Agriculture Reference
In-Depth Information
up to 40% if left unchecked (Ragsdale et al. 2007). In September, alates are pro-
duced that return to
Rhamnus
spp., where mating occurs and eggs are laid.
Initial studies at KBS LTER and elsewhere found that
A. glycines
was attacked
by a wide diversity of native and previously established predators (Fox et al. 2004,
2005; Rutledge et al. 2004) and parasitoids (Kaiser et al. 2007, Pike et al. 2007) with
the potential to suppress
A. glycines
population growth. This provided the opportu-
nity to ask several important questions:
(1) Is
A. glycines
primarily limited by top-down or bottom-up forces?
Top-down forces represent the influence of higher trophic levels such
as predation, whereas bottom-up forces represent the influence of lower
trophic levels such as plant vigor or defense mechanisms.
(2) How do predators and parasitoids interact in the
A. glycines
-soybean
system, and does intraguild predation (predation of potentially competing
predators and parasitoids) alter the outcomes of these enemy interactions?
(3) Is predation/parasitism sufficient to cause a trophic cascade, whereby
predators suppress herbivore prey, leading to increased crop yield?
(4) How does the occurrence of this new food source affect established
coccinellid communities?
(5) How does landscape structure interact with enemy communities to alter
A. glycines
population dynamics?
Studies addressing these key questions were conducted at KBS as well as in com-
mercial soybean fields in Michigan and throughout the U.S. North Central Region
and are discussed below.
Top-Down vs. Bottom-Up Effects
In a series of studies conducted in the MCSE, Costamagna and colleagues explored
the impact of crop management and natural enemies on soybean aphid population
dynamics (Costamagna and Landis 2006; Costamagna et al. 2007a, b). By contrast-
ing soybean aphid population growth in the Conventional, No-till, and Biologically
Based systems (Table 8.1), they were able to examine a full range of potential
bottom-up influences (fertility, soil moisture, induced host defences, etc.) that could
be generated under realistic soybean growing conditions. In addition, by exclud-
ing natural enemies from selected plots, Costamagna et al. contrasted the relative
importance of top-down and bottom-up forces for keeping aphid populations in
check. They found that predation reduced initial aphid establishment by ~30% in
24 hours and that, overall, top-down influences provided a 4- to 7-fold suppression
of aphid populations (Fig. 8.6).
In contrast, these investigators found no evidence for significant bottom-up
forces across the range of agricultural practices, that is, there were no agricultural
practices that differed in their abilities to check aphid populations in the absence
of predators. The natural enemy community at KBS is dominated by generalist
predators (lady beetles, anthocorid bugs, syrphid fly larvae) and generalist aphid
parasitoids (Braconidae). Coccinellids appear particularly important for controlling
aphids, and intraguild predation—where predators attack other predators who are
