Multiple decrement life tables of Cephus cinctus Norton (Hymenoptera: Cephidae) across a set of barley cultivars: The importance of plant defense versus cannibalism

Authors: ACHHAMI, BUDDHI - Montana State University; PETERSON, ROBERT - Montana State University; SHERMAN, JAMIE - Montana State University; Reddy, Gadi V.P.; WEAVER, DAVID - Montana State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2020
Publication Date: 9/11/2020
Citation: Achhami, B.B., Peterson, R.K., Sherman, J.D., Reddy, G.V., Weaver, D.K. 2020. Multiple decrement life tables of Cephus cinctus Norton (Hymenoptera: Cephidae) across a set of barley cultivars: The importance of plant defense versus cannibalism. PLoS ONE. 15(9):e0238527. https://doi.org/10.1371/journal.pone.0238527.
DOI: https://doi.org/10.1371/journal.pone.0238527

Interpretive Summary: Wheat stem sawfly successfully colonizes ancestral wild grass hosts through domesticated spring and winter wheat cultivars. To mitigate economic loss, an integrated pest management approach has been recommended. Estimating causes of mortality by barley cultivars will provide a foundation to estimate cultivar-specific mortality and may allow for the development of cultivars that cause consistently greater levels of larval mortality. Overall the results suggest that cannibalism and plant defense were the two major causes of mortality for pre-diapause larvae across study sites. Further, irreplaceable mortality due to these two factors has a significant impact that reduces the size of pre-diapause larval populations of wheat stem sawfly, which drives the population growth the following year. Thus, capitalizing on cultivar traits that cause both a greater proportion of antibiotic neonate mortality and those that result in obligate cannibalism when attracting more females to oviposit are critical considerations to reduce economic losses caused by this species.

Technical Abstract: Accurately estimating cause-specific mortality for immature insect herbivores is usually difficult because feeding stages are exposed to abiotic and biotic mortality factors, causing cadavers to simply disappear before cause of mortality can be recorded. Also, herbivores are often highly mobile on hosts, making it difficult to follow patterns for individuals through time. In contrast, the wheat stem sawfly, Cephus cinctus Norton, spends its entire egg, larval, and pupal period inside a host stem. Therefore, with periodic sampling stage-specific causes of mortality can be ascertained. Consequently, we examined C. cinctus mortality in eight barley, Hordeum vulgare L., cultivars in two locations in Montana from 2016 to 2018 by collecting stem samples from stem elongation to crop maturity at weekly intervals, and collecting overwintered barley stubs the following spring and summer from the same plots. If larvae were present, we examined larval status—dead or alive—and categorized dead individuals into one of 5 mortality categories: plant defense, cannibalism, parasitism, pathogens, and unknown factors. We used multiple decrement life tables to estimate cause-specific mortality and irreplaceable mortality (the proportion of mortality from a given cause that cannot be replaced by other causes of mortality). Plant defense (antibiosis) caused 85.7 ± 3.6%, cannibalism (governed by antixenosis) caused 70.1 ± 7.6%, parasitism caused 13.8 ± 5.9%, unknown factors caused 38.5 ± 7.6%, and pathogens caused 14.7 ± 8.5% mortality in the presence of all causes of mortality. Similarly, irreplaceable mortality due to plant defense was 22.3 ± 6.4%, cannibalism was 29.1± 4.2%, unknown factors was 6.2 ± 1.8%, pathogens was 0.9 ± 0.5%, and parasitism was 1. 5 ± 0. 6%. Antibiosis traits primarily killed newly emerged larvae, while other traits supported more favorable oviposition decisions by females, increasing mortality by obligate cannibalism. Our results suggest that breeding barley for resistance to C. cinctus is a highly valuable tactic for management of this important pest.