Timothy hay field infected with Cereal Rust Mites; notice the curled leaves. Photo Credit: Andrew Frankenfield, Penn State Extension.
This cool-season pest has caused headaches for many timothy growers, particularly in Southeastern Pennsylvania where it seems to have spread to the majority of fields, reducing growth and crop quality. To determine whether this pest is active in your timothy fields, scout fields for signs of damage. Look for leaf blades that are rolled up tightly, rather than leaf blades that are flat and normally expanded. The feeding of the mites causes leaves to roll up, presumably to provide the mites with better protection and microclimate. The mites are microscopic and challenging to see even with good magnification.
Treatment is recommended if 25% of tillers show leaf curling within several weeks of green-up. Chemical options are limited, but Sevin XLR PLUS has a supplemental label allowing its use against mites on timothy in Pennsylvania. Spray coverage is important to control this pest. The product label states a range of 10-50 gallons per acre, suggesting the higher water volumes should be used for improved mite control. In general, one application at 2-3 pints per acre should provide control. Do not apply within 30 days of harvest. See the Cereal Rust Mite article for more details.