Antifeedant and growth disruption activities of five phytoecdysteroids in the red flour beetle Tribolium castaneum: structure-activity relationship

Phytoecdysteroids are analogs of insect hormones that control various vital functions, in particular, molting and metamorphosis. These phytoecdysteroids form part of the plant's defenses against insect pests. In the present work, we studied the effect of five phytoecdysteroids (20-hydroxyecdysone, Makisterone A, Ponasterone A, Polypodine B, Ecdysone) on larvae of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae). It is the most common pest of stored products. This study aimed to determine if small structural changes between these molecules could change their activity as antifeedants and growth disruptors in insect pests. Initially, we studied their effects on larval feeding behavior using a non-choice test by observing the presence of those larvae on the food pellets. Then we studied their toxicity on the insect development parameters by observing mortality percentages, pupation, and the emergence of adults. Our results indicate that all phytoecdysteroids possess antifeedant properties against T. castaneum larvae. This activity differs depending on the molecule and the concentration used. Compared to the control, 20-hydroxyecdysone exhibited a significant antifeedant activity, while polypodine B showed the lowest effect when incorporated to T. castaneum diet at the highest doses. Phytoecdysteroids caused significant disturbances in the growth and development parameters of T. castaneum, manifested by an inducted mortality, delayed pupation, reduced pupation, and reduced emergence of adults. From the study, we could conclude that the susceptibility of T. castaneum against phytoecdysteroids differs depending on the analog used. The minimal structural differences between these molecules could significantly affect their efficacy against this insect pest.