Assessing chemical control options and their effects on lesser clover leaf weevil, red clover pollinators and evaluating red clover seeding rates for optimal seed production and N fixation in Saskatchewan, Canada

Abstract

Red clover (Trifolium pratense L.) seed production in Saskatchewan is complicated by a series of agronomic and pest management issues. These include damage from the lesser clover leaf weevil (Hypera nigrirostris F.; LCLW), insecticide toxicity to bee community and unidentified optimal red clover seeding rate and biological N fixation. The research presented in this thesis had two objectives. First, to evaluate the effectiveness of the insecticides deltamethrin and cyantraniliprole in controlling LCLW in red clover in both field and laboratory-based evaluations, as well as to assess potential impacts on the bee community. The second objective was to determine the optimal red clover seeding rate for both seed production and N fixation. Examined under the field conditions, both insecticides were effective at controlling LCLW. However, deltamethrin provided rapid pest suppression in 24 hours after application. Rearing LCLW larvae on treated plants under laboratory conditions showed similar results where both insecticidal materials significantly reduced LCLW numbers in 10-12 days after treatment. The abundances of LCLW were always negatively related to seed yield regardless of treatment and initial weevil pressure. Seed yield was significantly lower in untreated controls whereas no difference was found between deltamethrin and cyantraniliprole application. In contrast, no yield response was found when LCLW abundance was lower than four larvae per 10 shoots. Unexpectedly, the bee community was not affected by either treatment and was not associated with seed yield. In this study, red clover was able to compensate for the range of seeding rates between 2.5 and 10.5 kg ha-1 without seed yield and N fixation losses, whereas only a 4.5 kg ha-1 rate had higher biomass production and N fixation value than a 0.5 kg ha-1 rate under adequate precipitation conditions. In contrast, no seeding rate effects were found in a site with limited water availability. Regardless of site, biomass was always positively correlated with N fixation; however, no association was found between seed yield and N fixation. Collectively, this thesis reveals that for single cut red clover seed production in Saskatchewan, a seeding rate of 4.5 kg ha-1 is optimal as it generated greater seed yield and N fixation than 0.5 kg ha-1, whereas higher seeding rates had no explicit benefits. It also demonstrated that LCLW management is likely more important to seed production than the risk of harm to bee the community. Finally, while not fully quantified, results in this thesis indicate that drought and water stress can substantially influence red clover seed yield.