Reproductive biology and overwintering success of the Western Bean Cutworm

Principal Investigator

Jeremy McNeil

Research Institution

The University of Western Ontario

External Funding Partners

Ontario Bean Growers (applicant – Growing Forward 2)

This project was funded in part through Growing Forward 2 (GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario.

Project Start

April 2015

Project End

October 2017

Objectives

Determine if both sexes of the Western bean cutworm (WBC) produce long distance sex pheromones.
Evaluate the effect of temperature/humidity on respiration of prepupae in the spring and fall.
Evaluate the effect of soil temperature/humidity on survival in field cages.

Impact

The enhancements to monitoring techniques which are better matched to the WBC biology and better reflect potential crop damage will allow corn farmers to manage crops more economically by using insect control products when threshold targets are reached.
The understanding of overwintering survival parameters will provide the basis for a model to predict the effect of abiotic conditions (such as climate) on the overwintering survival for different areas where the WBC is found.

Scientific Summary

Western bean cutworm (WBC; Striacosta albicosta) is an important agricultural pest of corn and edible dry beans. Historically the WBC has been limited to the western part of North America. However, in the last decade it has expanded its range and is now found in the Great Lakes region each summer. The sex pheromone had been identified and pheromone traps have been used as a monitoring tool. However, it was noted that trap catch data provide a very poor indicator of subsequent damage in either beans or corn. WBC injury has also been shown to increase incidence of fungal diseases in corn ears. Better management of the pest will not only reduce direct losses due to herbivory but reduce the potential for mycotoxins in the grain, particularly deoxynivalenol (DON).

This project investigated the gaps in understanding of reproductive and overwintering biology of WBC that should help explain the increased densities of this insect in Ontario. The objective of this study is twofold. First, to determine if both male and female moths produce a long distance sex pheromone. The captures from traps baited with randomly chosen males and females were not very high, but the preliminary results suggested that both males and females do capture conspecifics, although males captured very few females.

And second, to examine the overwintering biology/physiology of western bean cutworm prepupae. We established a major field plot with 90 cages (about 1m in depth), some with probes at different depths so that we can monitor seasonal changes in temperature. This information will provide a data base for future models predicting year to year changes in the population density of local populations. In 2015, we seeded individual cages with overwintering prepupae and determined temporal mortality curves and determine resource utilisation throughout the winter for an early (1 Sept 2015) and late (25 Sept 2015) cohort. We sampled three cages from the two cohorts (10 prepupae/cage) at the end of October, November and January. The early cohort burrowed deeper (20 cm) and by midwinter had about 20% survival. In contrast the later cohort pupated at a shallower depth (14 cm) but had a higher midwinter survival (48%). However, by April there were no survivors in either cohort. The soil temperatures never approached the super cooling point, so overwintering mortality was not due to freezing. We set out the same field plot and for 2016-2017, but had increased the number of cages. We excavated a subsample of cages in October, 2016 and noted several things. In 2016 prepupae only dug down to an average depth of 10 cm compared with 17cm in 2015. Also, while over 50% were still alive nearly 25% were already pupating. Less than 2% were alive in the spring of 2017 and the resulting adults were generally deformed. In early October 2017 we excavated the first samples from the 2017-2018 overwintering experiment less than 15% were still alive. In 2016 and 2017 this high mortality was associated with the very hot weather in September and early October and suggested that under such conditions infestations are due to immigrants and not local populations.

In light of the observed differences in burrowing depths between early and late cohorts, as well as between years we examined the burrowing of newly formed prepupae at 10, 20 and 30°C in sandy-loam and clay-loam soils. The higher the temperature the deeper larvae went (11, 18 and 21 cm respectively) before pupating in sand-loan soil, while there was no significant temperature effect in the clay-loam with all being found within 5-10 cm of the soil surface. Thus soil type could influence the long term overwintering conditions and significantly influence overwintering survival. In both soil types, regardless of the depth burrowed, the mass of individuals within the overwintering cells declined with increasing temperature. The field experiments of 2016 and 2017 suggest a somewhat more complex interaction between soil type and air temperature, and the researcher is planning additional laboratory experiments to examine this is greater detail.

The continued studies on prepupal mortality, together with our earlier results on cold hardiness, will provide a solid basis for the development of a model to predict the effect of abiotic conditions on the overwintering survival for different areas where the WBC is found. This will be integral to the development of a sound management strategy for this species, and allow for improved adherence to OMAFRA recommendations regarding action thresholds and use of integrated pest management strategies.

Project Related Publications