Development of best use practices on commercial colonies of Bombus impatiens on crops in Delaware

In comparing treatments by total foraging of a colony through the season all colonies placed with shaded treatment yielded higher total foraging counts than those placed in the sun (Figures 2,3), ( F2, 99=8.31, P=0.0005). Each colony’s peak forage counts and the date of the peak was extrapolated and compared by treatment and field. No difference between treatments was detected (Figure 4), (F2, 98=1.22, P=0.2986). The day in which an individual colony performed their peak forage was calculated to indicate the length of time the colony was still actively building up before decline in activity began. When compared by treatment, days to peak forage was not significant (Figure 5), (F2, 98=1.39, P=0.2532). Two foraging counts were conducted on the same day each week that aimed to reflect morning and afternoon foraging. We found that colonies generally foraged significantly more in the morning hours (before 11am) than they did in the afternoon (after 11am). Total season counts of morning foraging were measured against total season afternoon foraging counts, by treatment. Difference was seen by treatment, by time of day and by their interaction, treatment x time of day foraging activity of the colony was higher in the morning hours than in the afternoon, but the quad treatment affected the level of the differences seen (Figure 6), (treatment: F5, 198=4.43, P=0.0131, time: F5, 198=8.54, P=0.0039, treatment x time: F5, 198=4.29, P=0.0149).

Natural shade and shade structure treatments had higher average weights than those in the sun (Figure 7). Change in weight from the beginning of the season to the peak weight showed how much a colony gained at the height of the season. The natural shade had the highest weight change, followed by shade structure treatments, then the sun (Figure 8), (F2, 99=21.07, P<0.0001). The number of days it took for the colony to reach its peak weight was tested against its treatment and field. All shaded treatments took a longer time to reach their peak weight than those colonies in the sun (Figure 9), (F2, 93=16.59, P<0.0001).

At the end of the season when each colony was determined to be 'dead', the colony was collected and analyzed. The number of total cells counted in each colony was compared against the treatment that it was given and the field in which it was placed (Figure 10). Both shaded treatments had more total cells than sun treatments (F2, 91=12.08, P<0.0001). The number of bees that remained in the colony were counted with each post mortem analysis and compared to the colony’s site and treatment. More dead, adult bees were left in the colonies with sun treatments than the colonies with two shade treatments (Figure 11), (2011: F4, 31=5.85, P=0.0013, 2012: F2, 97=5.95, P=0.0037).

Average weekly temperature (Figure 12) and relative humidity (Figure 13) of the colonies did not differ in colonies with different treatments, (temperature: F2, 61=0.34, P=0.715, relative humidity: F2, 20=3.30, P=0.0579).

Watermelon surveys were conducted that determined fruit set by distance to bumble bee quad. One plant was chosen at 5 different distances per quad. The total number of fruit counted during both surveys was compared by distance the chosen plant was from quad (number of fruit on a plant 10 meters from quad, 20 meters, 30, 40, 50 (Figure 14) (F16, 268=0.33, P=0.8575).

The variety of placement treatments including the various designs of the shade structures provided by Koppert Biological Systems© caused differences in the performance of various metrics assessed during the field season. These metrics tested colony productivity by assessing forage rates, colony weight and cell production within the colony. All treatment placements generally provided a condition for the colony that was superior to a sun placement in relation to productivity tests results. These results align with the need for bumble bees to maintain their nest temperature at 30?C. This temperature is maintained by bumble bees producing heat at low ambient temperatures or fanning the nest when the weather gets too warm (Goulson 2010, Heinrich 2004, Vogt 1986). These temperature-regulating actions require energy, thus, energy spent regulating temperature is not available to optimize colony productivity (Heinrich 2004). The less energy available decreases the allotment of energy for nectar and pollen foraging and overall reduces intrinsic worth to growers. Indeed, in most five minute foraging counts, weight analyses, and nest post mortem analyses, the shaded colonies foraged more, weighed more, or performed in a way that indicated higher productivity, than colonies placed in the sun. Further, the more worker bees needed to thermoregulate a colony, equates to less bees available for brood maintenance or other duties needed for the colony (Vogt 1986).

Interestingly, we found that when counting dead, adult bees in the post mortem analyses, sun treated colonies left more bees than their shaded counterparts. Perhaps this is evidence that energy expended for colony hygienic behavior decreases when more energy is needed to maintain colony temperatures. In a healthy colony, dead, adult bees are most likely removed to reduce inevitable occurrences of disease when chronically exposed to the dead (Cremer et al. 2007). Evidence of dead immature bee removal was witnessed throughout the experiments, especially after delivery of the commercial quads to the field. Larvae that did not survive the trip were removed by worker bees and dropped outside of the colony. The removal of deceased adult bees is speculated but was not specifically witnessed during this project.

Through these experiments we can conclude that commercial bumble bee units must be placed under some shade to increase longevity and overall productivity throughout the growing season. This statement is displayed in the watermelon temperature figure, Figure 12. This figure demonstrates the ability for bumble bees to thermoregulate their nests, despite the differences in sun exposure. However, as the season progresses the bumble bees in the sun stop regulating the temperature of their colony earlier in the season than the other two treatments. The temperature regulation stops presumably because the colony is no longer active and thus is no longer foraging. The temperature of the sun treated colonies increases while the other two treatments continue on, regulating the temperature of the colony.
Overall, the experiments testing colony productivity and colony placement presents much evidence that shaded colonies expend less energy and expend less general worker resources thermoregulating the colony, during the summer growing months. This relaxation from maintaining colony temperature allows for the energy to be directed towards productivity as witnessed by the increase in foraging and weight in colonies placed in shade.

Pesticides were detected in the bees and the wax of each sample tested (see Table 1). LD50 were taken from Pesticide Properties Database (PPDB), a tool developed by the University of Hertfordshire in the UK, unless otherwise indicated. The LD50s are for honey bee contact or oral exposure for 48 hours as bumble bee exposure has not been investigated to the same extent in which it has been explored in honey bees. We find these LD50s inapplicable to our study for a few reasons. First, as bumble bees are larger than honey bees, they are known to consume more pollen and nectar on a daily basis than honey bees. If pollen and nectar contain pesticide residue, bumble bees will likely come into contact with these chemicals at a faster rate, by the simple fact that adult bumble bees eat larger quantities per day than honey bees. Second, in our study within the watermelon field, bumble bees foraged on more flowers per minute than honey bees. Therefore, bumble bees can be exposed to larger quantities of pesticides due to their increased visitation rates. Further, as stated above, the LD50s are applicable for an exposure of just 48 hours (PPDB). In our experiment, the fields in which our bumble bees foraged were generally sprayed weekly; and the chemicals found in the substrate tested were from a season’s worth of these weekly sprays. Thus, the commercial bumble bees are generally exposed to pesticides over a much longer time than is indicated in the given LD50s. Also there is growing evidence that exposure to multiple pesticides at a time creates synergisms leading to possible additive effects to honey bees. Much of the possibility for synergism has been shown between fungicides and neonicotinoids and pyrethroids (Johnson et al. 2010), all of which were found in the pesticide analysis. Bees are often exposed to more than one pesticide at a time within the field and are therefore subject to this multiple pesticide synergism. Much of these tests have been conducted in the lab rather than in practical applications in the field and thus field studies testing synergism are warranted for the future (Iwasa et al. 2003, Johnson et al. 2009, 2010, Pilling and Jepson 1993). Currently, the US EPA is working on developing studies to test chronic honey bee exposure. At this time, there are no formal guidelines developed to conduct chronic toxicity tests on honey bees, and to the best of our knowledge, there is no information available looking at bumble bee chronic exposure to these chemicals. Therefore, due to lack of scholarly literature and lack of funds that would allow for our own colony level pesticide analyses (rather than field) we have no real idea of the effects of the chemicals on the pollination ability of the commercial bumble bees within the field. Further, we call for such future analyses to be considered and conducted, as we believe not enough is known about chronic exposure of pesticide sprays to pollinators both commercially managed and native. In the meantime, as some pesticides detected in this study are those sprayed during many mosquito control programs we call for the creation of bumble bee registry programs. The bumble bee registry can be run in conjuncture with existing state honey bee registries for the notification of mosquito abatement sprays to protect the purchased commercial bumble bee pollinators.

• References cited
• Graphs representing results from the field and Table showing pesticide esposure levels