The effect of the pathogenic fungus, Beauveria bassiana on tomato leafminer Tuta absoluta (Lep.: Gelechiidae)

Document Type : Complete paper

Authors

• Zahra Rahmati Habibabadi
• Nafiseh Poorjavad

Department of Plant Protection, College of Agriculture, Isfahan University of Technology, Isfahan, Iran.

Abstract

Keywords

• Biological control
• Cumulative mortality rate
• Fungal conidia

Main Subjects

• Biocontrol of pests

Extended Abstract

Introduction

The tomato leafminer, Tuta absoluta (Lep: Gelechiidae) is one of the most destructive pests of tomatoes in greenhouse and outdoor cultivation. Although the main method of its control relies on the use of chemical insecticides, its biological control is increasingly being considered as a safe and environmentally friendly method. The entomopathogenic fungus, Beauveria bassiana is a promising agent in the biological control of Lepidopteran pests. Unlike other microbial agents, this fungus does not need to be ingested by insects to cause disease due to the ability of direct penetration through the host's cuticle. The high pathogenicity and rapid germination and sporulation of B. bassiana have caused it to be used limitedly to control T. absoluta, previously. The aim of this study is to evaluate the impact of B. bassiana on survival and development time of different life stages of T. absoluta under laboratory conditions.

Materials and Methods

Different life stages of the pest, including eggs, four larval instars and pupae, were infected with nine conidial suspensions of B. bassiana (10² to 10¹⁰ conidia per ml) by using leaf-dipping method. Mortality of eggs, each larval and pupal stages was calculated after seven days of infection to obtain the LC50 values. Also, the survivors were followed until the end of life, and their life span were calculated and their survival curves were using Kaplan–Meier estimates (GraphPad Prism program). The experiment was carried out in 25 ± 1°C and  60 ± 5% relative humidity, and photoperiod; 16L:8D. Polo plus software was used to calculate LC50 and LT50. Data variance was analyzed using One-Way ANOVA method and Tukey's test was used to compare the means.

Results and Discussion

The results showed that the lowest LC50 value is related to the egg stage and then the pupal stage is more sensitive. The sensitivity of larvae to pathogenic fungi decreases significantly with increasing larval age.  The high sensitivity of young larvae to pathogenic fungi can be related to their cuticular fine structure and weak immune system compared to older larvae.  Also, in pupae that do not have diapause, such as the pupa of T. absoluta, it seems that the high rate of metabolism causes less activity of the immune system and greater sensitivity of this stage to pathogens. Although the length of the embryonic period was not affected by the fungus infection, the length of the larval and pupal periods decreased significantly with the increase in the concentration of B. bassiana. In such a way that the length of the pupation period decreased from 6.73 days in the control treatment to 5.33 days in the 10¹⁰ concentration. It seems that the use of B. bassiana can quickly reduce the amount of juvenile hormone, and as a result, the larva-pupa and pupa-adult molting is done earlier, which ultimately reduces the duration of these stages. The survival rate curves after infection up to the pupal stage at different larval ages showed that the survival rate increases with the age of the treated larvae. Also the increase in the concentration of B. bassiana and the increase in time, the survival rate decreases, so that the lowest LT₅₀ value corresponds to the concentration of 10¹⁰ in first instar larvae.