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2004-08-25 WS & CC Meeting
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2004-08-25 WS & CC Meeting
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<br />BEETLE LIFE CYCLE <br /> <br />European Elm Bark Beetle <br /> <br />The European elm bark beetle overwinters in the larval <br />stage under the bark of dead or dying elm wood. Pupation occurs <br />In spring and adu~ beetles emerge in June. Cool weather may <br />delay emergence. After emergence, the adults fly to nearby elm <br />trees to feed in the crotches ofsmall branches. It is during feeding <br />that fungus spores can be introduced into the large springwood <br />(water -conducting) vessels of healthy trees (figure 4). lithe tree <br />encountered is not an elm, the beeties do not feed but continue <br />to fly until an elm is reached or until they die. In this way the <br />beetles and the fungus are occasionally dispersed up to several <br />miles. After feeding in healthy trees, the adult beetles seek <br />suitable breeding sites under the bark of recently dead or dying <br />elm trees or logs. Elm trees or logs dead for reasons other than <br />Dutch elm disease can serve as suitable breeding sites. Adu~s <br />burrow into the bark and excavate tunnels for egg laying in the soft <br />inner bark and adjacent wood. The egg tunnels of the European <br />species run paralleltothewoOd grain. Eggs hatch and larvae feed <br />at right angles tothe egg-laying tunnel to produce a charaoteristic <br />pattern (figure 5). <br /> <br /> <br />Figure 5. The egg-taying galleries of the European bark <br />beetles run parallel to the grain of the tree, with the larval <br />feeding tunnels at right angles to the main gallery. <br /> <br /> <br />Figure 4. Eurpoean elm bark beeUes <br />feed inside the crotch of the branch, <br />spreading the fungal spores of Dutch <br />elm disease <br /> <br />A second generation of adult beetles is pro- <br />duced in July, August, or September, with the time of <br />emergence depending on when the eggs where laid, <br />the moisture content olthe wood, and weather. Trees <br />with severe wilt after spring leafing are capable of <br />producing second generation adu~s by July 15. Slow- <br />wilting trees and trees infected later in the season <br />produce beetles later in the summer. However, condi- <br />tions within a tree vary, making it possible for more than <br />one group of second generation adults to emerge from <br />the same tree. After emergence, the second genera- <br />tion adults, like their first generation counterparts, fly to <br />healthy trees to feed. After feeding, the second <br />generation adu~s seek elm material suitable for breed- <br />ing to construct egg-laying tunnels. Some of the <br />resulting larvae will become the overwintering popula- <br />tion; others, because of favorable weather, produce a <br />partial third generation of adult beetles in October <br />(figure 6). <br /> <br />Native Elm Bark Beetle <br /> <br />Native elm bark beetles In Minnesota normally <br />have one generation a year and overwinter as adults. <br />However, they can overwinter as larvae (grubs) where <br />beetle population levels are high and brood wood is <br />abundant. Thus sanitation programs appear to force <br />the beetle to winter as an adult <br /> <br />Native adults overwinter in bark or bark crevices of elms, often wilhin the first six inches at the base <br />of hea~hy elm trees. They become active in April as the weather warms and some may seek healthy elms <br />for feeding, an important factor in early-season disease transmission. But some seek dead and dying elm <br /> <br />4 <br />
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