Susceptibility of Drosophila suzukii larvae to the combined administration of the entomopathogens Bacillus thuringiensis and Steinernema carpocapsae

Abstract

Non-native pests are often responsible for serious crop damage. Since Drosophila suzukii has invaded North America and Europe, the global production of soft, thin-skinned fruits has suffered severe losses. The control of this dipteran by pesticides, although commonly used, is not recommended because of the negative impact on the environment and human health. A possible alternative is the use of bio-insecticides, including Bacillus thuringiensis and entomopathogenic nematodes, such as Steinernema carpocapsae. These biological control agents have a fair effectiveness when used individually on D. suzukii, but both have limits related to different environmental, methodological, and physiological factors. In this work, we tested various concentrations of B. thuringiensis and S. carpocapsae to evaluate their efficacy on D. suzukii larvae, when administered individually or in combination by using agar traps. In the combined trials, we added the nematodes after 16 h or concurrently to the bacteria, and assessed larvae lethality from 16 to 48 h. The assays demonstrated a higher efficacy of the combined administration, both time-shifted and concurrent; the obtained data also showed a relevant decrease of the time needed to kill the larvae. Particularly, the maximum mortality rate, corresponding to 79% already at 16 h, was observed with the highest concentrations (0.564 µg/mL of B. thuringiensis and 8 × 102 IJs of S. carpocapsae) in the concurrent trials. This study, conducted by laboratory tests under controlled conditions, is a good starting point to develop a further application step through field studies for the control of D. suzukii.

Drosophila suzukii Susceptibility to the Oral Administration of Bacillus thuringiensis, Xenorhabdus nematophila and Its Secondary Metabolites

“…In a previous paper, we evaluated the effects of the administration of the nematode S. carpocapsae with Bt either singularly or combined to SWD larvae [ 9 ]; the results of the combined administration, when compared to the treatments with single bioinsecticides, revealed a significant increase in effectiveness both in terms of larval mortality rate and reduction in treatment time. The best results were obtained when the larvae were treated with the two entomopathogens simultaneously.…”

“…However, the efficacy of these pathogens, when used in single, does not always provide satisfactory results; in addition, their efficacy often requires high concentrations of the pathogen, which can interfere with the life cycle and affect other beneficial organisms present in the environment [ 48 , 49 ]. The reduction of the administered concentrations and the improvement of the rapidity of action, could be achieved by combining different bioinsecticides, thus by exploiting the possible synergistic or additive potential of the applied pathogens [ 9 , 50 , 51 , 52 ].…”

“…To verify the effects of various bioinsecticides on SWD larvae, the agar trap method was used as described by Mastore et al [ 9 ]. Larvae were passively fed on substrate, and susceptibility was evaluated by a single oral administration of spores and crystals of Bt and Xn bacteria, and Xn secretions.…”

“…A number of papers described single or combined administrations of nematodes, microorganisms, and secretions, even under different temperature conditions [ 20 , 21 , 22 ], and, in some cases, the results obtained are positive, i.e., the combinations significantly increase the effectiveness of the biocontrol method. In a previous work [ 9 ] we observed that the presence of Steinernema carpocapsae combined with Bt subspecies kurstaki, enhanced the lethality of SWD larvae, when administered concurrently or time shifted. The obtained data showed a relevant decrease of the time required to reach a high kill rate, being 16–24 h with concentrations of 0.564 µg/mL and 8 × 10 2 IJ/mL for Bt and S. carpocapsae , respectively.…”

“…These nematocomplexes at the infective juvenile (IJ) stage can penetrate the target insect inside the infested fruit [ 6 , 7 ]. In some studies, in which immune defenses of the insect target have also been investigated [ 8 , 9 ], nematodes have been shown to be quite effective, although semi-field and field studies are very scarce [ 10 ]. Moreover, the isolated bacteria symbionts, Xenorhabdus spp.…”