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BEETLE LIFE CYCLE European Elm Bark Beetle The European elm bark beetle overwinters in the larval stage under the bark of dead or dying elm wood. Pupation occurs In spring and adu~ beetles emerge in June. Cool weather may delay emergence. After emergence, the adults fly to nearby elm trees to feed in the crotches ofsmall branches. It is during feeding that fungus spores can be introduced into the large springwood (water -conducting) vessels of healthy trees (figure 4). lithe tree encountered is not an elm, the beeties do not feed but continue to fly until an elm is reached or until they die. In this way the beetles and the fungus are occasionally dispersed up to several miles. After feeding in healthy trees, the adult beetles seek suitable breeding sites under the bark of recently dead or dying elm trees or logs. Elm trees or logs dead for reasons other than Dutch elm disease can serve as suitable breeding sites. Adu~s burrow into the bark and excavate tunnels for egg laying in the soft inner bark and adjacent wood. The egg tunnels of the European species run paralleltothewoOd grain. Eggs hatch and larvae feed at right angles tothe egg-laying tunnel to produce a charaoteristic pattern (figure 5). Figure 5. The egg-taying galleries of the European bark beetles run parallel to the grain of the tree, with the larval feeding tunnels at right angles to the main gallery. Figure 4. Eurpoean elm bark beeUes feed inside the crotch of the branch, spreading the fungal spores of Dutch elm disease A second generation of adult beetles is pro- duced in July, August, or September, with the time of emergence depending on when the eggs where laid, the moisture content olthe wood, and weather. Trees with severe wilt after spring leafing are capable of producing second generation adu~s by July 15. Slow- wilting trees and trees infected later in the season produce beetles later in the summer. However, condi- tions within a tree vary, making it possible for more than one group of second generation adults to emerge from the same tree. After emergence, the second genera- tion adults, like their first generation counterparts, fly to healthy trees to feed. After feeding, the second generation adu~s seek elm material suitable for breed- ing to construct egg-laying tunnels. Some of the resulting larvae will become the overwintering popula- tion; others, because of favorable weather, produce a partial third generation of adult beetles in October (figure 6). Native Elm Bark Beetle Native elm bark beetles In Minnesota normally have one generation a year and overwinter as adults. However, they can overwinter as larvae (grubs) where beetle population levels are high and brood wood is abundant. Thus sanitation programs appear to force the beetle to winter as an adult Native adults overwinter in bark or bark crevices of elms, often wilhin the first six inches at the base of hea~hy elm trees. They become active in April as the weather warms and some may seek healthy elms for feeding, an important factor in early-season disease transmission. But some seek dead and dying elm 4