Lethal infection thresholds of Paenibacillus larvae for honeybee drone and worker larvae (Apis mellifera)

Area of Science:

• Veterinary Entomology
• Apiculture Science
• Microbial Pathogenesis

Background:

• American Foulbrood (AFB) is a devastating brood disease in honeybees (Apis mellifera) caused by Paenibacillus larvae.
• Understanding differential susceptibility between honeybee castes is crucial for disease management strategies.
• Previous research has not fully elucidated sex-specific responses to AFB infection in larvae.

Purpose of the Study:

• To compare the mortality rates and bacterial loads between honeybee drone and worker larvae infected with Paenibacillus larvae under controlled conditions.
• To investigate the role of larval size in determining lethal thresholds for American Foulbrood.
• To assess the quantitative differences in Paenibacillus larvae infection dynamics between sexes.

Main Methods:

• Controlled in vitro infection of honeybee drone and worker larvae with Paenibacillus larvae.
• Monitoring of mortality onset and cumulative mortality over 8 days post-infection.
• Quantification of Paenibacillus larvae cell numbers and lethal titres in deceased larvae using quantitative real-time PCR (qPCR).

Main Results:

• Drone larvae exhibited a 1-day delay in mortality onset and 10% lower cumulative mortality compared to worker larvae.
• Higher Paenibacillus larvae cell numbers were observed in drone larvae than in worker larvae.
• Differences in bacterial titres between sexes were correlated with larval body size, suggesting size as a key factor for AFB tolerance.

Conclusions:

• Larval size, rather than sex, appears to be a primary determinant of lethal thresholds for American Foulbrood.
• Drone larvae demonstrate a resistance phenotype similar to worker larvae, with variations explained by body size differences.
• Findings suggest that larval size plays a critical role in honeybee tolerance to Paenibacillus larvae infection.