Integrating Natural Products - Genetic Resources for Control of Varroa Jacobsoni, a Parasitic Mite of the Honey Bee

Integrating Natural Products - Genetic Resources for Control of Varroa Jacobsoni, a Parasitic Mite of the Honey Bee

Summary

Our project focuses on integrating mite resistant stocks of honey bees with other treatment modalities, including natural products and non-chemical methods. We evaluated: 1) several stocks of mite resistant bees; 2) a new formulation of formic acid; 3) a mite control device called a screen bottom-board; 4) the effect of screen bottoms on formic acid efficacy; and 5) drone-comb mite traps. We prepared for: 1) a final evaluation of mite resistant bees; 2) an evaluation of the effect of screen bottoms on mite levels in package and over-wintered colonies; and 3) an evaluation of the number of drone-comb treatments required for effective mite control with that method.

Objectives/Performance Targets

OBJECTIVES: 1. evaluate honey bee stocks that are partially resistant to the parasitic honey bee mite, Varroa destructor; 2. optimize formic acid for control of Varroa mites; 3. evaluate drone brood as a ‘trap crop’ for control of Varroa mites.

Accomplishments/Milestones

Objective 1: Mite resistant bees:
1. Evaluation of RUSSIAN stock: Using a cooperator’s bees, we compared colonies headed by queens reared from inbred RUSSIAN stock that were allowed to mate naturally with unselected drones. We established a population of 96 colonies, half resistant and half unselected. All queens were reared in a common apiary and allowed to mate naturally with unselected drones. In the fall, we evaluated the mite/bee ratios, but found no significant difference between treatment groups (0.008 ± 0.008 M/B in colonies with unselected stock compared to 0.012 ± 0.004 M/B in colonies headed by RUSSIAN queens. The cooperator reported that colonies headed by RUSSIAN queens were prone to swarming, a highly undesirable trait.

2. Evaluation of SMR stock: We attempted several evaluations of the SMR stock from USDA-ARS. We reared virgin queens from inbred SMR breeder queens and from an unselected stock of bees. In one case, we provided a cooperator with 60 virgins of each type. His job was to introduce them to small colonies and evaluate them over the summer. He reported that 59/60 SMR queens were rejected or disappeared within a few weeks of introduction. With the unselected stock, 59/60 were accepted and performed very well (we did not collect data because there was no SMR treatment group for comparison). Since all queens were reared in the same queen-raring colony and handling thereafter was identical, we assume that the level of inbreeding in the SMR virgins (since they come from inbred SMR stock, all virgins produced from this stock are still inbred – the workers they produce using semen from unrelated and unselected drone sources are F1 hybrids). In a second trial, we provided a cooperating beekeeper with 90 queens, which were introduced with about 50% success. However, shortly thereafter, the beekeeper passed away quite unexpectedly (he was only 51 and in the peak of health), therefore, we were not able to collect data on those hives.

We had somewhat better success in introducing SMR queens in our own colonies at Dyce Lab. Of 90 virgins introduced during the fall (45 SMR and 45 CONTROLS), we retained about half of the SMR queens the following spring. In August, we took ether roll samples from the colonies. CONTROL colonies had an ether roll count of 10.79 ± 1.58 mites, compared to 11.83 ± 1.64 mites in SMR colonies (P > 0.50).

3. Plans for coming season: We plan one final evaluation of the RUSSIAN and SMR stocks. We will requeen 96 colonies of one of our cooperators operations. One third will receive CONTROL queens from unselected stock, 1/3 will receive RUSSIAN queens from commercial suppliers, and 1/3 will receive SMR queens, also from a commercial supplier. We will evaluate these colonies for mite levels in the fall. We will also evaluate them for the SMR trait (suppressing mite reproduction). High performing colonies will be transferred to our larger breeding population for further evaluation.

Objective 2: Formic Acid:
1. New bottle formulation of formic acid: We evaluated a new formulation of formic acid. The existing product has a problem with the packaging. The acid dissolves the packaging, rendering the product dangerous to handle. The new formulation makes use of a heavy duty plastic bottle to eliminate this problem. We evaluated this product in two apiaries, each with approximately 24 colonies. We found that the new product provided 86.33 ± 3.21 % control, compared to 55.77 ± 3.21 % mite mortality in control colonies (P < 0.0001). The level of control achieved with this product was a bit lower than previous studies, but was still within the range considered useful. The lower efficacy was probably was due to the low rate of evaporation (39.16 ± 0.04% in one apiary and 37.78 ± 0.04 % in the other apiary). This low rate appears to be due to the dimensions of the product which result in a low evaporative surface-area to volume ratio. 2. Effect of screen bottom boards on efficacy of formic acid treatment for mite control: There is considerable interest among beekeepers in a device known as a screen bottom board. This device replaces the traditional solid bottom board on which the hive rests. The theory is that many mites fall off of mites and land on the bottom board. If it is solid, they can return to the bees, but if it is screened, they fall through and out of the hive. However, there is concern that the open bottom on the screen bottom boards may reduce the efficacy of formic acid because it is heavier than air and may simply flow out of the hive before having an effect on the mite population. We tested our own proven formulation of formic acid on 24 colonies, 12 with traditional bottoms and 12 with screen bottoms. We obtained 86.50 ± 3.1 % mite mortality in hives with screen bottoms and 87.58 ± 3.1 % in hives with traditional bottoms (P > 0.50). Again, these results are probably due to the fact that the evaporation rate of formic acid was relatively low during this unusually cold period (48.13 ± 1.9 % in colonies with solid bottoms and 54.33 ± 1.9 % in colonies with screen bottoms), but still within the acceptable range. These results also provide a good estimation of the effectiveness of formic acid in cold weather. Previous fall applications had been conducted when the weather was much warmer. This past fall, October was unusually cold. The efficacy of the formic acid was reduced, but still sufficient for a fall treatment.

Objective 3: Drone brood ‘Trap Crop’: Mites reproduce in brood cells of honey bees and prefer drone brood to worker brood by a factor of 5 or 10 to 1. Drone foundation makes it possible to produce solid combs of drone cells. These drone combs can be placed in colonies; and after they are full of capped drone brood and mites, they can be removed, frozen to kill the mites, and then returned to the colony for cleaning and another round of treatment. In our first study, we have measured the size of the colony (Combs of Bees) as an indicator of the effectiveness of this method in managing mites. In colonies with 3 drone comb treatments during the summer, we estimated a cluster size of 6.85 ± 0.94 Combs of Bees, compared to 2.36 ± 0.89 % Combs in colonies without drone comb treatment (P < 0.002). This suggests that this method is highly effective in protecting colonies during the critical time of the season, the late summer, when honey production is still in full swing, but when the colonies cannot be treated with pesticides. This past season, we prepared 96 colonies for a full scale evaluation of this method this coming season. We plan to measure mite levels, honey production and cluster sizes as indicators of the value of the drone comb trap method in treatment groups receiving different numbers of drone-comb treatments during the summer. Objective 4: Screen bottom boards: We have evaluated screen bottom boards to determine their effect on mite levels, honey production and wintering success: 1. Trial 1: During the summer of 2001, we conducted our first evaluation of the screen bottom boards. We found that the screen bottom had no significant effect on fall cluster size (9.26 ± 0.55 Combs of Bees for the colonies with the solid bottom board and 8.46 ± 0.54 Combs of Bees in the colonies with the screen bottom boards (P > 0.50). Similarly, we found no significant effect of screen bottom boards on honey production (69.07 ± 5.45 kg in colonies with regular bottom boards and 63.34 ± 5.28 kg in colonies with the screen bottom boards (P > 0.50)). Finally, we did not detect a significant difference in mite levels (mite/bee ratio) between the two treatment groups (0.087 ± 0.018 M/B in colonies with the solid bottoms and 0.11 ± 0.018 M/B in colonies with the screen bottom board (P > 0.50)). A M/B ratio of 0.10 is considered very high.

Trial 2: During the summer of 2002, we continued our evaluation of the screen bottom boards. We found that the screen bottom had no significant effect on fall cluster size (6.38 ± 0.77 Combs of Bees for the colonies with the solid bottom board and 5.73 ± 0.74 Combs of Bees in the colonies with the screen bottom boards (P > 0.50). Similarly, we found no significant effect of screen bottom boards on honey production (39.08 ± 5.59 kg in colonies with regular bottom boards and 36.83 ± 5.59 kg in colonies with the screen bottom boards (P > 0.50)). Finally, we did not detect a significant difference in mite levels (mite/bee ratio) between the two treatment groups (0.036 ± 0.008 M/B in colonies with the solid bottoms and 0.021 ± 0.008 M/B in colonies with the screen bottom board (P > 0.25)).

Trial 3: Our previous studies have used package bees to start colonies in the spring time. Typically, mite levels do not grow as fast in package colonies compared to fully-developed, over-wintered colonies. Therefore, we are over-wintering our current colonies involved in the screen bottom board experiment. We will establish 24 new package colonies this spring in the same apiary. This will allow us to repeat the experiment, but this time with an emphasis on any difference between these two types of colonies.

Impacts and Contributions/Outcomes

Objective 1: Mite resistant bees: Our evaluations for RUSSIAN and SMR queens have been disappointing. We had anticipated that the F1 colonies produced from RUSSIAN or SMR queens would exhibit about half the level of resistance as the inbred stocks. We have no explanation for the poor performance (mite loads) of the bees we tested. The poor acceptance of RUSSIAN and SMR virgins is likely due to inbreeding in the breeder population.

Objective 2: Formic Acid: Our studies on formic acid continue to increase our understanding of this product under a variety of environmental circumstances. The new bottle formulation is unlikely to be used because of the low evaporation rate resulting from the low SA/V ratio. However, the manufacturer is working on new packaging and hopes to have a safe product available soon. Once this packaging problem is solved, we anticipate widespread adoption of the product by beekeepers, especially in the north.

Objective 3: Drone brood ‘Trap Crop’: This method looks extremely promising, especially for the majority of beekeepers who generally run fewer than 25 colonies. We do not believe that larger beekeepers will be able to use this method because it increases the amount of labor required to manage the hives. A beekeeper with >250 hives simply could not use this method. Nonetheless, for the small beekeeper, this method promises to reduce winter loss by helping to maintain colony health during the active season. We will evaluate this method this coming season using a large population of colonies. We will measure mite loads, cluster sizes and honey production as part of our evaluation.

Objective 4: Screen bottom boards: To date, and despite considerable promotion in the popular bee journals, we have found no evidence that would lead us to recommend that beekeepers use screen bottom boards for mite control, or anything else. While our results have been negative, they have value because they provide the best data on the efficacy of the devices currently available. We plan one final year of evaluation (including in the study the effect of the device on package vs wintered colonies).

Collaborators: