Penn State Extension


Cereal Rust Mite

Abacarus hystrix (Nalepa)

Significant losses in timothy from feeding by the cereal rust mite have been reported in Lancaster, Lebanon, Dauphin and York counties over the past two years. It is likely that the mite also has been causing losses in other counties across the state. Yield loss estimates range from 30-70%.

In Maryland, problems have been seen for the last 10 years. When first observed during the early 1990's, the mites were only found to infest 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, by Dr. Ronald Ochoah, a USDA-ARS mite specialist.

A. hystrix has been infesting timothy for some time in Pennsylvania, but due to its small size growers have attributed its subtle injury symptoms to other agronomic problems. Based on grower contacts and surveys conducted by extension personnel in Maryland, Delaware, and Pennsylvania, the range of cereal rust mite infestations appears to be expanding into virtually every acre of timothy grown in the region.

Timothy hay is a very profitable cash crop produced on an estimated 170,000 acres in Pennsylvania and its value can exceed that of alfalfa. It is primarily marketed to the horse industry at premium prices ranging from $75 to $180 per ton. Gross revenues for this crop range between $216 to $315 per acre.

Timothy production in Pennsylvania currently does not meet the horse industry's needs; so considerable quantities of timothy are imported into the state. For this reason a decrease in timothy production in the state results in economic losses to growers and higher prices for imported hay for horse owners.

Description & Life Stages

Adult rust mites are very small (<1mm). They are spindle-shaped, with four legs and may be white, yellow or orange. You will need a 10 to 20X hand-lens to see them. Evidence of their presence on the plant is off-colored foliage and leaf or bud abnormalities. Large mite populations often produce many elongate, white shed skins.

The mites overwinter in the adult and egg stages. Eggs from overwintering adults are deposited in the leaf vein grooves. The eggs appear as very small spherical yellow balls.

The immature stages are similar to the adult, but smaller. Immatures begins hatching in March, with the peak adult populations being reached peaking in April. Damage is most evident in April and May.

As leaves unfold, eggs and the immature stages are distributed higher in the canopy. Adult mites move downward into the plant crown, where they prefer to feed on the youngest tissues of the plant. The mites undergoes numerous generations per year, with a generation time of 16-18 days (at 20o C).

Although development is reduced, mite stages are active during the winter in the crowns of host plants. Unlike most pest mite species, this mite prefers cooler temperatures and tends to be less active during warmer summers months.


The cereal rust mites feed on bulliform cells at the base of grooves on the adaxial leaf surface, causing direct injury to timothy. Symptoms appear as retarded growth, stunting, and plant discoloration. Injured plants appear to be drought stressed even when adequate moisture is available for plant growth. As with most grasses, the edges of timothy leaves roll together when the plant is under moisture stress, as seen in the above picture. No other pest species or agronomic factor is more important to timothy production at this time.

Severe mite infestations have two negative impacts on Pennsylvania growers. Feeding injury causes substantial yield losses and reduces hay quality because of a brown discoloration. Horse producers are reluctant to buy hay that is not the normal color of quality timothy.

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. The presence of RMV and AMV in the USA, however, has not been detected. Although symptoms of timothy mite feeding injury resemble a viral infection, no associate disease has been confirmed. Nonetheless, if these foreign viruses should enter the US, there exists the potential for transmission to forage grasses and wheat through timothy mite infestations.

Integrated Pest Management

Grower's should watch fields in early- to mid-March for the presence of small round eggs in the leaf veins. Although, scouting procedures have not been fully developed, it is always a good idea to take a representation sample across the field. Populations can begin in isolated pockets. A few mites can rapidly develop into an economic problem.

Economic Thresholds - There are no established economic thresholds for the pest. Treatment is recommended, however, in fields with a previous history of cereal rust mites and/or when 25% of the plant tillers exhibit curled tips on the new leaf blades within several weeks following green-up. Research is underway to develop better monitoring plans and economic threshold. The following tables provide a rough estimate of the returns from a miticide application.

Table I. Pennsylvania Economic Loss With and Without the Use of Sevin XLR Plus - 2000 Season (data provided by Dr. Marvin Hall, Department of Agronomy at Penn State University)

(A) Based on High/Low Year Data

Gross Revenue/Acre

Net Revenue/Acre

High Year

Low Year

High Year

Low Year

Without Proposed Miticide*





With Proposed Miticide**





(B) Based on High/Low Year Data

Gross Revenue (avg)

Net Revenue (avg)

Without Proposed Miticide



With Proposed Miticide



* Without Proposed Miticide: Based on 30% yield reduction (information from conversations with producers and consultants).
** With Proposed Miticide : No yield reduction from mites and cost of production of $10 per acre with one application of Sevin.

Management Strategies

The only known and registered method of managing the pest is a properly timed application of Seven XLR PLUS. Research trials at the University of Maryland in 1999 and 2000 indicated that Sevin XLR PLUS applied at 3 pts. (1.5 pounds a.i. per acre) per acre provides effective control of cereal rust mites in timothy. Because mite stages are concealed in the leaf blade grooves, a spray volume of 20 or more gallons with a ground applicator is required to achieve adequate coverage and penetration to the mites.

Higher water volumes should be used for improved mite control. Apply approximately 4 weeks after green-up in the spring. This usually occurs around early to mid April in the eastern and central counties of Pennsylvania where most timothy is grown. Only one application per cutting at 3 pts per acre of formulated material is allowed per season and it must be applied more than 30 days before harvest or grazing.

Use information:
Common Chemical Name carbaryl
Trade name Sevin XLR PLUS
Formulation 4F
EPA Registration Number 264-3
Manufacturer Aventis CropScience
Crop/Site Affected: Timothy - Statewide

Carbaryl (Sevin XLR PLUS) is an N-methyl carbamate, which controls a wide spectrum of insect pests on many field, fruit and vegetable crops.


Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authored by: Del Voight - Capitol Regional Agronomy Team Dennis Calvin - Associate Professor of Entomology Marvin Hall - Professor of Crop & Soil Sciences


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