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More bugs, with a nod to diseasesAugust 29, 2001

Juneberry sawfly

About 60 species of sawflies in the genus Nematus have been recorded as defoliators of broadleafed trees in eastern forests. Several have black heads, black legs, and longitudinal black stripes composed of closely spaced dots. One such species was observed defoliating Juneberry just south of Brainerd. Its body was yellow with six longitudinal black stripes, and there were black ventrolateral black bands and black dots on its ventral side. The dorsal side of its abdominal end was also black. It was about 1/2 inch long and one sawfly in the collection had spun a silken cocoon by August 20th. Most of these Nematus sawflies overwinter as prepupae in silken cocoons in the duff, then change into adult sawflies in June. After mating, the females deposit eggs in slits on the undersides of leaves. Willow, poplar, birch, hornbeam, locusts and alder are their major sources of food.

Striped alder sawfly on birch

Larvae of the striped alder sawfly, Hemichroa crocea, were reported feeding on paper birch near Grand Rapids and Tower in mid-August. This is another introduced species. The larvae is yellowish green has a shiny black head and a dark-brown or black subdorsal stripe on each side of the body running the length of the body. The larvae feed in groups and can consume the entire leaf except for the larger veins. Their primary host is alder but they will also become epidemic, feeding on birch and willow. There are two generations per year.

Wither the willows: Willow blight

Willow blight also called 'willow scab and black canker' has been reported in Carlton and Itasca Counties this summer.

Symptoms and disease development:

Infected leaves are killed rapidly and become blackish.

Leaves remain attached to the tree but eventually dry up and fall off.

The fungi move into the petiole and twig causing shoot dieback.

Black lesions and cankers develop on twigs, shoots and branches.

Shoots and leaves and entire trees take on a scorched or burnt appearance.

Damage early in the season is often more severe in the lower part of the crown.

The disease, willow blight, is actually caused by two fungi, Venturia saliciperda and Glomerella miyabeana, that occur together. These fungi were imported from Europe and first reported in eastern North America about 1927. The disease apparently spread through northeastern Minnesota in the early 1980's when large numbers of willows in windbreaks and yards were reached and killed.

Willow blight is most apparent in years with wet spring weather which favors spread of the disease. Control on shade and ornamental trees consists mainly of pruning out and destroying diseased twigs and branches during the dormant season to reduce inoculum and further spread of the disease. High levels of infection two or three years in a row may kill trees of any size.

Spruce with the Midas touch

Spruce needle rust

A golden cast to Colorado Blue spruce trees in northern Minnesota is due to the orange-colored spores of the spruce needle rusts (Chrysomyxa ledi and C. ledicola). The spores are exposed as the fungal blisters on current year's needles burst. In some areas, the majority of the susceptible Colorado Blue spruce are infected. Needle rust is also very common in northwestern counties on white spruce. In other areas miles away the trees may show no sign of infection. This is probably due in part to the lack of alternate hosts for the fungus. Indeed this is why this rust is usually seen only in the northern areas of the Minnesota. The alternate hosts are commonly Labrador tea and leather leaf, both native heaths and prominent members of sphagnum bog communities. Norway and Black Hills appear to be quite resistant, white spruce is somewhat resistant, but Colorado Blue and black spruce are very susceptible. Not only is the black spruce susceptible but grows in the same habitat as the alternate hosts.

The fungi of spruce needle rusts require an alternate host to complete their life cycles. Spores released from infected spruces during late summer infect the native heaths and overwinter on them. The following spring the heaths return the favor and release spores that infect young spruce needles.

The shedding of the infected needles from the spruce occurs after sporulation, August to September. The majority of current year's needles may be lost. And although this makes the tree unsightly, it will survive. Repeated attacks over consecutive years, which usually does not happen, will slow the growth of the tree but rarely kill it.

Spruce needle rust: closeup

Fungicidal treatment is prophylactic and ineffective, removal of alternative hosts is near impossible, so cultivation of resistant spruce species is encouraged. Luckily these infestations are not usually annual events and therefore control methods are rarely needed. The best thing you can do for your trees is to keep them well watered. Avoid using sprinklers though because they get the needles wet and lead to needle and twig disease problems, such as, needle rust. Keep the weeds mowed near the trees so the winds can dry the needles better and prevent infection. Mulching around yard trees keeps the soil moist and also keeps competing weeds and grasses away from the trees.

Spruce bud scale (the insect)

spruce bud scale (extreme closeup)

Spruce bud scales were observed in Clearwater County on windbreak spruce trees. These are really hard to discern since they look a lot like normal spruce buds. Heavily infected trees are weakened by their feeding, which causes needle loss. When artificial control is needed, insecticides have proven effective.

The scale develops through one generation each year and overwinters as immature individuals clustered around terminal buds, their feeding tubes inserted into the sap stream. They resume feeding in the spring and mature in early summer. At this time, the female scales are 3 mm in diameter. The eggs are formed under the scale. As they hatch, the young crawlers move out to the tips of the twigs, where they settle, insert their fine feeding tubes and feed until late fall.


Pine tortoise scale

In early August, one of the Forest Health Unit's specialists responded to a southern Crow Wing County homeowner's request for help in determining what had caused the death of three of his jack pine saplings and was causing dieback of another four jack pines. Flies, bees and wasps were rapidly flying around and crawling on the twigs of the four live jack pines, but they were not damaging the trees. On close examination, they were observed to be feeding on the "honeydew" secreted by countless numbers of pine tortoise scale insects (Toumeyella parvicornis). Only female scales excrete this sugar-rich honeydew as they suck plant sap. Black sooty mold also was utilizing the honeydew. Young white and red pines within six feet were not bothered by this insect, but the twigs of the infested jack pines were so covered with scale that no bark could be seen.

Tortoise scales are so named because the reddish brown coverings of the female scale resemble turtle shells (oval and strongly convex). Male scales are long, narrow, flattened and whitish in color. After feeding for about a month the immature males enter a brief pupal or resting stage, then emerge with wings in early August, fly about in search of females without ever feeding and die within a day or two. Mated females enter a state of hibernation with the onset of cool weather in the fall. After overwintering on the twigs, they resume feeding as about 500 eggs develop in their body cavities. In late June, male and female nymphs emerge and begin feeding by inserting their slender mouth parts through the bark of twigs and suck sap. Very soon their legs become functionless and they remain fixed at their feeding sites. Wind and birds landing on twigs disperse these insects to other trees. Infestations of pine tortoise scale are not usually of long duration, partly as a result of heavy predation by larvae of lady beetles and unfavorable weather. Acephate (Orthene) is one of several insecticides that provides satisfactory control.

Pine tortoise scale prefers jack pine, is found in nearly every eastern state, occurs as far west as the Dakotas and ranges north into southeastern Manitoba and westward into other prairie provinces. It is one of several species soft scales (family Coccidae) that are common pests of ornamental trees throughout most of the world. There about 90 species in the United States.

Red-headed Jack pine sawfly

Very light defoliation of Jack pines was observed in scattered Jack pine stands in southern Beltrami County in late July. If summers are long enough, there can be two generations per year. Feeding by the first generation takes place from early June to mid-July and the second generation is active between early August and mid-September. The first generation only consumes old foliage. In years with two generations, both current and old foliage is consumed. The larvae are strongly gregarious, feeding in large colonies of 125 individuals or more per branch. Full grown larvae have a greenish-white body with dark stripes and square black spots along the sides and, of course, red heads.