these models outperformed the previously published 1999 model, which overpredicted the spread on the northern, western and eastern fronts by 2001 (Fig. 8.3c). The results of these models suggest that landscape diver- sity affects the spread of rotation-resistant individuals primarily to the north and east, while wind is a limiting factor to dispersal on the western and southern fronts.
Results suggest that the conclusions based on a linear model using proportion of extra vegetation as the key parameter are likely to be robust.
Thus, we hypothesize that, as the landscape diversity represented by the proportion of non-maize and non-rotated soybean vegetation in a geo- graphical region increases, the rate of regional spread of the rotation- resistant WCR decreases over several years.
management had contributed to the evolution of this phenotype. We created a simple model of adult behaviour, population genetics and genetic expression of behaviour in a landscape with different levels of maize–soybean rotation (Onstad et al., 2001b). The landscape consists of four plant patches: maize grown every year (continuous maize fields), a maize patch that follows a non-maize patch in a rotation (first-year maize fields), and two non-maize patches, one preceding maize in a rotation (soybean) and the other never being rotated to maize (extra vegetation).
Normal monophagous individuals emerge from the natal maize patch and distribute themselves (and their eggs) across the two maize patches according to the relative area of each maize patch. Polyphagous (rotation- resistant) individuals distribute themselves across all patches according to the proportional area of each patch in the whole region. When expres- sion of polyphagy is additive, heterozygotes move to all crop types but are biased towards maize.
The original model describing the evolution of behavioural resistance (Onstad et al., 2001b) used a fitness cost for feeding in soybean. Recent data of Mabry (2002) demonstrate that there is no fitness cost. In an improved model (Onstad et al., 2003b), the simple fitness parameter was replaced with population dynamics from the model of rootworm adapta- tion to transgenic maize (Onstad et al. 2001a) described later in this chapter. Ignoring gender, fecundity was set to 220 eggs per female and overwintering mortality at the egg stage was set to 50% (Godfrey et al., 1995). We assume density-dependent mortality occurs as competition for feeding sites among neonate larvae and is applied following overwinter- ing mortality and any mortality due to toxins.
The rotation level, R, is the sum of the proportional areas of rotated maize and soybean, which are always equal in the model. The baseline simulation involves a 2-year rotation with the landscape defined as R = 0.85, extra vegetation is 0.05 and the proportion of continuous maize is 0.10. The initial frequency of the allele for polyphagy and resistance is 0.0001. Figure 8.4 shows how the resistance allele frequency changes over 20 years. The allele for rotation resistance reached 3% in years 13, 3 and 3 and 50% in years 14, 6 and 5 when the resistance allele is reces- sive, additive or dominant, respectively. A dominant allele for resistance permits the frequency of the resistance allele to increase the fastest, but, after several years, the resistance allele frequency is actually greatest when expression of rotation resistance is additive or recessive.
Figure 8.5 shows how the rotation level influences the resistance allele frequency after 15 years. As R increases and the proportion of extra vegetation decreases, the allele frequency for rotation resistance increases as expected. The allele frequency for rotation resistance declines as the proportion of continuous maize or extra vegetation increases. The model suggests that evolution of polyphagy and behavioural resistance may have resulted from selection on a single gene that could only develop where grower practices involved high levels of rotation (R > 80% of land- scape). In less diverse landscapes, crop rotation selects for the expansion
160 D.W. Onstad et al.
of host preferences and reduction in fidelity to maize by adults. Diverse landscapes may delay resistance to crop rotation depending on the fitness costs and the nature of the genetic system.
Onstad et al. (2003b) used the improved model to indicate that only three management scenarios prevent or delay the development of WCR resistance to crop rotation compared to a 2-year rotation scenario (Fig.
8.6). (We did not consider soil insecticide use in first-year maize.) One scenario maintains the 2-year rotation but replaces the current soybean varieties with cultivars that repel 90% or more of the WCR adults before egg laying occurs. The second is a 3-year rotation with regular soybean and wheat, which is unattractive for oviposition. In this scenario, the pro- portion of land planted to soybeans, wheat and rotated maize is 0.30 while the proportion of land planted to continuous maize is 0.10. Wheat that repels or never attracts 90% of WCR adults is always planted before maize. The third scenario involves transgenic maize planted in a 2-year rotation with soybean. Transgenic rotated maize kills 90% of WCR larvae hatching in rotated maize. We did not model the use of transgenic maize in continuous maize fields.
When the allele for rotation resistance is recessive, these strategies kept the resistance allele frequency fixed at 0.0001 (Fig. 8.6). The use of repellent soybeans was able to slow the evolution of resistance signifi- cantly when the expression of rotation resistance was additive, but not as well when the allele for resistance was dominant (Fig. 8.6). The use of
Heterogeneous Landscapes and Variable Behaviour 161
Fig. 8.4. Resistance allele frequencies over 20 years with a 2-year rotation (R = 0.85) and an initial resistance allele frequency of 0.0001 when the allele for rotation resistance is recessive (X> y), additive (x= y), or dominant (Y> x).
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
x= y
Y> x
X> y
Y allele frequency
0 2 4 6 8 10 12 14 16 18 20
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transgenic rotated maize or a 3-year rotation with unattractive wheat were the most effective management strategies for slowing the evolution of resistance. Each of these strategies kept the resistance allele frequency at low values (≤0.02) over 15 years with each type of gene expression (Fig.
8.6). Only the repellent soybean and transgenic maize were economically valuable as well and, of these two technologies, only transgenic maize is close to commercialization. This model did not consider resistance to transgenic maize during the 15-year time horizon.