Lethal and Sublethal Toxicity of Beta-Carboline Alkaloids from Peganum harmala (L.) against Aedes albopictus Larvae (Diptera: Culicidae)

Plant-derived agents are powerful bio-pesticides for the eco-friendly control of mosquito vectors and other blood-sucking arthropods. The larval toxicity of beta-carboline alkaloids against the Asian tiger mosquito, Aedes albopictus (Skuse) (Diptera: Culicidae), was investigated under laboratory conditions. The total alkaloid extracts (TAEs) and beta-carboline alkaloids (harmaline, harmine, harmalol, and harman) from Peganum harmala seeds were isolated and tested in this bioassay. All alkaloids were tested either individually or as binary mixtures, using the co-toxicity coefficient (CTC) and Abbott’s formula analysis. The results revealed considerable toxicity of the tested alkaloids against A. albopictus larvae. When all larval instars were exposed to the TAEs at 48 h post-treatment, the mortality of all larval instars varied in a concentration-dependent manner. The second-instar larvae were the most susceptible to different concentrations of TAEs, and the fourth-instar larvae were more tolerant to TAEs than the second-instar larvae. Especially, the third-instar larvae exposed to all alkaloids also showed that all doses resulted in an increased mortality of the third-instar larvae at 48 h post-treatment, and the toxicities of the tested alkaloids in a descending order were TAEs > harmaline > harmine > harmalol, with the LC₅₀ values of 44.54 ± 2.56, 55.51 ± 3.01, 93.67 ± 4.53, and 117.87 ± 5.61 μg/mL at 48 h post-treatment, respectively. In addition, all compounds were also tested individually or in a 1:1 ratio (dose LC₂₅/LC₂₅) as binary mixtures to assess the synergistic toxicity of these binary combinations against the third-instar larvae at 24 and 48 h post-treatment, respectively. The results demonstrated that when tested as a binary mixture, all compounds (especially TAEs, harmaline, and harmine) showed their synergistic effects, exceeding the toxicity of each compound alone. Interestingly, the obtained data further revealed that the TAEs at sublethal doses (LC₁₀ and LC₂₅) could significantly delay the larval development and decrease the pupation and emergence rates of A. albopictus. This phenomenon could be helpful in order to develop more effective control strategies for different notorious vector mosquitoes.