ISSUE 5 June 12, 2008
TENT CATERPILLARS IN NORTH DAKOTA
Three species of tent caterpillar occur in North Dakota: eastern tent caterpillar (Malacosoma americanum), forest tent caterpillar (M. disstria), and prairie tent caterpillar (M. californicum ssp. lutescens). Host plant damage by these moth species is caused by the larvae, or "caterpillars." Several hardwood hosts may be used, depending on the species.
Eastern tent caterpillar prefers chokecherry, though occasionally it will use other hardwood species. Larvae of eastern tent caterpillar are gregarious and construct tent-like nests of silk in the forks of branches and smaller trees (see photograph). Tents are used as shelter or as resting places.
Larvae forage during the day in new foliage on nearby branches. Larvae feed for 6 to 8 weeks and are about 2 inches long when mature. Larvae (see photograph) are black and rather hairy, with a whitish-yellow stripe down the middle of the back, narrow broken orange stripes just to either side of the whitish mid-stripe, and lateral white and blue markings.
Larvae disperse when mature and spin cocoons in sheltered places. Adult moths appear in late June and early July. Females lay eggs in a band-like cluster of 150 to 350 eggs around a small twig and cover the eggs with a frothy excretion called spumaline. Eggs overwinter and larvae emerge in the spring; thus, there is one generation per year. Larval feeding disfigures ornamental plants but usually does not result in permanent damage unless the feeding is severe. Tents and masses of larvae are unsightly. Eastern tent caterpillar populations usually peak every 10 years.
Forest tent caterpillar utilizes a wide variety of hosts, including ash, aspen, basswood, birch, cottonwood, elm, maple, and oak. Larvae emerge in the spring from overwintered eggs. Emergence coincides with the flush of host plant foliage. Larvae feed for 5 to 6 weeks and are about 2 inches long when mature. Larvae are identified by keyhole-shaped spots along the midline of the back and by broad bluish lateral bands. Unlike other tent caterpillars, forest tent caterpillar does not form a tent. Instead, larvae gather and spin silken mats on branches. Larvae tend to feed in wandering masses. Mature larvae form silken cocoons and adult moths emerge about 10 days later. Females deposit 150 to 200 eggs around small twigs and cover them with spumaline. Light defoliation has little effect on tree growth, but severe feeding can affect growth and cause twig mortality. In North Dakota, outbreaks of forest tent caterpillar typically last for 2 to 4 years.
Prairie tent caterpillar can utilize a variety of hardwood host, though chokecherry is its preferred host. Prairie tent caterpillar is the most common tent caterpillar species in North Dakota. Prairie tent caterpillar overwinters in the egg stage and larvae emerge in the spring with the flush of their host plant foliage. Larvae feed for 6 to 8 weeks and are about 2 inches long when mature. Larvae are black with a white mid-line stripe broken into dashes and light blue lateral stripes also broken into dashes. Like eastern tent caterpillar, larvae of prairie tent caterpillar form silken tents in the forks of branches and small trees and feed on nearby foliage. Mature larvae spin cocoons in curled leaves or in leaf litter. Adult moths emerge in mid-summer. Females lay eggs near the base of the host plant in the ground. Damage by larvae is similar to that of eastern tent caterpillar.
Control of all tent caterpillar species should target larvae. Actively feeding larvae are easily controlled with conventional foliar insecticides including acephate, carbaryl, imidacloprid, or any of several pyrethroids. Biorational treatments include Bacillus thuringiensis (Bt), insecticidal soap, and pyrethrin. Boiling water can also be poured directly on tents that contain larvae. Tents also may be physically removed and destroyed.
Extension Entomolgy Research Specialist
FOLIAR DISEASES OF TREES
With the extended cool, wet period experienced in Fargo and other areas around the state, foliar diseases of trees are expected to appear. Common foliar diseases that are favored by cool, wet weather include Rhizosphaera needle cast and Stigmina needle cast on spruce, anthracnose diseases on several deciduous hosts (such as ash, oak), and others.
On spruce, needle cast diseases cause symptoms that include premature loss of inner needles (never the current season’s growth, and typically not the last two year’s of growth, except in severe cases). Needle cast diseases on spruce must be diagnosed accurately, since other non-disease (abiotic) factors can cause similar symptoms. Without an accurate diagnosis, a management strategy that is ineffective might be recommended. Below is a standardized report compiled by the NDSU Plant Diagnostic Lab that you might receive if you submit a spruce that is diagnosed with Rhizosphaera needle cast disease:
"The spruce that you submitted has been evaluated. This tree is infected with Rhizosphaera needle cast of spruce, caused by the fungus Rhizosphaera kalkhoffii. This disease can lead to extensive loss of interior needles, with 1-3 years worth of needles only present at the ends of the branches. Affected trees are typically closely planted in windbreaks or in multiple plantings in yards. Early symptoms of the disease are yellowing, brown, red-brown, or purple-brown interior needles. Fungal fruiting bodies on infected needles can be seen with a magnifying glass or hand lens. The fruiting structures are black and grow through the normally white, circular stomates in rows on the needles. New, current year needles become infected in June and July and typically discolor the following spring. These fruiting bodies resemble another fungus (Stigmina lautii) that reportedly causes another needle cast on spruce, so differentiating between these two diseases is important since management strategies may differ. On this particular sample, only fungal bodies of Rhizosphaera kalkhoffii were detected.
Rhizosphaera needle cast usually develops in protected areas of the tree where the humidity level is higher than in exposed trees or parts of trees. If not controlled, the disease may spread up the entire tree over a period of years, as long the environment continues to be conducive for disease development.
Control of Rhizosphaera needle cast with a fungicide may be warranted if the infection is severe and if the tree is a highly valued specimen. Effectiveness of protective fungicides depends on two properly timed applications per year for two consecutive years. The first application should occur when the new shoot growth is 1/2 to 2 inches in length or when needles are 50% their normal length, followed by a second application three to four weeks later or soon after needles are fully elongated. The application of fungicide must be repeated for two consecutive years to be effective. If not, no control will occur.
Research conducted by James Walla (NDSU forest pathologist) indicates that the fungicide chlorothalonil (sold as Daconil 2787® or Bravo®) provides good control for this disease. Properly timed application of the fungicide, as described above, with sufficient coverage over a two year period can result in eradication of the fungus that is present on the tree."
NDSU Plant Diagnostician