No Scent, No Sex For The Japanese Beetle

UC Davis chemical ecologists, led by Walter Leal, have isolated, identified, cloned and expressed a pheromone-degrading enzyme in the Japanese beetle (Popillia japonica) that could lead to important applications in controlling the invasive pest that has threatened U.S. agriculture since 1916. Damages in the larval and adult stages cost more than $450 million annually in the United States, according to the U.S. Department of Agriculture.

The research, aimed at exploring new frontiers in pest control and funded by the USDA’s National Research Initiative and the National Science Foundation, probes the male Japanese beetle's sophisticated sense of smell and how it distinguishes between two sex pheromones.  An insect detect smells on sensilla in its antennae.

In the Japanese beetle, two olfactory receptor neurons (ORNs) housed in the same structure of the antenna, the sensilla placodea, are highly sensitive, said Leal, a professor of entomology at UC Davis.   One detects the pheromone, R-japonilure, emitted by a female of the same species. The other, S-japonilure, tunes into a female of the closely related Osaka beetle. Both beetles are native to Japan and share the same habitat.

The S-japonilure serves as a behavioral antagonist or a stop signal."If the Japanese beetle smells the other species, it shuts down," Leal said. "It's like a stop sign, the pheromone being green and the behavioral antagonist, red."

Previous studies by the Leal group showed that the Japanese beetle uses an enzyme or protein in its antenna to inactivate pheromone by degrading the compound. Leal and his postdoctoral researcher Yuko Ishida isolated that enzyme, PjapPDE, from more than 100,000 antennae.

The UC Davis study shows how the enzyme, PjapPDE, interacts with the pheromones. "We've found that it degrades the pheromones more rapidly," Leal said. "Kinetic studies indicate that PjapPDE is

involved in the fast inactivation of the pheromone, R-japonilure, and slower degradation of the behavioral antagonist.”

Leal's goal is to find ways and means to slow down pheromone degradation by inactivating the enzyme, and “cause males to be unable to detect the pheromone and find females.”

What's unusual about Japanese beetle's pheromone: The chemical molecules are "mirror images" that can take either a left-handed or right-handed form, Leal said.  "If you look in the mirror, they mirror each other. The one on the right looks like the one on the left, the pheromone being a green signal, and the behavioral antagonist, a red signal."

Their findings could lead to better pest control methods, scientists agree.  First detected in the United States in 1916, the Japanese beetle was initially found in a nursery near Riverton, N.J.  It has now infested some 22 states east of the Mississippi River and is spreading west. Isolated infestations have popped up in California, Wisconsin and Oregon.

In its larval stages, the beetle is considered the most widespread turfgrass pest in the United States. The adult, about one-fourth-inch long with a shiny metallic green body and bronze-colored wings, feeds

on foliage and fruits of several hundred species of fruit trees, ornamental trees, shrubs, vines, and field and vegetable crops, including apples, plums, apricots, cherries, peaches, grapes, roses, soybeans, Japanese maples and crape myrtles. Adult damage commonly appears as skeletonized leaves and larval damage as dry spots on lawns, golf courses and in pastures.

Leal worries that if the pest gains a foothold in California, it could be the next Mediterranean fruit fly or light brown apple moth. State agricultural officials have found hundreds of hitchhiking Japanese beetles at airports over the past five years.  "Unfortunately, California would provide a favorable climate and abundant food supply for the Japanese beetle," Leal said.

Debby Tanouye, branch chief of Plant-Pest Detection/Emergency Projects, California Department of Food and Agriculture, said today that the Japanese beetle is not a big problem in California due to safeguards firmly in place: border station inspections, detection trapping and high hazard trapping and aircraft inspections. “In the last five years, the lowest number collected at high hazard locations and aircraft was 165, with a high of 927.”

“We have had three large eradication projects (Sacramento County 1961, San Diego County 1972 and Sacramento County 1983),” Tanouye said. “We have had three isolated treatment programs (Sacramento 2002, Long Beach, 2006 and San Diego 2007).”

“We have not detected any this year; however, the season is just starting,” she said. “The first inspection of aircraft and traps is scheduled for this week.”

CDFA inspects passenger and cargo aircraft from June through October for any hitchhiking Japanese beetles. 

Wendell Roelofs, a chemical ecologist at Cornell University, Ithaca, N.Y., and a member of the National Academy of Sciences, described the UC Davis research as cutting edge science and "amazing."

"A few years ago in Japan, Walter Leal began his one-man assault on identifying sex pheromones of agriculturally important insect species," Roelofs said. "He then moved to the cutting edge of research on all aspects of this communication system and has now, once again, become a pioneer in characterizing an enzyme that is involved in deactivating pheromone compounds that have already interacted with antennal pheromone receptors.  It is another step in understanding this amazing mating signaling system in insects."

Chemical ecologist Coby Schal of the University of North Carolina, Raleigh, said the PNAS paper presents "important and novel analyses of the specificity of the pheromone degradation process. This contribution is of general interest to researchers interested in olfaction, not only in insects, but in all animals."

"Although odorant signal termination is a critical process—especially in insects flying to a pheromone source, because the sensory system of the responder must be reset on a millisecond time-scale as the male navigates the complex intermittent pheromone "plume"—no other research group has carried out the thorough analysis conducted by Ishida and Leal," Schal said.

Schal called the PNAS paper "the most detailed analysis to date of the involvement of a pheromone-degrading enzyme in differential inactivation of the native pheromone and its chiral behavioral antagonist of the Japanese beetle."

UC Davis entomologist Bruce Hammock said the research is important for the pest management control. "Regulation of chemical mediators such as pheromones by degrading enzymes is increasingly seen in multiple biological systems," Hammock said.  The findings "may provide a biochemical basis for speciation in this interesting group of insects."

For their studies, Leal and Ishida obtained Japanese beetles from researchers in Nebraska and Ohio, where the insect is a serious pest. Ohio-based Mike Klein, USDA-ARS, and professor Fred Baxendale and graduate student Chelsey Wasen of the University of Nebraska, Lincoln, collected the males through pheromone-baited traps.

Said Klein: "The Japanese beetle feeds on over 400 plants, including grapes, citrus and most ornamentals.  Attempts to control the beetle if it becomes established in California would cost millions of dollars and disrupt biological control programs now in place for many other pests."

The work is supported by Leal’s five-year USDA grant, "The Molecular Basis of Pheromone Degradation in Japanese Beetle: Exploring New Frontiers in Pest Control," awarded him in 2003.

Leal will present a lecture on his work at the Aug. 24-29 meeting of the Brazilian Society of Entomology, Uberlandia.  A native of Brazil, Leal studied and did research in Japan for 16 years before accepting a faculty position at UC Davis in 2000. His postdoctoral researcher is a native of Japan. In Japan, the Japanese beetle is not a serious pest and in the United States due to natural enemies.

Known for his innovative research on how insects detect smells and communicate within their species, Leal last year received the International Society of Chemical Ecology's Silverstein-Simeone Lecture Award. Over the last two decades, Leal has identified and synthesized complex pheromones from many insects, including scarab beetles, true bugs, longhorn beetles and moths.