Bee Vectoring: Development of the Japanese Orchard Bee as a Targeted Delivery System of Biological Control Agents for Fire Blight Management

Abstract

Fire blight, which is caused by the bacteria Erwinia amylovora, remains one of the most important diseases limiting the productivity of apple and pear orchards in the United States. In commercial orchards, in-season fire blight management relies exclusively on the use of antibiotic treatments (such as streptomycin and oxytetracycline) and on bacterial biocontrol agents whose efficacy is limited. We hypothesize that the efficacy of the biocontrol agents can be greatly enhanced through targeted delivery to flowers, which serve as initial infection courts, using the Japanese orchard bee, Osmia cornifrons. Many factors, such as the synchrony of life cycle with plant phenology and specificity to pomaceous plants, suggest that O. cornifrons could be an excellent vector of the biocontrol products during bloom in pome tree fruits. However, deployment of this pollinator species to deliver biocontrol agents for fire blight control has not been attempted previously due to the lack of an efficient system to pack the bodies of the bees exiting nesting tubes with the biocontrol products. In this study, we design and test a dispenser system to facilitate the use of O. conifrons as a vector for commercially available biocontrol products for fire blight control. The effectiveness of O. conifrons to deliver biocontrol agents to flowers, and to effect secondary dissemination from treated to untreated flowers is also evaluated in greenhouse experiments. We found that the O. conifrons bees were able to use the nest dispenser designed for the delivery of biological control products, and are effective in vectoring and delivering the Bacillus subtilis-based biological control product (Serenade®) to apple blossoms. We also found that the O. cornifrons were effective in secondary inoculation of this biological control product to newly-opened flowers. These findings suggest the potential use of commercially available O. conifrons and other orchard bees in targeted delivery of biological control products for fire blight, and possibly other diseases, in different fruit crops.

Distribution of a model biocontrol agent (Serenade® MAX) in apple and pear by mason bees and bumble bees

“…The successful secondary transmission of Bacillus amyloliquefaciens by bumble bees and mason bees contributes to the spread of the BCA to open, noninoculated flowers, which can improve not only the coverage after initial spraying applications when sufficient pollinators are present but also reduce the spraying frequency of BCA. Although we have not tested whether these bacteria were still alive and effective, other studies (such as Joshi et al ., 2020) showed the self‐perpetuating character of this entomovectoring system. However, the extent to which this can improve coverage in commercial fields remains to be tested as the number of flowers, initial coverage and bees to flower ratios may differ from our experimental setup.…”

“…(2009) in which O. cornuta and A. mellifera were able to deposit on average 9.6 × 10 3 and 1.4 × 10 3 CFU Biopro® ( B. subtilis strain BD170) within their first flower visit, accompanied with 14.7% and 33.7% of non‐colonized flowers, respectively. Overall, the numbers of B. amyloliquefaciens spores isolated from the flowers of the receiver trees 1 h after inoculation were rather low, certainly, compared to the mean amount of CFUs of Serenade® MAX deposited by the Japanese orchard bees O. cornifrons on crabapple flowers (1.9–4.2 × 10 5 CFUs/per flower) by secondary transmission (Joshi et al ., 2020). This difference is possibly due to the time that bees were able to forage on BCA inoculated trees, which was 1 h in our study compared to 24 h in the study by Joshi et al .…”

“…This difference is possibly due to the time that bees were able to forage on BCA inoculated trees, which was 1 h in our study compared to 24 h in the study by Joshi et al . (2020). In our study the duration of the experiment was kept rather limited to 1 h to avoid flowers on the receiver trees being visited by multiple pollinators, which could have diluted the presence of BCA on each flower, while in the study by Joshi et al .…”

“…However, no significant lethal effects (20%) were observed by topical contact with a lower and similar concentration as used in our study (3.8 × 10 6 CFUs/mL vs. 4.2 × 10 6 CFUs/mL, respectively). Furthermore, dry topical exposure to Bacillus subtilis had no deleterious impact on bumble bees (Mommaerts and Smagghe, unpublished data), and this has already been safely used in other pollinator‐vector experiments with honey bees and mason bees (Dedej et al ., 2004; Maccagnani et al ., 2009; Joshi et al ., 2020). However, this does not completely rule out that dry applications are safe for bees, as for instance exposure of Bombus terrestris to powder containing 10 7 spores of Metarhizium anisopliae strain F52 (Bio1020) per gram was harmless, while a 100‐fold higher spore concentration (10 9 spores per gram powder) was highly toxic (Smagghe et al ., 2013).…”

“…As a model BCA, we worked with Serenade® MAX (Bayer CropSciences, Belgium), a dry powder formulation of spores of Bacillus amyloliquefaciens (ex Fukumoto 1943) Priest et al 1987 strain QST 713, which is used for controlling various crop diseases. It was previously wrongly identified and named as Bacillus subtilis strain QST 713 (Mommaerts et al, 2009;Joshi et al, 2020) but are in fact sister species (but see Zhang et al, 2016;Fan et al, 2017). Serenade® MAX was used as dry powder (7.13 × 10 9 CFU/g) in the dispenser experiment, while in the other experiments, the product was diluted in demi water to a final concentration of 4.2 × 10 6 CFU/mL or 0.575 kg/ha (1000 L/ha sprayed in standard orchard).…”