Distinct colony boundaries and larval discrimination in polygyne red imported fire ants (Solenopsis invicta)

Genome-Wide Analysis of the Odorant Receptor Gene Family in Solenopsis invicta, Ooceraea biroi, and Monomorium pharaonis (Hymenoptera: Formicidae)

Olfactory systems in eusocial insects play a vital role in the discrimination of various chemical cues. Odorant receptors (ORs) are critical for odorant detection, and this family has undergone extensive expansion in ants. In this study, we re-annotated the OR genes from the most destructive invasive ant species Solenopsis invicta and 2 other Formicidae species, Ooceraea biroi and Monomorium pharaonis, with the aim of systematically comparing and analyzing the evolution and the functions of the ORs in ant species, identifying 356, 298, and 306 potential functional ORs, respectively. The evolutionary analysis of these ORs showed that ants had undergone chromosomal rearrangements and that tandem duplication may be the main contributor to the expansion of the OR gene family in S. invicta. Our further analysis revealed that 9-exon ORs had biased chromosome localization patterns in all three ant species and that a 9-exon OR cluster (SinvOR4–8) in S. invicta was under strong positive selection (Ka/Ks = 1.32). Moreover, we identified 5 S. invicta OR genes, namely SinvOR89, SinvOR102, SinvOR352, SinvOR327, and SinvOR135, with high sequence similarity (>70%) to the orthologs in O. biroi and M. pharaonis. An RT-PCR analysis was used to verify the antennal expression levels of these ORs, which showed caste-specific expression. The subsequent analysis of the antennal expression profiles of the ORs of the S. invicta workers from the polygyne and monogyne social forms indicated that SinvOR35 and SinvOR252 were expressed at much higher levels in the monogyne workers than in the polygyne workers and that SinvOR21 was expressed at higher levels in polygyne workers. Our study has contributed to the identification and analysis of the OR gene family in ants and expanded the understanding of the evolution and functions of the ORs in Formicidae species.

Assessing colony elimination in multicolonial ants: Estimating field efficacy of insecticidal baits against the invasive dark rover ant (Brachymyrmex patagonicus)

BACKGROUND

A frequent goal of pest management strategies targeting social insects is total colony elimination. Insecticidal baits are highly effective at controlling social insect pests, although their ability to provide total colony elimination has only been well studied in a few species. Genetically testing colony elimination in many urban pest ants can be challenging due to indistinct colony boundaries observed in unicolonial, invasive species; however, some pest ants, such as the dark rover ant (Brachymyrmex patagonicus), maintain strict colony borders through aggression towards non-nestmates. Each of these distinct colonies can be identified using molecular markers, allowing for the tracking of individual colonies pre- and post-treatment to measure colony density. While counting the number of foraging workers to assess treatment efficacy may suffice in some cases, it offers little insight into the colony-level impacts of a treatment.

RESULTS

Using microsatellite markers, distinct rover ant colonies were identified and tracked around residential structures before and after the application of an imidacloprid bait. The number of foraging ants at the treated structures was reduced by an average of 83.0% over a 28-day observation period. Baiting also significantly reduced the total number of colonies present. At the treatment structures, only ~25% of the original colonies remained at the end of the study. Colonies with foraging trails <1.5 m from a bait station had a higher chance of being eliminated.

CONCLUSION

Using insecticidal baits against B. patagonicus can be highly effective at colony elimination; however, with such small foraging ranges and high colony densities, proper placement is required to ensure enough bait is properly positioned to treat all colonies affecting a structure. © 2022 Society of Chemical Industry.