Evaluation of Local Pathogenic Fungi, Boric Acid, and their potential synergism for control of the European Fire Ant, Myrmica rubra (L.)

Yan, Shicia

' The European Fire Ant, Myrmica rubra, is an exotic species in the USA and is potentially damaging to local ecosystems. No research has been conducted on control of this species, especially with biological agents. Cadavers of M. rubra were colleted during the 2002 2003 field season at Mount Desert Island (MDI), Maine. Both entomopathogentic fungi, Beauveria bassiana and Metarhizium anisopliae, were widely distributed throughout sample sites in the island, with 9.5% and 1.2% of the field collected cadavers sporulating with these fungi, respectively. Five isolates of B. bassiana and one isolate of M. anisopliae were successfully mass-cultivated on SDA media, and their virulence on M. rubra workers were compared with GHA, a commercial strain of B. bassiana. Two isolates were significantly less virulent than GHA, in terms of median lethal concentration (LC50) and median lethal limit (LT50). Three isolates caused higher mortality and there was no virulence difference compared with GHA. The LC50 values of these three isolates ranged from 5.75 x 105 to 6.46 x 105 conidia/ml, 10 days post infection. At a concentration of 1.0 x 106 conidia/ml, their LT50 values ranged between 9.84 to 9.93 days. However, no difference was observed among the five isolates, as well as GHA, in the proportion of cadavers that sporulated. Higher conidia concentration caused greater sporulation. At a concentration of 1.0 x 106 conidia/ml, approximately 65% of the cadavers sporulated for all isolates. The effects of boric acid and sucrose concentrations on consumption and subsequent mortality of M. rubra workers were assessed under laboratory conditions. The higher boric-acid concentration led to less consumption, indicating avoidance behavior or repellency with higher boric-acid concentrated baits. There was no difference in mortality between ants fed the control and 0.1% boric-acid baits, while at 1.0% and 5.0% boric acid, mortality was significantly higher from the first day after application. The median lethal time (LT50) increased with decreasing boric acid concentration from 1.8 days at 5% boric acid to 118.8 days at 0.1%. The most effective boric acid concentration ranged from 0.25 to 0.5% with the LT50 ranging from 14.5 to 42.3 days. Bait consumption was significantly lower with the 20% and 30% sucrose concentration, compared with 10% concentration. No significant interaction in consumption was observed between boric acid and sucrose, but an interaction effect was observed for mortality. There was no mortality difference for three sucrose concentrations at control and the low rate of boric acid bait (0.1%). However, with an increasing rate of boric acid concentration (>0.25%), lower sucrose-concentration bait (10%) caused higher mortality. Potential synergism between boric acid and M. anisopliae for M. Rubra workers was also investigated under laboratory conditions. There was no synergism between boric acid and M. anisopliae on consumption of bait or subsequent mortality. However, there was a synergistic effect on M. anisopliae sporulation of cadavers. Sporulation increased with M. anisopliae concentration (105 conidia/ml and 106 conidia/ml) for ants exposed to 0.5% boric acid bait, but not for ant exposed to 0.25% or no boric acid. '