Forest entomology research

Naturally produced attractants of the small oak bark beetle, a potentially important vector of the oak wilt fungus in Minnesota

By: Angie Ambourn and Dr. Steven Seybold

Departments of Entomology and Forest Resources

University of Minnesota

Oak wilt is a vascular wilt disease that kills thousands of oaks annually across the United States. The disease is spread overland by insect vectors, which include sap beetles and oak bark beetles. The small oak bark beetle, Pseudopityophthorus minutissimus is one of these suspected of vectors. Although the vector status of the oak bark beetle has yet to be confirmed in Minnesota, in other states, these beetles are directly associated with declining oak trees, and in particular have been observed breeding, ovipositing, and feeding in twig crotches, leaf petioles, and small branches in oak-wilt killed trees. This suggests that oak bark beetles may spread the fungus to healthy trees and possibly initiate new infection centers in Minnesota. In order to better understand the biology of P. minutissimus we used naturally produced aggregation pheromones to determine the sex that produces the aggregation pheromone and the seasonal flight period. We also dissected newly colonized branches to determine the pioneer sex for this species.

Methods

Newly colonized northern red oak, Quercus rubra, branches were collected throughout the season from windfallen trees, in the Carlos Avery Wildlife Management Area, near Forest Lake in Anoka Co. Branches were brought back to the lab and all galleries dissected. Bark was pulled back to reveal the galleries, the adult beetles were removed, counted, and sexed. The percentages of each sex present in the galleries were determined.

Response to aggregation pheromones of P. minutissimus was measured using log baits of northern pin oak, Quercus ellipsoidallis, and red oak, Quercus rubra that contained live insects surrounded by sticky traps. The study was conducted at a residential property in Roseville in Ramsey Co. Each replicate contained branches that were approximately 13 cm in length x 3-5 cm in diameter. Each branch had 50 holes drilled in it and live P.minutissimus were inserted into the holes and covered with screening to prevent escape. Each of 12 replicates contained the following treatments:

  1. 50 male P. minutissimus
  2. 50 female P. mintussimus
  3. 50 female P. minutissimus placed in the holes first followed by 50 male P. minutissimus
  4. Control, which contained holes with no beetles

The log baits were placed on top of 122 cm wooden dowels, and the dowels were spaced in a line approximately 3.7 m apart. Each bait was randomly assigned to a position in the trap line and was re-randomized for every replicate in every trial. Cylindrical sticky traps were built to surround the log baits. Replicates were taken down approximately every 4 to 6 days and insects were removed from the sticky traps with a solvent. All catches were counted and the beetles were sexed. Responses were expressed as the number of beetles trapped per day.

Results

A total of 228 galleries were dissected from newly colonized branches (Table 1). One hundred and seventeen of those galleries (51.3%) contained single females whereas three galleries (1.3%) contained single males. However, 97 galleries (42.5%) contained both males and females. In these instances the galleries were already fully constructed suggesting that the dissection was made after the male had already joined the female in the galleries. Our finding that females initiate the galleries is contradictory to experiments in southern Ohio and Wisconsin that showed that male P. minutissimus make the initial attack and create the galleries.

A total of 2,040 P.minutissimuswere trapped in the pheromone attraction study, (973 males and 1067 females for an overall sex ratio of 1: 1.1). Peak flight of P. minutissimus occurred in late May 2002 (Figure 1). Fig 1 and Figure legends ( first half) There was secondary peak in late June, perhaps marking the flight of the re-emergent parents in 2002. A small number of beetles were also trapped in early September, suggesting that P.minutissimushas approximately 2 generations per year in Minnesota. This agrees with data from Wisconsin. Across all treatments, both male and female P. minutissimus were most attracted to log bait traps containing 50 females (Figure 2), suggesting that it is the female that creates the aggregation pheromone and thus colonizes the habitat first.

Future Directions

Chemical extracts have been prepared from female and male colonized branches to identify potential attractants. If a synthetic pheromone can be formulated, we will then test this pheromone in oak wilt centers and perform fungal isolations from dispersing beetles to better understand the biology and the role of P.minutissimus in the overland spread of C. fagacearum.

Table showing number of individual females, males or combinations of Pseudopityophthorus minutissimus recovered following dissections of newly colonized branches.
Table 1. Number of individual females, males or combinations of Pseudopityophthorus minutissimus recovered following dissections of newly colonized branches.

Response of Pseudopityophthorus minutissimus - total of all trap catches/replicate/day.
FIG 1. Seasonal flight response (mean trap catch/replicate/day) of Pseudopityophthorus minutissimus to all treatments (May 24 to September 9, 2002). Treatments were: 1) Control; 2.) Quercus ellipsoidalis or Quercus rubra branch artificially colonized with 50 males and 50 females (females were put it artificially drilled emergence holes first followed by males); 3) Q. ellipsoidalis or Q. rubra branch artificially colonized with 50 males, and 4.) Q. ellipsoidalis or Q. rubra branch artificially colonized with 50 females. In most cases only one block of treatments during a week were used except May 29- June 1 and June 6-June 9 when three blocks were placed out in a completely randomized block design. One these two dates, an average of trap catch was taken over the three replicates Maximum trap catches were 239 males (May 29-June 1, replicate 1) and 230 females (June 1, replicate 2).

Mean trap catch for male and female oak bark beetles.
FIG 2. Flight response (mean trap catch per replicate+ se) of male and female P. minutissimus to 1) Control ; 2) Quercus ellipsoidalis or Quercus rubra branch artificially colonized with 50 males and 50 females(females were put it artificially drilled emergence holes first followed by males); 3) Q. ellipsoidalis or Q. rubra branch artificially colonized with 50 males, and 4.) Q. ellipsoidalis or Q. rubra branch artificially colonized with 50 females. All treatments had 12 replicates over the course of the entire experiment except treatment 2 (11 replicates). The first trapping week (May 24 to May 28, 2002) consisted of treatments 1,2, and 3.