BUG OF THE MONTH
Poplar
Twiggall Fly (Hexomyza schineri)
Picture by Whitney Cranshaw Colorado State University.
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Picture by Joseph L. Sandoval
Adults are stout, shiny, dark flies about 0.!6 inch long. The developing gall fly is a greenish-yellow
maggot. This insect causes
galls
on the current season’s twigs. Obscured
by leaves, the original galls rarely are noticed until leaves fall in
autumn. However, the galled tissues
continue to grow and swell. Ultimately,
galls become large knots on trunks and larger branches, giving the plants a
gnarled, bonsai-like appearance. During
subsequent years, the galled area is incorporated into the growing twigs and
branches and ultimately may appear as large swollen bands on trunks and
branches. Although these old injuries
produce a permanent disfigurement, they do not seem to threaten tree
health. Galling is most common on
younger trees that produce a lot of succulent new growth.
Once a
tree has been attacked the galled tissues continue to grow and swell, and the
galled areas are incorporated into the growing twigs and branches and may
ultimately appear as large swollen bands on trunks and branches.
Poplar twiggall fly damage has been noted on Quaking Aspens in and around Santa Fe, since around 1994 according to Patrick Torrez, Santa Fe County Extension Agent.
Life Cycle: The poplar
twiggall fly overwinters within the gall as a full-grown, yellow-green maggot.
Picture by Whitney Cranshaw Colorado State University. Picture by Joseph L. Sandoval
Pupation occurs within the gall in late
winter or early spring. The majority of
the pupae then drop to the ground. At
the time that new growth forms, the adult flies emerge from the pupae and
become active. During the day, they
rest and sun themselves on leaves. After
mating, females move to developing twigs and insert eggs into the stems. The larvae hatch from these eggs and produce
the distinctive swelling in response to their feeding. Areas below buds appear to be particularly
flavored sites for galls. As the stems
continue to grow, the area where eggs were laid becomes increasingly
swollen. At first, the swelling
involves a fairly indistinct enlargement.
However, within two months, the full sized gall is usually present. The developing gall fly maggot grows slowly
within the gall all summer. It is
difficult to find until late summer and fall, when it grows rapidly, filling a
small cavity within the swollen area of the twig. Individual galls typically contain two to three larvae. There is one generation per year.
Pacific Northwest Insect Management
Handbook.
<http://weeds.ippc.orst.edu/pnw/insects?24LAN04.dat>
Picture by Whitney Cranshaw Colorado State University.
Picture by Joseph L. Sandoval
Picture by Joseph L. Sandoval
Picture by Joseph L. Sandoval