Agriculture Reference
In-Depth Information
for corn. Finally,
C. munda
was the only species that appeared to prefer the edge
habitats. Within soybean where
A. glycines
was particularly abundant in 2001, only
C. septempunctata
and
H. axyridis
responded numerically to the presence of the
aphid (McKeown 2003).
A Working Model of Pest Suppression
Through many of the above observations, KBS LTER researchers have developed
a working model of how soybean aphid suppression occurs. At the time soybean
aphids first arrive in soybean fields, resident predators such as anthocorids and
carabids—coupled with the feeding of more transient predators such as coccinel-
lid adults—result in the elimination of some incipient aphid colonies, and more
commonly the repeated suppression of those colonies that do establish. Sustained
predation pressure, in conjunction with declining host suitability later in the season,
can suppress aphid population growth. This effectively reduces food resources for
the subsequent generation of natural enemies and may also reduce the numbers or
fitness of aphids as they overwinter. Alternatively, if there are insufficient preda-
tors, or if aphid immigration overwhelms the predators' capacity to suppress their
growth, aphid colonies will grow to the point that they themselves begin to produce
alates and aphid abundance in the crop field may reach outbreak levels. Such aphid
outbreaks provide a nearly unlimited food source for subsequent natural enemy
generations and may increase natural enemy numbers (Fig. 8.8) and their overwin-
tering fitness (Heimpel et al. 2010).
A useful analogy is to consider the incipient aphid colonies in a field as “spot
fires” and generalist natural enemies as somewhat inefficient “firefighters.” The fire-
fighters continually find these spot fires and attempt to extinguish them. Sometimes
they succeed in completely eliminating a colony, but more frequently, a few aphids
are left behind. Under the right conditions, these “embers” may rekindle and allow
the colony to persist and grow. If colonies reach sufficient size that they themselves
begin to shed “sparks” (alate aphids), the field may soon become a “wildfire” (aphid
outbreak) that the predators are unable to control. Alternatively, with sufficient
numbers of predators, even if individually inefficient, a predator community may be
able to keep aphid numbers low for an extended period of time. This holding action
delays aphid population growth into the later season when conditions become less
favorable for population outbreaks to occur.
Landscape Effects on Soybean Aphid Suppression
The preceding analogy allows us to ask: What types of landscapes support a
sufficient community of “firefighters” to result in effective soybean aphid sup-
pression? Gardiner et al. (2009b) studied the impact of landscape structure on
aphid-suppression services in soybean. In particular, they examined the community
of mobile coccinellids that have repeatedly been shown vital to aphid suppression.
Their studies demonstrate that these predators are responsive to landscape structure
and that landscape diversity within 1.5 km of a soybean field is strongly related to
the level of soybean aphid suppression. Landscapes with high proportions of land
