We review the postulated threatening processes that may have affected the decline in the eastern population of the monarch butterfly, Danaus plexippus L. (Lepidoptera: Nymphalidae), in North America. Although there are likely multiple contributing factors, such as climate and resource-related effects on breeding, migrating, and overwintering populations, the key landscape-level change appears to be associated with the widespread use of genetically modified herbicide resistant crops that have rapidly come to dominate the extensive core summer breeding range. We dismiss misinterpretations of the apparent lack of population change in summer adult count data as logically flawed. Glyphosate-tolerant soybean and maize have enabled the extensive use of this herbicide, generating widespread losses of milkweed (Asclepias spp.), the only host plants for monarch larvae. Modeling studies that simulate lifetime realized fecundity at a landscape scale, direct counts of milkweeds, and extensive citizen science data across the breeding range suggest that a herbicide-induced, landscape-level reduction in milkweed has precipitated the decline in monarchs. A recovery will likely require a monumental effort for the re-establishment of milkweed resources at a commensurate landscape scale.

Monarch butterfly (Danaus plexippus) populations are in decline in agricultural landscapes, in which genetically modified crops that are resistant to herbicides ('Roundup Ready') have resulted in the decimation of milkweed (Asclepias spp.) hosts over large areas due to the increased use of glyphosate. Movement is the key ecological process linking individual fitness traits to the utilization of sparse resources distributed across landscapes with emergent population level consequences. Often, movement ecology is highly simplified or even abstracted into a simple rate of flow between populations (i.e. a metapopulation) separated by a hostile 'matrix'. Whereas, we can gain important insights into the population dynamic as a whole if we explore movement as an explicit, complex, behavioural process in which the matrix is not simply a void. We developed a spatially explicit individual-based model to describe host-seeking behaviour over the lifetime of a monarch butterfly, which utilizes hosts both aggregated in patches and scattered across the wider landscape as a substrate for laying eggs. We examine the simulated movement distances and spatial population distribution (eggs laid) as a result of different movement rules (directionality), perceptive distance (ability to find) and landscape configuration (how milkweed is distributed). This indicates the potential consequences of cleaning up the matrix (i.e. the obliteration of non-crop vegetation with Roundup) and changing habitat configurations at a landscape scale on individual movement behaviours and the emergent number of eggs laid, essentially the birth term in any population model. Our model generates movement distances of the order of 12 km commensurate with summer breeding monarchs and suggests that milkweed removal has reduced egg laying by up to 30%. We suggest possible amelioration strategies.The matrix in each landscape either contains a low background density of hosts (matrix with hosts) or is devoid of hosts (empty matrix).

Plants of the Brassicaceae are defended from feeding by generalist insects by constitutively-expressed and herbivory-induced glucosinolates (GS). We induced Arabidopsis plants 1, 16 and 24 h prior to allowing neonate larvae of the generalist Helicoverpa armigera to feed on whole plants for 72 h. These plants were subsequently retested with another group of neonates for a further 72 h. We used wild-type A. thaliana Col-0, and mutant lines lacking indolic GS, aliphatic GS or all GS. We hypothesized that larvae would not grow well on defended plants (WT) compared to those lacking GS, and would not grow well if plants had been primed or fed on for longer, due to the expected induced GS. There was survivorship on all lines suggesting H. armigera is a suitable generalist for these experiments. Larvae performed less well on wild-type and no indolic lines than on no aliphatic and no GS lines. Larvae distributed feeding damage extensively in all lines, more so on wild type and no-indolic lines. Contrary to expectations, larvae grew better on plants that had been induced for 1 to 16 h than on un-induced plants suggesting they moved to and selected less toxic plant parts within a heterogeneously defended plant. Performance declined on all lines if plants had been induced for 24 h, or had been fed upon for a further 72 h. However, contrary to expectation, individual and total GS did not increase after these two treatments. This suggests that Arabidopsis plants induce additional (not GS) defenses after longer induction periods.

Cotton growing landscapes in Australia have been dominated by dual-toxin transgenic Bt varieties since 2004. The cotton crop has thus effectively become a sink for the main target pest, Helicoverpa armigera. Theory predicts that there should be strong selection on female moths to avoid laying on such plants. We assessed oviposition, collected from two cotton-growing regions, by female moths when given a choice of tobacco, cotton and cabbage. Earlier work in the 1980s and 1990s on populations from the same geographic locations indicated these hosts were on average ranked as high, mid and low preference plants, respectively, and that host rankings had a heritable component. In the present study, we found no change in the relative ranking of hosts by females, with most eggs being laid on tobacco, then cotton and least on cabbage. As in earlier work, some females laid most eggs on cotton and aspects of oviposition behaviour had a heritable component. Certainly, cotton is not avoided as a host, and the implications of these finding for managing resistance to Bt cotton are discussed.

Brassicaceae (Cruciferae) are ostensibly defended in part against generalist insect herbivores by toxic isothiocyanates formed when protoxic glucosinolates are hydrolysed. Based on an analysis of published host records, feeding on Brassicas is widespread by both specialist and generalists in the Lepidoptera. The polyphagous noctuid moth Helicoverpa armigera is recorded as a pest on some Brassicas and we attempted to improve performance by artificial selection to, in part, determine if this contributes to pest status. Assays on cabbage and kale versus an artificial diet showed no difference in larval growth rate, development times and pupal weights between the parental and the selected strain after 2, 21 and 29 rounds of selection, nor in behaviour assays after 50 generations. There were large differences between the two Brassicas: performance was better on kale than cabbage, although both were comparable to records for other crop hosts, on which the species is a major pest. We discuss what determines “pest” status.