The presence of this mite as a significant factor of timothy losses has been reported in Lancaster, Lebanon, Dauphin, and York counties over the past 2 years; however, this pest has most likely been present for a longer period of time but gone unnoticed. Some yield loss estimates range from 30-70%.
In Maryland, problems have been seen for the last 10 years. When the problem first occurred in the early 1990’s in Maryland, the mites only infested the variety Climax, but high populations of the mite have recently been encountered on other varieties. In 1999, the mite was officially identified as the cereal rust mite, A. hystrix, by Dr. Ronald Ochoah, a USDA-ARS mite specialist. This is the first record of this species in Pennsylvania.
A. hystrix has been infesting timothy for some time, but due to its small size, growers have attributed its subtle injury symptoms to agronomic reasons. Based on grower contact and surveys by extension personnel in Maryland, Delaware, and Pennsylvania, the range of cereal rust mite infestations is expanding; virtually every acre of timothy grown in central and water Maryland and southeastern Pennsylvania is infested to some degree.
Description
Adult rust mites are very small (< 1 mm). They are spindle-shaped with four legs and may be white, yellow, or orange. You will need a hand lens to see them. To check for mites, look for off-color foliage, leaf, or bud abnormalities. Use a 10X or 20X hand lens. Large mite populations often produce many elongate, white shed skins. The mites feed on bulliform cells at the base of grooves on the upper surface of the leaf. Eggs are deposited in the grooves, and both eggs and immature stages become distributed higher in the canopy as leaves unfold. Adult mites move downward in the plant crown, where they prefer to feed on the youngest tissues of the plant. The mite undergoes numerous generations per year (a generation time of 16-18 days at 20°C) and there is no known diapause stage. Although development is reduced, mite stages are active during the winter in the crowns of its host plants.
Injury
The feeding of A. hystrix causes direct injury to timothy, which results in retarded growth, stunting, and discoloration. No other pest species or agronomic factor is more important as a constraint in timothy production that the cereal rust mite. Severe mite infestations have two negative impacts on local growers. Feeding injury causes substantial yield losses, as much as 50%, and also reduces hay quality by the brown discoloration. Horse producers are reluctant to buy hay that is not the normal color of quality timothy.
As a side note, A. hystrix is also known to vector ryegrass mosaic virus (RMV), a serious disease of temperate grasslands, and may be a vector of agronpyron mosaic virus (AMV), a minor disease of wheat and other grasses. These diseases cause substantial losses to pasture production in other parts of the world, especially in Europe. However, the presence of RMV and AMV in the USA has not been detected. The symptoms of mite feeding injury on timothy resemble the typical symptoms of a viral infection; however, no associate disease has been confirmed. Nonetheless, if these foreign viruses enter the US, there exists the potential for their virulence on forage grasses and wheat due to the abundance and wide distribution of the mite vector.
Time of Attack
Adults and eggs are present over winter, and the adult hatch begins in March with the peak adult population in April. Damage is most evident in April and will continue into May.
Scouting
Growers should observe fields in early to mid March and look for the presence of small, round eggs in the grooves of the timothy leaf surface.
Treatment is recommended in fields with a previous history of cereal rust mites and/or when 25% of the plant tillers exhibit curled tips of the new leaf blades within several weeks following green-up. Research is underway to develop a monitoring plan and threshold levels for economic justification of treatment.