Arboriculture & Urban Forestry 33(5): September 2007 345 SE) per meter squared [10 (±1.0 SE) per ft2] for white ash. McCullough and Siegert (2007) sampled 71 green and white ash trees killed by A. planipennis and reported that, on aver- age, roughly 89 to 105 A. planipennis could develop per meter squared (8 to 10 per ft2). At least six trees we surveyed had more than 50% dieback but less than 40 A. planipennis exits or woodpecker attacks per millimeter squared (4 per ft2). Although not all surveyed trees were debarked, a similar pattern was observed in other sites. If tree trunks become heavily infested before phloem in the canopy is exhausted, trees will succumb at lower A. pla- nipennis densities than might otherwise be expected (McCul- lough and Siegert 2007). We also observed a few trees with extensive dieback that had virtually no exit holes or wood- pecker attacks. Once these trees were debarked, it was ap- parent that nearly 100% of the phloem had been consumed, but virtually all larvae died as second or third instars because of intraspecific competition. The negative y-intercept derived for our green ash model likely reflected trees that exhibited some canopy dieback but few or no A. planipennis exit holes or woodpecker attacks. Smitley et al. (2006) reported a significant correlation (R2 0.59) between canopy dieback and density of A. pla- nipennis exit holes in a survey of ash trees growing near highway exits in southeast Michigan. They counted exit holes visible on the tree trunks but did not examine branches in the canopy, which are usually colonized by A. planipennis before the trunk becomes infested (Cappaert et al. 2005c). A scat- terplot of their data suggests that they observed a lower density of exit holes relative to canopy dieback than we observed, probably because they did not attempt to assess A. planipennis emergence in the canopy. For example, Smitley et al. (2006) recorded almost no exit holes in trees with 30% or less dieback. In contrast, we found up to 30 exit holes and woodpecker attacks per meter squared (3 per ft2) in trees with 30% or less dieback. Woodpeckers can play an important role in regulating populations of phloem-feeding or wood-boring beetle larvae (Anderson 1944; Nash et al. 1951; Barter 1957; Akers and Nielsen 1990; Fayt et al. 2005) and appear to be an important natural enemy of A. planipennis in North America (Cappaert et al. 2005b; Fuester and Schaefer 2006; Marshall et al. 2006; Strazanac 2006). Woodpeckers are known to feed on native buprestids such as A. anxius (bronze birch borer) with pre- dation rates reportedly ranging from very low to 50% of A. anxius larvae (Anderson 1944; Nash et al. 1951; Barter 1957; Akers and Nielsen 1990). Overall, woodpecker attacks ac- counted for an average of 35% of the A. planipennis in the trees we surveyed. Other native natural enemies of A. pla- nipennis in Michigan appear to be much less effective. Bauer et al. (2006) reported less than 2% of larvae were infected with pathogenic fungi, and less than 1% of larvae and 0.5% of eggs were killed by parasitoids. Even in its native range in China, less than 7% of A. planipennis larvae, on average, were killed by hymenopteran parasitoids (Liu et al. 2003). Woodpeckers were present at all of the sites we surveyed and attacks were noted on trees with relatively low, moderate, and high A. planipennis densities. At least three species of wood- peckers have been observed preying on A. planipennis in Michigan (Lindell et al. unpublished data), but little is known about their functional or numerical responses to this invasive pest. More research is warranted to address woodpecker be- havior and habitat use and effects of woodpecker predation on A. planipennis dynamics. The observational data reported here provide a framework for future studies that will be needed to identify mechanisms that underlie differential host preference by A. planipennis. There are clearly differences in host susceptibility or vulner- ability between North American ash species and Asian ash species, which have a long coevolutionary history with A. planipennis. In an experimental plot established in southeast Michigan in 2003, for example, European and North Ameri- can ash species sustained more A. planipennis attacks and higher mortality than Manchurian ash, a species native to Asia (Herms et al. 2005). Liu et al. (2003) reported that in the 1960s, when a plantation of North American white ash was established in China, an outbreak of A. planipennis erupted and all of the trees were killed. Within North America, numerous factors are likely to af- fect A. planipennis host selection and should not be consid- ered mutually exclusive. Several studies have shown that al- though A. planipennis is capable of attacking healthy trees in North America, it prefers trees stressed by girdling or other biotic or abiotic agents (McCullough et al. 2005, 2006; Po- land et al. 2006). This is consistent with observations made by Liu et al. (2003) who noted that in its native range, A. planipennis appears to function as a secondary pest, typi- cally attacking stressed trees. Agrilus species native to North America such as A. bilineatus and A. anxius exhibit an analo- gous pattern, similarly typically attacking only hosts that are severely stressed by drought, defoliation, disease or injury (Anderson et al. 1944; Ball and Simmons 1980; Cote and Allen 1980; Dunn et al. 1986; Muzika et al. 2000; Herms 2002). In addition, native Agrilus species and A. planipennis dem- onstrate an ovipositional preference for rough-barked portions of trees (Barter 1957; Loerch and Cameron 1984; Anulewicz 2006). Many varieties of green ash have rough, flaky bark even as young trees, whereas most varieties of white ash produce relatively smooth bark until they reach maturity. Blue ash trees, however, were less preferred or less suitable hosts than either green or white ash despite their rough textured bark. Host selection by A. planipennis is also likely affected by secondary compounds in ash leaves, bark, or phloem encountered by beetles as they feed on foliage or search for oviposition sites (Haack and Slansky 1986). Po- land et al. (2006) found that traps baited with a blend of ©2007 International Society of Arboriculture
September 2007
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
