Julie A. TILLMAN, Glenn L. HOLBROOK-2, David L. WOOD-1,
Coby SCHAL-2, Gary J. BLOMQUIST, and Steven J. SEYBOLD
Department of Biochemistry, MailStop 330, University of Nevada,
Reno, NV 89557-0014 USA
1-Division of Insect Biology, Department of Environmental Science,
Policy, and Management, University of California, Berkeley, CA 94720 USA
2-Department of Entomology, North Carolina State University,
Box 7613, Raleigh, NC 27695-7613 USA
The aggregation pheromone of western populations of the pine engraver beetle, Ips pini (Say) (Coleoptera: Scolytidae), is composed of an enantiomeric blend [>95%-(-)] of ipsdienol (2-methyl-6-methylene-2,7-octadien-4-ol). Under natural conditions in the field, feeding on host Pinus spp. phloem is generally regarded as a prerequisite for pheromone production and release by male Ips spp. However, previous studies in the laboratory have proved that ipsdienol can be produced by male I. pini and I. paraconfusus Lanier from vapor exposure to the host tree monoterpene myrcene. Subsequent interpretations of this result have led to the conclusion that myrcene, either inhaled or ingested during feeding on Pinus spp. phloem, may be the sole precursor to ipsdienol for male I. pini and I. paraconfusus. However, we have recently demonstrated that ipsdienol can also be produced de novo by male I. pini and I. paraconfusus.
We have used [1-14C]acetate radiotracer techniques with male I. pini to directly demonstrate that de novo production of the aggregation pheromone is stimulated by both host phloem-feeding and by topical juvenile hormone (JH) treatment. Phloem pre-fed and JH-treated beetles showed ~nine-fold and -ninety-fold increases, respectively, in the specific activity of ipsdienol relative to the untreated control. JH also dose-dependently stimulated the incorporation of (RS)-[244C]mevalonolactone into ipsdienol by male I. pini. This provides direct evidence for the use of the isoprenoid pathway in de novo biosynthesis, and, when combined with the results using [1-14C]acetate, provides evidence for one or more JH-regulated step(s) after mevalonate in the isoprenoid pathway. However, this evidence does not exclude the possibility of additional JH-regulated steps before mevalonate in the isoprenoid pathway.
The corpora allata are paired glands that are the site of JH biosynthesis in insects. In a current study, we are using the chemical allotectomizing agent precocene to confirm the regulatory role played by JH in de novo pheromone biosynthesis, and to elucidate the sequential relationship between phloem feeding, JH biosynthesis and/or release from the corpora allata, and ultimate de nova pheromone production. A second study compares in vitro JH synthesis from isolated corpora allata originating from unfed and pre-fed male and female insects. This work will also aid in the deterrnination of the temporal physiological relationship between phloem feeding and JH stimulation of de novo pheromone production.
Phloem feodhg on the new host tree by I. paraconfusus initiates a cascade of JH-coordinated physiological events that ultimately results in flight muscle degeneration and reproductive maturity under the bark of the host tree. Our results indicate that de novo pheromone production by male I. pini is another feeding stimulated, JH-coordhated step in the behavioral and/or physiological chain of events that ultimately lead to host colonization and reproduction critical to the survival and ecological success of Ips spp.