Insect declines have been reported widely and are expected to alter ecosystem functions and processes. Land-use change is recognised as a major cause of decline in insect biodiversity and abundance. Variation in local environmental drivers and the scale of available monitoring data have left large knowledge gaps in which taxa are declining and where declines are the greatest, and how these declines will impact ecosystems. We used 11 years (2006-2016) of monitoring data on 40 farms distributed over ~10,000 km2 in southern Québec, Canada, to quantify the impact of agricultural intensity on temporal trends in abundance and biomass of Diptera (true flies). There was a large difference in temporal trends between farms, which we found to be driven by agricultural landcover. Contrary to expectation, increases in abundance over time were greater in areas with higher agricultural intensity, especially with an increase in cereal crops. In contrast, declines in dipteran biomass were steeper in areas of higher agricultural intensity, although only with greater maize and soy production rather than cereals such as wheat. Variation in forest cover around farms had the least effect on trends. We found steeper declines in biomass per total number of Diptera with increasing agricultural cover, suggesting the presence of community turnover toward smaller-bodied flies with lower individual biomass. Our results reveal further complexities in insect trends driven by changes in land-use and show the importance of long-term monitoring and the use of multiple indicators for understanding biodiversity change.

Many control methods target agricultural pests’ reproductive traits and so, knowledge of these traits is crucial. The seedcorn maggot, Delia platura (Diptera: Anthomyiidae), is reported as a cosmopolitan polyphagous pest species which can be found in high numbers in numerous crops. Two morphologically identical genetic lines of D. platura (H- and N- lines) with distinct distribution ranges were recently discovered. While many biological traits have been described for D. platura, no study to date has been conducted on the life history strategies and reproductive behaviors of its two lines. Using laboratory-reared colonies originating from the Montérégie region in Québec, this project investigates the effect of group composition (sex-ratio and density) on the mating success and pre-oviposition period of the two D. platura lines. We found a substantial increase in mating success with increasing proportion of males within mating groups for both lines while group density had negligeable effects. These results corroborate reports of D. platura high-density swarms in which sex ratios are usually male-biased. The pre-oviposition period decreased as the ratio of males to female increased at low density only for the N-line while the opposite trend was observed at high density for both lines. These results suggest differences between the mating systems of these two lines, with the H-line females being choosier towards potential mates than those of the N-line. We also describe reproductive traits of both lines along with their implications for integrated pest management strategies such as the sterile insect technique or the release of individuals carrying pathogens, both of which must take into account the high degree of sexual selection present in such group contexts. Keywords: reproductive biology, mating dynamics, seedcorn maggot, swarms

Animals are expected to select a breeding habitat using cues that should reflect, directly or not, the fitness outcome of the different habitat options. However, human-induced environmental changes can alter the relationship between habitat characteristics and their fitness consequences, leading to a maladaptive habitat choice. The most severe case of such nonideal habitat selection is the ecological trap, which occurs when individuals prefer to settle in poor-quality habitats while better ones are available. Here we studied the adaptiveness of nest box selection in a tree swallow (Tachycineta bicolor) population breeding over a 10-year period in a network of 400 nest boxes distributed along a gradient of agricultural intensification in southern Québec, Canada. We first examined the effects of multiple environmental and social habitat characteristics on nest box preference to identify potential settlement cues. We then assessed the links between those cues and habitat quality as defined by the reproductive performance of individuals that settled early or late in nest boxes. We found that tree swallows preferred nesting in open habitats with high cover of perennial forage crops, high spring insect biomass, and high density of house sparrows, their main competitors for nest sites. They also preferred nesting where the density of breeders and their mean number of fledglings during the previous year were high. Additionally, we detected mismatches between preference and habitat quality for several environmental variables. The density of competitors and conspecific social information showed severe mismatches, as their relationships to preference and breeding success went in opposite direction under certain circumstances. Spring food availability and agricultural landscape context, while related to preferences, were not related to breeding success. Overall, our study emphasizes the complexity of habitat selection behavior and provides evidence that multiple mechanisms may potentially lead to an ecological trap in farmlands.