United Forces of Botanical Oils: Efficacy of Neem and Karanja Oil against Colorado Potato Beetle under Laboratory Conditions

Neem and karanja oil are the most promising botanical insecticides in crop protection nowadays. Given that information about the insecticidal abilities of these oils is lacking, the aim was to explore the effects of neem and karanja oil binary mixtures. The insecticidal activity of NeemAzal T/S (Trifolio-M GmbH, Lahnau, Germany) (neem oil), Rock Effect (Agro CS a.s., Česká Skalice, Czech Republic) (karanja oil), and their binary mixes (at 1:1, 1:2, and 2:1 volume ratios) against the larvae of the Colorado potato beetle (CPB; Leptinotarsa decemlineata) was studied. In our bioassays, a synergistic effect of the mixtures, which was dose-dependent, was observed for the first time against this pest. The most effective blend was the 1:1 ratio. Its efficacy was more or less the same as, or even greater than, the neem oil alone. The LC(50) of neem oil two days after application was (0.075 g·L(−1)) and the LC(50) of the mixture was (0.065 g·L(−1)). The LC(50) of karanja oil was (0.582 g·L(−1)), which was much higher than the LC(50) of neem oil. The LC(90) of neem oil five days after application was (0.105 g·L(−1)) and the LC(90) of the mixture was (0.037 g·L(−1)). The LC(90) of karanja oil was (1.032 g·L(−1)). The results demonstrate that it is possible to lower the doses of both oils and get improved efficacy against CPB larvae; nevertheless, further verification of the results in field conditions is necessary.