Reproductive traits, identity and host preferences of two genetic lines of the seedcorn maggot, Delia platura (Diptera : Anthomyiidae)

Future climate change scenarios predict an exacerbation of agricultural pest damage worldwide. Growers are stuck between a rock and a hard place namely the increasing capricious demands of consumers and pressure from pesticide companies to use their products. There has been increasing interest in developing safe alternatives to chemical pesticides which rely heavily on accurate pest species identification and a profound knowledge of the biological traits of such targeted species. One such alternative is the sterile insect technique which consists of releasing sterile individuals that mate with the targeted population but produce infertile eggs, effectively decreasing population levels over time. One obvious impediment to such technique is to release individuals that do not have the capacity to mate with the targeted population due to reproductive incompatibilities. Such situation may arise when dealing with misidentified cryptic species. If the released individuals look the same as the targeted population morphologically but are reproductively incompatible due to being unique biological entities, the technique is bound to fail. This technique has been employed with success for the onion maggot, Delia antiqua, and cabbage maggot, Delia radicum (Diptera: Anthomyiidae), in western Montérégie, Québec. There has been rising interest amongst growers to target another Delia species: the seedcorn maggot, Delia platura. However, two genetic lines (the H- and N- lines) have recently been discovered within D. platura. These lines differ in distribution patterns with the only region of overlap being in eastern Canada. Additionally, field data suggest differences in phenological patterns and relative proportions in different crops between the lines in western Montérégie. Using laboratory-reared colonies originating from the Montérégie region, the present project aims at better understanding the biological traits of the two D. platura lines along with evaluating their reproductive compatibility by comparing different reproductive isolation mechanisms. The initial phase of the project (Chapter 2) consisted of contrasting the reproductive traits of the H- and N-lines. As many dipteran species mate in swarms whose dynamics are highly influenced by group sex ratio and density, we evaluated the effect of these two variables on the mating success, pre-oviposition period and fecundity of the two D. platura lines. By experimentally varying a mating group’s sex ratio and density, we observed an increase in female mating probability as the ratio of males to females increased while group density had negligeable effects. N-line females had a higher mating probability than H-line females in all treatments suggesting that H-line females are choosier towards potential mates than N-line females. This high degree of choosiness may thus be a relevant pre-mating isolation mechanism between the lines. 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. The results corroborate field observations of D. platura mating in swarms (which are typically male-biased) and suggest difference in the mating system of the two lines. These differences may affect inter-line mating success along with control methods that target mating such as the sterile insect technique. Using a group composition that optimized mating probability for both lines as determined in Chapter 2 (30 males : 2 females), we conducted inter-line crosses in Chapter 3 to evaluate their reproductive compatibility. We uncovered asymmetrical pre-mating isolation between the lines with H-line females rarely mating with N-line males while N-line females readily mated with H-line males. However, the eggs laid in this type of cross hatched at a 25% lower rate than the inter-line crosses suggesting either post-mating pre-zygotic or post-zygotic isolation. The larvae that successfully hatched from this type of cross had a high degree of developmental success according to the proxies evaluated: survival to adult emergence from puparia, larval and pupal developmental time, pupal weight, and adult sex ratio. Thus, we uncovered no evidence of post-zygotic isolation in this type of cross from the larval hatching to adult emergence stages. Our results point to a partial reproductive isolation between the two lines, suggesting that they are in fact two unique biological entities which could in turn influence the development of species-specific control methods for either line. We thus propose the terminology ‘biotype’ to designate the H- and N- lines to emphasize their biological differences within the agronomic and scientific community. As our results suggest that the agronomic community may actually be dealing with two biological entities, in Chapter 4 we aimed to evaluate their respective host associations by quantifying their degree of acceptance and preference when subjected to yellow onion, sweet corn, soy, radish, broccoli and a soil control in no-choice and choice experiments. In a no-choice setting, the N-line had a higher degree of acceptance on sweet corn compared to the control. While the H-line had a higher degree of acceptance on yellow onion after 24 hours, more eggs were generally laid on each of the 5 crops after 48 hours compared to the control. When subjected to a choice between the substrates, the N-line laid more eggs on all crops except yellow onion than on the control whereas the H-line laid more eggs on each of the 5 crops tested. Although females did not distribute their eggs randomly among substrates, neither biotype showed a high degree of preference in the choice experiment. Additionally, the experimental setting of the choice experiment stimulated females of both biotypes to lay more eggs than in the no-choice experiment. Our results suggest differences in ovipositional behavior between the two biotypes, at least under laboratory conditions. This thesis has demonstrated that the H- and N- lines of D. platura are two unique biological entities that should be taken into account when developing control methods. Specifically, efficient use of the sterile insect release technique would consist of targeted releases in areas where swarms of either biotype are formed and releasing individuals that are of the same biotype as the targeted population. Considering that different stimuli appear to affect oviposition in the two biotypes, further investigation on the specific role of these stimuli will help predict the presence of the biotypes in fields and thus allow for more targeted releases. It is of my optimistic opinion that the upcoming years will prove to be highly fruitful in the development of environmentally safe control methods for both biotypes of Delia platura.

Name thesis:degree_name

M. Sc.

Level thesis:degree_level

Maîtrise

Discipline thesis:degree_discipline

Biologie

Grantor dc:publisher

Université de Sherbrooke

Year dc:date.issued

2022

Author dc:creator

• Bush-Beaupré, Allen

Advisors dc:contributor.advisor

• Savage, Jade
• Bélisle, Marc

• Agricultural entomology
• Species delimitation
• Reproductive behavior
• Entomologie agricole
• Délimitation des espèces
• Comportement reproductif

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/2.5/ca/

Language dc:language.iso

en

Handle dc:identifier.uri

http://hdl.handle.net/11143/19927

OAI identifier oai:identifier

oai:usherbrooke.scholaris.ca:11143/19927