Laboratory assays were conducted to evaluate deterrency and contact toxicity of six essential oils to the Argentine ant, Linepithema humile (Mayr), and the red imported fire ant, Solenopsis invicta Buren. In choice tests, both Argentine ants and fire ants crossed barriers treated with multiple rates of basil, citronella, lemon, peppermint, or tea tree oil less frequently than paired control barriers. Eucalyptus oil did not prevent movement of either species at any of the rates tested. In continuous exposure assays, citronella oil killed 50% of Argentine ants in 34.3 min and was the only treatment to cause 100% Argentine ant mortality after 24 h. Argentine ant mortality after 24 h was 89.8% with peppermint oil and 85.7% with tea tree oil, with the remaining treatments having mortality not significantly different from the control. Only citronella oil caused significant mortality of red imported fire ants, with 50.6% of the ants being dead after 24 h of continuous exposure.

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