The biology and management of bark beetles in old growth pine forests of Itasca State Park

Executive summary part I: the bark beetle community
Pine trees at Itasca Park can be colonized by more than 10 species of bark beetles (Scolytidae), of which the most common are Ips pini, I. grandicollis, and I. perroti. These three species differ in their relative abundance and life history. Flying adults of I. grandicollis are most abundant in May and most of the population seems to have only one generation per year. In contrast, I. pini has 2 -3 generations per year at Itasca, and flying adults are most abundant in early September. I. perroti has 1 - 2 generations per year at Itasca and flying adults are most abundant in August. During 1998, across 24 trapping stations scattered through the old growth pine forests at Itasca, average cumulative captures for I. pini, I. grandicollis, and I. perroti were 142, 30, and 16 beetles • trap array -1 • year-1. By virtue of having multiple generations per year, I. pini has the greatest capacity for rapid increases in population size following blowdowns. Other things being equal, late summer populations of I. pini probably represent the greatest threat to pine trees at Itasca. However, the early season flight of I. grandicollis may be the greatest threat to trees that have been weakened by winter damage or spring fires. Both I. pini and I. grandicollis tend to increase in abundance when windstorms blow down suitable host trees. In 1997, the local abundance of I. pini was about three-fold higher within sites that sustained blowdowns, but was no higher than background levels at sites 200-300 m away from the blowdowns. In 1998, the abundance of I. grandicollis was elevated by about two-fold within blowdowns and at sites 200-300 m away from blowdowns. The restricted spatial scale of population responses to blowdowns indicates that beetle control programs or risk mitigation measures in response to blowdowns could be effective when conducted on a scale of hectares and need not be conducted across many square kilometers. One other practical implication is that forests outside the park suffer little risk of outbreaks due to beetle populations within the park, and vice versa.

Except that they both responded to blowdowns, there was little correlation between the local abundance of I. grandicollis and I. pini, suggesting that different factors influence their populations, possibly predators. The community includes several specialist predators that might act as natural controls on the abundance of Ips. In order of decreasing abundance, these include Thanasimus dubius (Coleoptera: Cleridae), Platysoma cylindrica, and P. parallelum (Coleoptera: Histeridae). All of these predators appear to be univoltine (one generation per year) with the flying adults being most common in May and June. The absence of predators later in the summer must contribute to increased population growth in I. pini.

The abundance of predators was positively correlated with the abundance of I. grandicollis (perhaps because I. grandicollis are the most abundant prey when T. dubius adults are feeding and reproducing in May) but negatively correlated with the abundance I. pini (perhaps because T. dubius predation limits the abundance of I. pini). Interspecific competition is another potential control on bark beetle abundance. The three Ips species rely on the same food resources, which are frequently limiting. If I. pini is the most significant forest pest at Itasca, I. grandicollis and I. perroti can be thought of as natural control agents of I. pini. Alternatively, if I. grandicollis is equally likely to attack live trees, then mass-trapping of I. pini may allow compensating increases in the abundance of I. grandicollis (because of relaxed competition from I. pini) and provide no net benefits in reducing tree mortality.

Three-trap arrays baited with three different combinations of commercially available pheromone blends provide an efficient means of quantifying the abundance of Ips and their predators at Itasca. In Chapter 3, we recommend a simple, inexpensive sampling program to monitor year-to-year trends in bark beetle populations at Itasca. This would provide a valuable tool for assessing current risks from bark beetles and adjusting the timing of forest management practices as appropriate. The logistics of this program would be especially easy because of the availability of traps and trained personnel at Itasca.



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