eGarden © 2004 Ed Mues

Two Conifer Pests: One Long Here, One a 1992 Arrival

Our needled evergreens serve as landscape focal points, property line definers, Christmas tree candidates, as well as windbreaks, privacy screens, living fences, and as a valuable timber crop.

HERE FOR A LONG TIME: One particular pest, the white pine weevil, has been around since the turn of the twentieth century. It exists, as is often the case, wherever its favorite host, the eastern white pine tree, exists. It is a native pest, and when insect populations are high, or trees are weakened by drought, storm damage, excess moisture, or other factors, Norway spruce, Colorado blue spruce, red pine, jack pine, pitch pine, Scotch pine and even Douglas fir can be attacked. The white pine weevil (Pissodes strobi) prefers to attack trees in full sun and at least three feet in height. It is considered to be the most destructive insect pest of eastern white pines in North America.

The adult weevil is a diminutive creature, just barely 1/4 inch in length. Close examination would reveal the characteristic snout-like beak that is common to most weevils. The rust colored adult lives throughout the winter in the duff beneath infested trees. They have burrowed in for protection. Though difficult to see, on warm sunny days in early spring (March to mid-April) they crawl or fly to the central leader, the uppermost vertical shoot, also called excurrent growth habit, where they feed on terminal buds. From mid-April into May the females mate and begin laying eggs in the feeding injury sites. The eggs hatch in seven days or so and begin feeding on the inner bark in a downward movement. The mature larvae are nearly white, with a brown head and a slightly C-shape, nearly legless body, just slightly larger than the adult weevil.

The first evidence of attack is clear resin droplets around the central leader in early spring. This pitch flow is the indication that adults are feeding and preparing egg- laying spots for the coming month. Feeding damage is markedly identifiable by the withered, shepherd's crook appearance of the central leader by mid-June or a little later. If you look closely, you should be able to identify, remove, and destroy any infested tops you can reach before the larvae mature to adults. Try to get this done by the end of June. If you gently slice away the outer bark of the dead leaders, you should be able to see the small C-shaped weevil larvae in their wood chip cocoons. Left to their own, these larvae will pupate through July and into August and emerge as adults to enter the leaf litter at the base of the trees to overwinter.

The consequences of white pine weevil attack is mostly due to the larval feeding which kills the current years growth and shows up as wilting, drooping and dying of top growth. The result is a disfigured, crooked, limby and deformed tree, often with a forked top because two or more leaders replaced the central one killed off. The good news is that the tree is seldom killed. The consequences to the timber industry is not so severe except in the case of recurring annual damage. It is to the landscape, Christmas tree and other ornamental uses where the aesthetic value of specimens is lost, that the economic impact is most felt.

In manageable situations where the tree height is not an insurmountable problem to work on, the dead central leader can be cut out and removed. Select the strongest lateral branch from the whorl surrounding the dead leader and gently bend it to the vertical and splint it using tape or twist ties and a piece of strong dowel. It will become the new leader and you can fairly preserve the shape of your tree. I’ve successfully done this and it's hard to spot the point of attack and recovery, now.

FAR FROM HERE UNTIL 1992: The pine shoot beetle, from Europe and Asia, is the second most destructive insect pest of conifers in North America. This foreign invader has gotten lots of press in recent years because (a) it's an alien invader, and, (b) it's a killer. That is the real importance of this relative newcomer. We know about the attention paid to alien invaders from my previous columns. This guy, the common pine shoot beetle, Tomicus piniperda, was discovered in July 1992 on a tree plantation near Cleveland, Ohio. Between an eighth and three sixteenths of an inch in length, the pine shoot beetle most often attacks and kills Austrian and Scots pines.

There is a major quarantine involving this serious insect pest. Initially found in Illinois, Indiana, Michigan, New York, Ohio and Pennsylvania, it has already been found and verified in Maryland, Maine, New Hampshire, Virginia, Vermont, West Virginia, and Wisconsin. This is according to APHIS.

Its dispersal is aided and abetted by high winds, and, quarantines might only be a stopgap measure of limited success. It is a bark beetle! Herein lies some of the problem. The adult beetles spend the winters in lower stems and in already cut tree trunks. But, when still in an infested tree, and the tree succumbs quickly, it loses interest, and strikes again at another vulnerable candidate. It may launch many attacks in its lifetime.

It attacks lateral shoots of trees rather than the central leaders. The shoots become stunted, have slightly bent tips, the needles discolor. Close examination of shoots will reveal a hole ringed with resin drops and bits of frass (look like compressed wood chips) which the larvae eliminate as they feed. Prune out and destroy these shoots immediately. Look for the adult beetles and infested shoots in mid-June. If May is exceptionally warm and dry, start looking in May and early June. In cool, wet springs, symptoms my not appear until mid-July or later. Because the beetles breed in recently killed or dying pines, all affected should be destroyed by burning, chipping, or burying. As this pest continues to spread, important economic losses of pines will continue to mount.

Learning to tell the difference between these two can be very useful for homeowners, landscapers, nurserymen, and foresters. Once the villian is identified, measure can be taken to control it and its spread.

From The Garden of Ed. Submitted for publication in The Towne Crier on November 3, 2004

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