some cases, nursery plants may be more significantly impacted. In general, activity from rust mites may end on its own during the growing season as high temperatures return in the summer and fall. As such, depending upon the timing during the growing season, chemical management of rust mites may not be necessary. Additionally, due to the lack of information about the specifics of the biology and timing of the life cycles of rust mites, management using chemicals may be difficult.
In the springtime, scout hemlock needles for hemlock rust mites (Nalepella tsugifoliae) using a 10X hand lens to search for the spindle-shaped eriophyid mites. Populations of the hemlock rust mite may linger further into the summer if weather conditions are cool and dry. In general, rust mite species may be monitored for in the early spring throughout the cool periods of the growing season. Use magnification, and pay close attention to host plants that appear off color, such as bronzed, russeted, rusty, yellow, or grayish. Immature and adult rust mites may be visible with magnification, as well as white cast (shed) skins as the organisms develop.
Prune out and destroy individual, infested branches when possible. This may not be possible if the damage is concentrated high up in the canopy, or if the population is so abundant that the entire plant is impacted.
There are several good natural enemies reported for rust mites, such as lady beetles and predatory mites. For example, predatory mites and predatory insects are sometimes reported as natural enemies of hemlock rust mites (Nalepella tsugifoliae). While the exact details of the species involved, or the level to which they provide population reduction of rust mites is often not discussed, in general natural enemy activity within rust mite populations is noted in the literature (Johnson and Lyon, 1991).
Abamectin (NL)
Carbaryl (L)
Pyrethrins (L)
Pyrethrin + piperonyl butoxide (L)
Horticultural oil (L)
Spiromesifen (L)
Chemical management of rust mites may be difficult to time properly.
Active ingredients that may be applied systemically include: Abamectin (injection).
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
