A Promising New
Parasitoid Drills Down on
Emerald Ash Borers
By Melissa Mayer, Science Writer
Originally published on Entomology Today
The wasp flies through the forest, heavy with eggs,
following the scent of beetle infestation. She locates
a promising tree and lands, using sensory organs on
her legs to detect beetle larvae feeding below the
surface. She drills through the bark and deposits her
clutch. Within a few days, emerging wasp larvae will
feast on the soft body of the unlucky beetle larvae.
It reads like a horror story, but for researchers man-
aging the invasive emerald ash borer (EAB) beetle
(Agrilis planipennis), it's a biocontrol solution for
a problem plaguing ash forests in 35 states.
A report published in June in the Journal of
Economic Entomology offers data on the newest
parasitoid wasp released against the EAB beetle:
Spathius galinae. One of the authors, Dr. Jian
Duan of the Beneficial Insect Introduction Unit at
the U.S. Department of Agriculture's Agricultural
Research Service, says including this species in
the biocontrol lineup could be a game-changer.
"The addition of S. galinae to the current bio-
control arsenal will provide a whole spectrum of
protection for surviving ash trees," he says.
Beetle Invasion
The Asia-native EAB beetle hitched a ride on U.S.-
bound cargo sometime prior to its first detection in
the U.S. in 2002 and began feeding on the phloem
of local ash trees. The results have been devas-
tating; infested trees lose most of their canopy
within two years and die within three or four years.
Scientists decided to import the beetle's natural
enemies: parasitoid wasps. Between 2007 and
2012, they released three such species as bio-
control agents: Tetrastichus planipennisi, Spathius
agrili, and Oobius agrili. The results were mixed.
S. agrili's parasitism rate never reached 1 percent.
The other two species did better: O. agrili
clocked in at 1 to 32 percent, depending on the
release area, and T. planipannisi achieved 30 to
85 percent parasitism—but only in saplings.
Unfortunately, T. planipannisi's effective-
ness doesn't extend to larger trees. Its 1.5- to
2.5-millimeter ovipositor is simply too short to
reach larvae under bark thicker than 3.5 mm,
which is the case for more mature trees.
Enter Spathius galinae
Researchers then turned to S. galinae, which
boasts a substantial ovipositor, twice as long as
that of T. planipennisi. This enables the wasp to
reach larvae infesting much larger trees. >>
In this image, what appears to be an emerald ash
borer larva is in fact mostly Spathius galinae pupae.
The parasitoid larvae consumed all of the emerald
ash borer tissue except the head capsule before
forming their cocoons in the gallery. Photo by Jian
Duan, Ph.D., USDA-ARS
www.urban-forestry.com 31
Research Updates on Dutch Elm Disease,
Emerald Ash Borer, and Chestnut Blight
