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News from the UniversityJune 29, 2001

Entomology research

There are many different research projects currently underway in Dr. Steve Seybold's forest entomology laboratory at the University of Minnesota. The following are descriptions and updates of projects of three graduate students from the lab who are working in forests throughout the state; Kamal Gandhi works in the Gunflint trail area in Cook county, Darren Blackford works in white spruce plantations in many locations in the northern part of the state, and John Kyhl works in oak forests in and around the Twin Cities metropolitan area.

Responses of Sub-Boreal Beetles to a Severe Windstorm Event to Silvicultural Activities, and to Prescribed Fire in Northeastern Minnesota

by Kamal J. K. Gandhi

The July 4th 1999 severe windstorm in the Superior National Forest in northeastern Minnesota provides a unique opportunity to evaluate the interaction of forest insect populations with windstorm disturbances, and explore the short-term effects of post-wind-disturbance practices such as salvaging and prescribed burning on potential insect outbreaks and future tree damage and mortality. During the summers of 2000 and 2001, as a part of the multi-disciplinary team that includes personnel from the USFS North Central Research Station, Northeastern Area Forest Health Protection, Minnesota DNR, Superior National Forest Gunflint Ranger Station and University of Minnesota, we established 32 research plots stratified into two forest cover types (jack pine and aspen/birch). We compared beetle diversity, abundance and assemblage patterns between the undisturbed, severely wind disturbed (> 67% tree mortality), salvage-harvested forests, and burned forests. Within each research plot, we sampled litter-dwelling beetles using unbaited pitfall traps, and bark and wood-boring beetles with six species of trap-logs, Lindgren funnel traps baited with semiochemicals, and pitfall traps baited with pinenes and ethanol. We also established permanent vegetation plots to link changes in beetle assemblages with aspects of regeneration such as forest structure and composition.

Preliminary results indicate that bark beetles and wood-boring beetles do not necessarily increase their populations in one year-old, wind-disturbed forest stands. Instead, some common bark beetle species such as Ips pini and I. perturbatus were caught most often in the salvage-harvested stands. Similar trends were also observed for a number of wood-boring beetles such as cerambycid and buprestid species. Future studies will include an assessment of beetle colonization patterns of live, moribund, and dead trees within disturbed stands.


Spruce Budworm Project

by Darren C. Blackford

Minnesota has had a long history of defoliation by the eastern spruce budworm (SBW), Choristonuera fumiferana. In the early 1990's SBW began causing growth loss, top-kill, and mortality in increasing numbers of white spruce, Picea glauca, plantations in northern Minnesota. From 1998, SBW populations have progressively declined but continue to defoliate approximately 100,000 acres in Northern Minnesota. In cooperation with MN-DNR Forest Health personnel, we are conducting an extensive study of the population dynamics and impact of SBW in pure white spruce stands. We are measuring SBW population densities and defoliation levels in stands that have been divided into treated (thinned) and control (unthinned) plots. Another area of interest is the dispersal or movement of SBW in and between stands. It is thought that early instar larval dispersal influences SBW population distributions and mortality rates and that forest density influences early instar dispersal. A less dense white spruce stand might promote dispersal and increase mortality during dispersal relative to a more dense stand (i.e. one with higher basal area). We are testing this hypothesis by measuring dispersal loss of early (1st and 2nd) instars. We currently have 8 study plots located in Itasca, Beltrami, and Koochiching Counties.

2000 Season: Defoliation surveys were completed on 7 plots during the 2000 season. An average of 1,015 current year shoots was measured per plot for 7,109 shoots. These results indicate that defoliation levels are extremely low (Table 1). A total of 23 egg masses were found during the 2000 season throughout all plots, predicting a low level of defoliation for the 2001 season. Defoliation during the 2000 season in thinned stands was slightly lower than in unthinned stands (Table 2), however these data have not been analyzed statistically yet. As these results include stands that were thinned in fall of 1999, we expect there to be a more pronounced effect of thinning in the years to come.

Table 1. Level of defoliation on plots by location.

Plot name No defoliation Light 0-33% Moderate 34-66% Heavy 67-100%
Sam Welch's corner 67 29 4 1
Dora Lake 77 17 4 2
Johnson Landing 68 25 5 2
Powerline 97 3 0 0
Larson Lake 52 38 8 2
Smith Creek 46 37 10 7
Hill City 84 12 3 1
Mean defoliation, all plots 70 23 5 2

Table 2. Level of defoliation between thinned and unthinned plots

Plot type No defoliation Light 0-33% Moderate 34-66% Heavy 67-100%
Thinned 73 20 5 2
Unthinned 65 26 6 3

2001 Season: Preliminary data for the 2001 season indicate a slightly lower population density of SBW than during the 2000 season. However, one site (Smith Creek) had a similar population density in 2000 and 2001 whereas another site (Powerline) had a higher population density in 2001. Sticky traps were placed in all plots to measure SBW dispersal. In 5 of 8 plots we measured a 50 to 100% increase in dispersal loss in thinned stands vs. unthinned stands. These preliminary results support our hypothesis that a less dense stand might favor dispersal and hence dispersal-associated mortality. Pupal survey, egg mass survey, and a defoliation survey are yet to be completed for this 2001 season. Results from these surveys will be reported in an upcoming newsletter.


Life History of Principal Insect Vectors of the Oak Wilt Fungus

by John F. Kyhl

Surprisingly little is known about the life history of a group of beetles thought to spread oak wilt in Minnesota. The goal of this project is to research the life histories of some sap beetles (Coleoptera: Nitidulidae) implicated in the overland spread of oak wilt. We are using three different approaches to clarify the life history of nitidulid beetles implicated in the overland spread of oak wilt. They are 1) trapping and collecting beetles to identify overwintering sites and stages, 2) pheromone trapping to identify flight activity periods of various species, and 3) fungal mat removal to identify activity periods and determine population densities.

Trapping and Collecting: In the early spring, three species of nitidulids (Epuraea corticina, Colopterus

truncatus, and Glischrochilus quadrisignatus) emerged from the soil and duff collected from beneath oak wilt killed trees. Using emergence traps placed on the soil in the early spring, we found E. corticina adults emerging in May. In another series of experiments, C. truncatus and Prometopia sexmaculata were found overwintering in small numbers under the bark of oak wilt-killed trees. Collectively, these findings show that both habitats (soil and under bark) are acceptable overwintering locations for nitidulid beetle vectors of oak wilt. Given the low numbers of nitidulids beneath the bark, soil and leaf litter seems like a more common overwintering location at this point.

Pheromone trapping: Using the C. truncatus aggregation pheromone, we found that C. truncatus and other nitidulid species were first active in mid-April this year. Continued trapping showed that C. truncatus has a distinct flight activity period from mid-April to mid-June, which was very similar to patterns found last year (Figure 1). In additional pheromone experiments, we found that an alternative, less expensive pheromone formulation is equally attractive to C. truncatus, and that chipped oak tissue is not attractive to C. truncatus. Our work has demonstrated that an effective pheromone-based monitoring system can provide important information on the activity period of oak wilt vectors. Land managers, forest health professionals, and others can use this information to time pruning and other activities to avoid this period of high beetle activity.

Figure 1 - line chart

Figure 1. The seasonal flight period of C. truncatus in 2000 and 2001. In 2000, there was a peak in appearance in late April/early May, while in 2001 there is a large peak in mid/late April. Seasonal activity periods beyond July are not shown, since only one C. truncatus adult was caught in pheromone traps after this date.

Mat removal: Oak wilt fruiting structures (mats) were collected again this spring to further assess the life history of some nitidulid species. Mats have been collected during each spring and fall since late 1999. Previous work suggests that nitidulid species composition (of both adults and larvae) differs between spring and fall mats. Statistical analyses to clarify these differences are underway.