Landscape - Rust Mites on Conifers

Landscapers may have seen some damage by rust mites on conifers this fall. They also can be problems in early spring. The following is an article on these pests and when to have them on the calendar to look for.

CONIFEROUS RUST MITES. Rust mites are tiny microscopic creatures that cause a silvering or bronzing to the plant needles they affect. Two species present in spring and fall are the hemlock rust mite (HRM) and the white pine sheath mite (WPSM). Hemlock rust mite mostly feeds on hemlock and occasionally on spruce. It feeds on the margins and upper surface of needles, turning them an olive-green to silver-green color. Premature needle drop can occur in heavy infestations. HRM can be found any time of year, but has population peaks in the early spring (around 265-471 GDD) and again in late October to Early November. Bloom of Cornus mas (Cornelian cherry dogwood) may coincide with the spring HRM peak.

White pine sheath mite feeds on white pine and inhabits the innermost portion of the needle bundle, where it causes spotting/”russetting” of needles. Excessive stunting of old needles may also result from its feeding. Damage from WPSM is often mistaken for air pollution, phytotoxicity, or seasonal needle drop when first observed. WPSM can be found in early spring but late summer-fall populations are also observed. Penn-Del IPM research group data and Longwood research show peaks around 275 GDD and again after 1000-2000 GDD (August to late September). Bloom of Acer saccharum (Sugar maple) may be a plant phenological indicator for WPSM.

Monitoring for rust mites is tricky. A hand lens is usually inadequate. Instead, beat sampling branches over a funnel and into a vial, filling the vial with water or rubbing alcohol afterward. View the vial contents in a dish with a 40x or higher dissecting scope (inexpensive scopes are fine). Rust mite eggs are slightly larger than the mites themselves and are often observed at the base of infected needles. Manage rust mites with sprays of 2% horticultural oil or a labeled product such as Avid (abamectin) or Forbid (spiromesifen). Many of the newer selective miticides have no affect on rust mites, so be sure to check the label prior to treatment. Naturally occurring biological controls such as predatory mites will eat rust mites, so critically evaluate the need for spraying before application.

Article by Casey Sclar, IPM Coordinator, Longwood Gardens in the UD Cooperative Extension Ornamentals Hotline, April 7, 2006.