•Synthetic alarm pheromone triggered significant EAG and behavior responses.•Twelve alkoxypyrazines generally elicited a dose-dependent EAG response.•The alarm responses triggered by 12 alkoxypyrazines were not significantly different at all test doses.•Food containing pyrazines attracted more fire ant workers in the first 15-min period than hexane control.The red imported fire ant, Solenopsis invicta produces an alarm pheromone component, 2-ethyl-3,6-dimethylpyrazine, and responds to its pyrazine analogs in a similar manner but at varied detection thresholds. Herein, the responses of fire ant workers by electroantennogram (EAG) and behavior were tested with twelve structurally-related oxygen-containing pyrazines (alkoxypyrazines) and the synthetic alarm pheromone. All tested compounds elicited a dose-dependent EAG response, with S. invicta responding greatest to the synthetic alarm pheromone. Chemical structure of pyrazines influenced the EAG response but not always alarm behavioral response. Among the 13 tested compounds, 7 compounds displayed significantly greater EAG response than 2-isopropyl-3-methoxypyrazine and 2-ethoxy-3-isopropylpyrazine at the dose of 1000 μg. Four of these 7 compounds, 2-ethyl-3,6-dimethylpyrazine, 2-methoxy-3-methylpyrazine, 2-ethoxy-3(5 or 6)-methylpyrazine, and 2-chloro-3-methoxypyrazine with characteristic substituents on pyrazine ring were further subjected to bait discovery bioassay. Hotdog bait containing pyrazines attracted significantly more fire ant workers in the first 15-min period, resulting in quicker recruitment to food block than hexane control. The potential of using alkoxypyrazines in fire ant control is discussed.Download high-res image (262KB)Download full-size image

A characteristic behavior in ants is to move rapidly to emission sources of alarm pheromones. The addition of ant alarm pheromones to bait is expected to enhance its attractiveness. To search for candidate compounds for bait enhancement in fire ant control, 13 related alkylpyrazine analogues in addition to synthetic alarm pheromone component were evaluated for electroantennogram (EAG) and behavioral activities in Solenopsis invicta. Most compounds elicited dose-dependent EAG and behavioral responses. There exists a correlation between the EAG and behavioral responses. Among the 14 tested alkylpyrazines, three compounds, 2-ethyl-3,6(5)-dimethyl pyrazine (1), 2,3,5-trimethylpyrazine (7), and 2,3-diethyl-5-methylpyrazine (12), elicited significant alarm responses at a dose range of 0.1–1000 ng. Further bait discovery bioassay with the three most active alkylpyrazines demonstrated that food bait accompanied by sample-treated filter paper disk attracted significantly more fire ant workers in the first 15 min period. EAG and behavioral bioassays with pure pheromone isomers accumulated by semi-preparative high-performance liquid chromatography demonstrated that 2-ethyl-3,6-dimethylpyrazine was significantly more active than 2-ethyl-3,5-dimethylpyrazine.

A method based on silica gel chromatography and GC-MS/GC-FID analyses was developed for the quantitation of alkaloidal compounds in imported fire ants, Solenopsis richteri, S. invicta, and their hybrid found in the southern United States. The cis and trans alkaloids from fire ant body extracts were successfully separated by silica gel chromatography, identified by GC-MS, and quantitated by GC-FID. Piperideine compounds were eluted together with the cis and trans piperidines, but were well-resolved on a nonpolar GC column. Eight pairs of piperidine isomers and 12 piperideines were quantitated. The ratios of trans alkaloids to corresponding cis isomers ranged from 87 to 378:1 in S. invicta and were significantly higher than in S. richteri and hybrid ants. The results were discussed in relation to the evolution of fire ant venom alkaloids and their role as host location cues for parasitic Pseudacteon phorid flies.

The alkaloid chemistry of the venom of hybrid fire ant, Solenopsis richteri × Solenopsis invicta, was investigated using silica gel chromatography and GC−MS techniques. In addition to most cis alkaloids of parental species, S. richteri Forel and S. invicta Buren, the cis alkaloid fraction of the body extract of hybrid fire ants also contains five significant new alkaloids. Hydrogenation of the cis alkaloid fraction yielded only five piperidines, 4′, 12′, 12, 20′, and 20. Sodium borohydride and lithium aluminum hydride selectively reduced C═N double bond in piperideine alkaloids to give a mixture of cis and trans piperidines. However, reduction of the five new components yielded several new peaks with much longer retention times and increasing molecular weights over 30. It is evident that the chemical identities of the five new peaks are quite different from those known piperidines or piperideines found in Solenopsis fire ants.