Agriculture Reference
In-Depth Information
since “time immemorial” (Barnes 1956). The
Hessian fl y has been a well-known pest in Ukraine
since 1847 and in Morocco, Algeria, and Tunisia
since the early 1900s.
The Hessian fl y was fi rst reported in North
America in 1777 and in New Zealand in 1888 and,
in both cases, probably traveled via ships that
carried soil or infested wheat stems. In North
America, the Hessian fl y is now found in all major
wheat-growing regions, from east coast to west
coast and from Georgia to the northern limits of
wheat production in Canada. Populations have
not been found in the wheat-growing regions of
southern Asia, such as India, or in Southeast Asia,
such as China. In the southern hemisphere, New
Zealand is the only country reported to have the
Hessian fl y.
pathogens, the Hessian fl y larva does not elicit a
visually obvious symptom, for example, yellow or
white lesions or twisted leaf growth. This means
that the plant must be carefully dissected to fi nd
the larva in its concealed feeding location, either
between two leaf sheaths at the base of a juvenile
plant or between a leaf sheath and stem in an older
plant.
Natural enemies also make signifi cant contri-
butions to population dynamics of the Hessian
fl y, especially during the one to two generations
that develop during late summer and autumn
(Barnes 1956). A large number of parasitoids
and predators are known both in areas where
the Hessian fl y originated (present-day Syria)
and in areas where it was introduced. Predators
and parasitoids commonly attack Hessian fl y
eggs, which remain on leaf surfaces for 3 to 5
days. Parasitoids also are common but do not
kill their host until the third larval instar, when
the Hessian fl y larva's damage to the plant is
complete. Thus, future crops have a reduced
risk of attack, but the current crop probably
does not benefi t from the presence of the
parasitoid.
A single Hessian fl y larva is suffi cient for infl ict-
ing damage to the wheat plant (Berzonsky et al.,
2003). However, the extent and type of damage
depends on the growth stage of the plant. During
the seedling stage, the larva (the developmental
stage that feeds on the plant) feeds at the base of
the plant between the leaf sheaths. Impact is
unusually severe because the larva manipulates
resource allocation within the plant, with resources
normally allocated to plant growth redirected to
insect growth (Anderson and Harris 2006; Harris
et al., 2006). This attack is associated with death
of the seedling or, at the very least, death of the
main stem. Yield is negatively impacted, with
60%-100% fewer seeds per plant produced.
During stem elongation, the larva feeds on the
stem just above the fi rst or second node. This
weakens the stem and produces the most easily
recognized symptom of Hessian fl y attack, that is,
broken culms. In most cases, seeds are produced
but they are smaller in number and shriveled. In
this case, both yield and milling quality are nega-
tively impacted.
Biology, plant damage, and
control methods
Relative to many other insect pests, the popula-
tion dynamics of the Hessian fl y are poorly under-
stood for a number of reasons. The fi rst is that
the Hessian fl y lives on many other grasses besides
wheat, with many of these grasses occurring
outside of managed agricultural fi elds (Barnes
1956; Harris et al., 2003). Larvae have been found
on grass species in seventeen genera of the tribe
Triticeae and one genus of the tribe Bromeae.
Some of these grasses are crop or forage plants,
including barley (
Hordeum vulgare
L.), rye (
Secale
cereale
L.), triticale (×
Triticosecale rimpaui
Wittm.), and brome grass (
Bromus
spp.), while
many others are common wild grass species of
Agropyron
,
Elymus
,
Hordeum
, and
Aegilops
,
including the wild progenitors of domesticated
wheat such as
Aegilops tauschii
. It is not known
whether the Hessian fl y populations on wild
grasses contribute to periodic outbreaks in com-
mercial wheat fi elds.
The second reason for our poor understanding
of Hessian fl y population dynamics is that, unlike
many other insect and microbial pests of wheat,
the Hessian fl y is rarely part of annual surveys of
wheat damage. This is because it is very diffi cult
to quickly score a wheat plant for the presence of
a Hessian fl y larva. Unlike many aphids and
