Agriculture Reference
In-Depth Information
Predators attacked parasitoids (thereby demonstrating intraguild predation), but
even when protected from predation, parasitoids were unable to provide economi-
cally significant levels of aphid control. In contrast, predators alone or in com-
bination with parasitoids were capable of suppressing aphids below economic
thresholds. Coccinellids were again identified as the key predators. These results
support theoretical predictions that key predators can provide strong herbivore sup-
pression even when they prey on species from other guilds within the natural enemy
community (Costamagna et al. 2008).
Modeling Population Growth
Modeling population dynamics can be a powerful tool for exploring scenarios that
may be difficult to investigate empirically. While simple models frequently suf-
fice, more complex species-specific models may be necessary to understand certain
phenomena. Costamagna et al. (2007b) used the results of predator exclusion cage
experiments at the KBS LTER and other sites to develop a series of models exploring
soybean aphid population growth. Using a simple model, Costamagna and Landis
(2006) estimated that in the absence of natural enemies the intrinsic rate of increase
for
A. glycines
was very high (
r
= 0.30-0.33), consistent with previous studies in
other portions of the aphid's exotic range (Indonesia). Subsequently, Costamagna
et al. (2007b) showed that
A. glycines
population growth could be more accurately
simulated by incorporating an intrinsic rate of increase that declines linearly with
time following soybean planting. They interpreted the decline in intrinsic growth
rate as a response to declining host quality (i.e., older soybean plants may become
less nutritious: a bottom-up control) that could interact with other mortality fac-
tors to play an important role in our understanding of overall aphid dynamics. For
example, generalist natural enemies that continually suppress colonies of aphids
may delay the growth of aphid colonies to a time when soybean growth is less
suitable for their reproduction (Rutledge and O'Neil 2006). In this way, the early
season impact of generalist predators becomes magnified by the later season impact
of declining host quality. Finally, Matis et al. (2009) extended the specific model
to more generally address the population dynamics of any organism specializing in
the exploitation of ephemeral resources.
Field-Level Response to Soybean Aphid
McKeown (2003) investigated the numerical response of the four dominant coc-
cinellids in the KBS landscape (
H. axyridis, C. septempunctata, C. maculata, and
C. munda
) to the presence of the soybean aphid and alternative prey at crop inter-
faces. Coccinellid predators were monitored for 18 weeks during the 2001 growing
season in a field near the MCSE where corn and soybean were planted in alternated
blocks (Fig. 8.7). Using an array of traps deployed within the two crops and at their
interfaces, they showed that each of the four species displayed a marked prefer-
ence for a particular habitat. Two species,
C. septempunctata
and
H. axyridis
, were
significantly more likely to be found in soybean than in corn. In contrast,
C. macu-
lata
, which is known to feed on corn pollen, exhibited an overwhelming preference
