Impact of a Microbial Pest Control Product Containing Bacillus thuringiensis on Brood Development and Gut Microbiota of Apis mellifera Worker Honey Bees.

To avoid potential adverse side effects of chemical plant protection products, microbial pest control products (MPCP) are commonly applied as biological alternatives. This study aimed to evaluate the biosafety of a MPCP with the active organism Bacillus thuringiensis ssp. aizawai (strain: ABTS-1857). An in-hive feeding experiment was performed under field-realistic conditions to examine the effect of B. thuringiensis (B. t.) on brood development and the bacterial abundance of the core gut microbiome (Bifidobacterium asteroids, Gilliamella apicola, the group of Lactobacillus and Snodgrasella alvi) in Apis mellifera worker bees. We detected a higher brood termination rate and a non-successful development into worker bees of treated colonies compared to those of the controls. For the gut microbiome, all tested core members showed a significantly lower normalized abundance in bees of the treated colonies than in those of the controls; thus, a general response of the gut microbiome may be assumed. Consequently, colony exposure to B. t. strain ABTS-1857 had a negative effect on brood development under field-realistic conditions and caused dysbiosis of the gut microbiome. Further studies with B. t.-based products, after field-realistic application in bee attractive crops, are needed to evaluate the potential risk of these MPCPs on honey bees.

Chronic High Glyphosate Exposure Delays Individual Worker Bee (Apis mellifera L.) Development under Field Conditions.

The ongoing debate about glyphosate-based herbicides (GBH) and their implications for beneficial arthropods gives rise to controversy. This research was carried out to cover possible sublethal GBH effects on the brood and colony development, adult survival, and overwintering success of honey bees (Apis mellifera L.) under field conditions. Residues in bee relevant matrices, such as nectar, pollen, and plants, were additionally measured. To address these questions, we adopted four independent study approaches. For brood effects and survival, we orally exposed mini-hives housed in the "Kieler mating-nuc" system to sublethal concentrations of 4.8 mg glyphosate/kg (T1, low) and 137.6 mg glyphosate/kg (T2, high) over a period of one brood cycle (21 days). Brood development and colony conditions were assessed after a modified OECD method (No. 75). For adult survival, we weighed and labeled freshly emerged workers from control and exposed colonies and introduced them into non-contaminated mini-hives to monitor their life span for 25 consecutive days. The results from these experiments showed a trivial effect of GBH on colony conditions and the survival of individual workers, even though the hatching weight was reduced in T2. The brood termination rate (BTR) in the T2 treatment, however, was more than doubled (49.84%) when compared to the control (22.11%) or T1 (20.69%). This was surprising as T2 colonies gained similar weight and similar numbers of bees per colony compared to the control, indicating an equal performance. Obviously, the brood development in T2 was not "terminated" as expected by the OECD method terminology, but rather "slowed down" for an unknown period of time. In light of these findings, we suggest that chronic high GBH exposure is capable of significantly delaying worker brood development, while no further detrimental effects seem to appear at the colony level. Against this background, we discuss additional results and possible consequences of GBH for honey bee health.