Chemical and Engineering New
03-26-07
Why Ladybugs Smell Bad
Methoxypyrazines are responsible for the odor released by spooked bugs
Iowa State University researchers have now completed an exhaustive study of
the chemicals responsible for the characteristic smell of the bugs' noxious
emissions. The finding, which was presented yesterday at the American
Chemical Society national meeting in Chicago, could lead to new strategies to
eliminate the offensive compounds and improve wine quality.
Jacek A. Koziel, an agricultural engineer, took up the project as a change of
pace from his usual work analyzing livestock odor (C&EN, Sept. 25, 2006, page
104). "I wanted to work with something different, with a different odor profile,
something that many people could easily identify with," he told C&EN. "Since
there were hundreds of lady beetles in my old office with leaky windows, I chose
them. They released this characteristic odor when I handled them or when in my
frustration I tried to swat them."
He enlisted his postdoc Lingshuang Cai as well as Iowa State entomologist
Matthew E. O'Neal, a ladybug expert, to work on the project. Cai presented the
team's findings before the Division of Agrochemicals.
The team studied the multicolored Asian ladybird beetle, which invaded the U.S.
early last decade. Homeowners encounter the bugs because they aggregate
inside buildings for shelter during winter, Cai noted. Wine growers cross paths
with them when the bugs feast on damaged grapes in vineyards.
Because humans' odor detection threshold for the odiferous ladybug compounds
is so low, "even a tiny amount in the wine will cause a great sensory impact on
human tasting," Cai said. "That's why these compounds are so noxious."
The extent of the resulting wine taint problem is unclear, Koziel noted. But "there
have been reports of vineyards going bankrupt because entire vintages were lost
due to this problem."
Cai began the project by placing ladybugs in a capped glass vial for a day and
then collecting the volatile compounds they released. She used gas
chromatography and mass spectrometry to separate and identify individual
compounds in the mixture. Then, a panel of human "sniffers" told her which of
those compounds were the most important contributors to the ladybug scent. Of
the 38 compounds identified, Cai determined that 2,5-dimethyl-3methoxypyrazine (DMMP), 2-isopropyl-3-methoxypyrazine (IPMP), 2-sec-butyl-3methoxypyrazine, and 2-isobutyl-3-methoxypyrazine play a major role.
The overall smell is a mixture of nutlike, green bell pepper, potato, and moldy
odors. At the concentrations present in ladybug emissions, the mixture is "really
stinky," Cai said.
The Iowa State researchers are not the first to assess ladybug emissions. But
Cai noted that they were the first to discover that DMMP is an important
contributor to the odor. "This study also provided the first unambiguous evidence
that IPMP is responsible for the characteristic odor of live" ladybugs, Cai said.
Koziel added that "we also are the first to observe a correlation between the color
of the beetles and the amount of methoxypyrazine found in the air about them."
Orange beetles release more of the compounds than do yellow beetles.
The Iowa State team's process also represents a "very sensitive, accurate
method for identifying the presence of these compounds," Koziel noted. "This
method will help those studying ways to prevent methoxypyrazines from
contaminating wine." The group recently published details of its findings online in
the Journal of Chromatography A (DOI: 10.1016/j.ch