Selección y caracterización de la patogenicidad de una cepa de Bacillus pumilus activa frente a la mosca de la fruta del Mediterráneo Ceratitis capitata (Diptera tephritidae)

Resumen

Ceratitis capitata is one of the most important fruit pests worldwide. The medfly is a polyphagous species that causes losses in almost 300 crops, which leads to huge economic losses. Entomopathogenic bacteria belonging to the genus Bacillus have been proven to be safe, environmentally friendly, and cost-effective tools to control pest populations. As no control method for C. capitata based on these bacteria has been developed, isolation of novel strains is needed. Here, we report the isolation of 115 bacterial strains and the results of toxicity screening with adults and larvae of C. capitata. As a result of this analysis, we obtained a novel Bacillus pumilus strain, strain 15.1, that is highly toxic to C. capitata larvae. The toxicity of this strain for C. capitata was related to the sporulation process and was observed only when cultures were incubated at low temperatures before they were used in a bioassay. The mortality rate for C. capitata larvae ranged from 68 to 94% depending on the conditions under which the culture was kept before the bioassay. Toxicity was proven to be a special characteristic of the newly isolated strain, since other B. pumilus strains did not have a toxic effect on C. capitata larvae. Here we also show approaches in the biology of the 15.1 strain, in its principal pathogenicity characteristics, and some aspects of its mode of action. Thus, toxicity assays of strain 15.1 against Medfly at different developmental stages were carried out, leading to the finding that activity is larvicidal with no significant effects on other stages. The action of the strain on the development of Medfly in various fruits is then described, showing promising effects in citrus. The strain was analyzed phylogenetically, showing similarities to B. cereus group strains. Finally, the bacterial community of the Medfly larval gut was analyzed under different conditions, showing that partial elimination of this microbiota did not affect the mortality rates caused by B. pumilus 15.1. To our knowledge, the entomopathogenic activity of B. pumilus against Medfly, and the isolation of a Bacillus strain pathogenic for C. capitata larvae are reported here for the first time. The results of our study have revealed that there are strains of Bacillus genus with unexplored activities and potential uses. We do not expect that B. pumilus will provide total control on C. capitata larvae, but the results of the present study suggest that B. pumilus 15.1 could be considered as a strong candidate for the biological control of C. capitata. Moreover, these findings represent an important starting point for the development of pest control strategies that could help to reduce economic losses in fruit crops. However, further studies are now required in order to determine the toxicity against insects from other orders, the biosecurity and validate its effectiveness in the suppression of Medfly populations under field conditions.