Investigations on residues of XenTari® (Bacillus thuringiensis subspec. aizawai) on greenhouse tomatoes

XenTari® (Bacillus thuringiensis (B.t.) subspecies aizawai) is an important biological plant protection agent for the control of Noctuidae larva on tomato fruits in greenhouses and belongs to the group of presumptive Bacillus cereus species. In general, food control agencies do not routinely differentiate between B.t. and B. cereus and a threshold of 105 colony forming units (cfu)/g fresh weight is applied for presumptive B. cereus in official food control. As no data exists on the expected residues of B.t. spores after application, residual experiments were conducted on tomatoes in greenhouses. In the greenhouse experiment, five applications of XenTari® were applied at weekly intervals. The concentration of B.t. spores on the tomato fruits ranged in all experiments between 4.9 × 104 and 8.5 × 104 cfu/g fresh weight. For single application of B.t., a maximum spore concentration of 4.7 × 104 cfu/g fresh weight was measured. None of the experiments reached the threshold for B. cereus of 1 × 105 cfu/g, although treatments were applied in a very narrow window. The findings were confirmed by additional laboratory experiments and by experiments conducted on a commercial tomato farm. To prove the degradation of B.t. spores under protected greenhouse conditions over time, a series of samples was taken after the last application over one week. Over all, the experiments demonstrated that the concentration of B.t. spores was reduced within one week to between 46% and 77% of the initial spore concentration. Therefore, in comparison to open field condition the degradation of B.t. spores under greenhouse condition was limited. When only the upper parts of the tomato plant were treated with XenTari® a distinct reduction of B.t. spores of up to 90% of B.t. spores with a concentration of 1.85 × 103 cfu/g fresh weight on the marketable tomatoes was achieved. DOI: 10.5073/JfK.2014.09.04, https://doi.org/10.5073/JfK.2014.09.04