By Dick Arnold, Ann Ray, and Jocelyn Millar
Pheromones are widely used for detecting, monitoring, and controlling insect pests, but they’ve seen little use for detection of endangered species. However, volatile pheromones show tremendous promise for monitoring endangered and threatened insect species, because traps baited with pheromones can be highly selective and very sensitive, targeting only the specific species of interest, and attracting individuals even at low population densities.
An example involving the federally-listed, threatened valley elderberry longhorn beetle (Desmocerus californicus dimorphus) illustrates the promise of these pheromones. VELB adults are only active for a few weeks per year, so visual survey methods can be extremely time-consuming, costly, and of little value. Furthermore, females are essentially indistinguishable morphologically from females of another subspecies which is not threatened (Desmocerus californicus californius). Because of these difficulties, adult emergence holes have been used as a surrogate detection method to demonstrate the presence or absence of the beetle. However, emergence holes are problematic because beetles may emerge from roots below the soil line, the holes may be modified over time (by birds or ants, for example), and because holes made by other insects may be mistakenly attributed to VELB.
When the U.S. Fish and Wildlife Service (USFWS) proposed to delist the VELB in 2012, it was decided that a more sensitive and less labor-intensive survey method was needed to reliably detect the beetle. In the spring of 2013, we used a sex-attractant pheromone of D. c. californicus called (R)-desmolactone to attract 33 VELB males. During a subsequent dose-response study in 2014, an additional 63 males were attracted to traps baited with (R)-desmolactone.
While that may not sound like a lot, the total number of beetles that we trapped (all live beetles were released) exceeded the total number of known specimens when the subspecies was recognized as threatened by the USFWS in 1980 — a graphic demonstration of the power of using pheromone-baited traps as a detection tool.
Our results demonstrated the effectiveness of the pheromone (R)-desmolactone to detect males, and it also showed that this pheromone could probably be used to more reliably document the true geographic range of the threatened VELB based on identifying adult males instead of counting emergence holes. Furthermore, these findings have important implications for the preservation and restoration of VELB habitat and re-establishment of VELB populations because our traps are designed to exclusively capture live males, allowing females to remain in the population and to reproduce.
Conservation lands in California’s Central Valley include extensive areas of habitat that have been restored to benefit the VELB. However, because current survey methods are unreliable, USFWS guidelines do not require landowners and managers to demonstrate that VELB have successfully recolonized conservation lands. Instead, the beetle is assumed to be present in restored habitat.
All three of our study sites included restored habitat, but VELB adults were detected at only two of them, suggesting that apparently appropriate habitat does not necessarily correlate with or guarantee presence of the VELB. Our results suggest that traps baited with (R)-desmolactone can more reliably demonstrate recolonization of restored or protected habitats, or of conservation banks that offer VELB credits.
In the fall of 2014, the USFWS retracted its proposal to delist the VELB as a threatened species. In the future, the sex-attractant pheromone should become an extremely useful survey tool for detecting and monitoring populations, and for assessing the effectiveness of habitat restoration and other conservation measures to benefit the beetle. In particular, it will provide far more accurate information and will be more cost effective than the currently used visual survey methods.
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Dr. Dick Arnold is the founder of Entomological Consulting Services, Ltd in Pleasant Hill, CA. The company specializes in assisting government agencies and private industry in environmental, forestry, and technical matters that deal with rare and endangered insects and other types of invertebrates, such as crustaceans, arachnids, and snails. Dr. Ann Ray is an entomologist who studies chemical communication and mating behavior in longhorned wood-boring beetles (Cerambycidae). She is an assistant professor of biology at Xavier University (Cincinnati, OH). Dr. Jocelyn Millar is an organic chemist who specializes in isolation, identification, and synthesis of insect semiochemicals and applications of chemical ecology to integrated pest management. He is a distinguished professor of entomology at the University of California, Riverside.