[Toxicity and sublethal effects of calcium cyanamide against susceptible strains of Aedes albopictus].

Abstract

Objective: To examine the toxicity and sublethal effects of calcium cyanamide against susceptible isolates of Aedes albopictus, so as to provide insights into rational use of calcium cyanamide for integrated management of Ae. albopictus.

Methods: The sublethal concentrations [30% lethal concentration (LC₃₀) and median lethal concentration (LC₅₀)] of calcium cyana mide against susceptible strains of Ae. albopictus were determined using the larval immersion test. With 100 mL of dechlorinated water as the control group, after the larvae of susceptible strains of Ae. albopictus were immersed in calcium cyanamide for 24 hours, the pupation rate, pupation duration, emergence rate, number of eggs laid, percentage of eggs hatched, and lifespan of Ae. albopictus were calculated and compared post-treatment with calcium cyanamide at different sublethal concentrations. The midgut tissues of larvae of susceptible strains of Ae. albopictus treated with 100 mg/L calcium cyanamide were sampled for pathological sectioning to observe midgut tissue damages. To evaluate the residual activity, 100 larvae of susceptible strains of Ae. albopictus were treated with 200 mg/L and 500 mg/L calcium cyanamide, and the mortality of larvae was calculated every 24 hour, with dead larvae replaced until no larval death.

Results: The regression equation for the toxicity of calcium cyanamide against larvae of susceptible strains of Ae. albopictus was y = -9.441 + 4.657x, with an LC₅₀ of 106.42 mg/L [95% confidence interval (CI): (94.64, 118.36) mg/L] and an LC₃₀ of 82.17 mg/L [95% CI: (94.64, 118.36) mg/L], respectively. After larvae of susceptible strains of Ae. albopictus were treated with sublethal concentrations (LC₃₀ and LC₅₀) of calcium cyanamide for 24 hours, there were reduced pupation and emergence rates of larvae (all P values < 0.000 1), prolonged pupal stage (both P values < 0.000 1), reduced numbers of eggs laid by survival female Ae. albopictus (both P values < 0.000 1), reduced percentages of eggs hatched by Ae. albopictus eggs (both P values < 0.000 1), and reduced median survival period of survival female Ae. albopictus (χ² = 9.36 and 20.33, both P values < 0.01) in the LC₃₀ and LC₅₀ groups relative to the control group. There was a numerical decline in the median survival period of survival female Ae. albopictus in the LC₃₀ groups relative to the control group (χ² = 2.42, P > 0.05), and there was a significant decline in the median survival period of survival female Ae. albopictus in the LC₅₀ group relative to the control group (χ² = 11.42, P < 0.01). Histopathological examinations showed severe damages to the midgut tissues of larvae of susceptible strains of Ae. albopictus, and residual activity assay revealed that the mortality of larvae of susceptible strains of Ae. albopictus was both 0 on day 32 post-treatment with calcium cyanamide at a concentration of 200 mg/L and on day 70 post-treatment with calcium cyanamide at a concentration of 500 mg/L, showing complete loss of the larvicidal activity of calcium cyanamide.

Conclusions: Calcium cyanamide is highly toxic against susceptible strains of Ae. albopictus, and calcium cyanamide at sublethal concentrations (LC₃₀ and LC₅₀) may inhibit growth, development, and reproductive capability of susceptible strains of Ae. albopictus, and shorten the lifespan of adult mosquitoes.