Bumblebees (Bombus spp.) are social organisms which reproduce at the level of the colony [see life history info here]. With colony cycles that last for many weeks, bumblebee colonies have high nutritional demands. When food (i.e., pollen and nectar) is scarce, population size can be negatively affected through decreased colony growth and reduced or failed reproduction. In my study area, northern Virginia, my own data show that annual drought conditions cause a steep decline in flowering plants in July, beginning a period of flowering plant scarcity that can last a month or more (Figure 1). Long-term studies suggest that mid-summer periods of flowering resource scarcity are becoming more pronounced due to climate change. This dynamic poses a potentially significant challenge for bumblebees with long colony cycles that extend into the late summer, and is of particular interest because among declining species, those with seasonal activities that extend into the late summer are disproportionately represented. One hypothesis regarding this trend is that late-season bumblebees are more prone to extirpation because late summer food resources are more limiting.
Figure 1: Floral resource abundance curves for 2010 and 2012 at Blandy Experimental Farm (Boyce, VA). In the study region, annual drought conditions result in floral resource scarcity starting in mid- to late July. In 2012, this period of dearth was somewhat delayed and mitigated by rainier conditions. Although a pulse of resources arrives in the fall, this peak is small relative to the maximum flower abundance of early summer. Note: in 2010, a pulse of flowers arrived in the fall; however, this is not graphically featured here. The Common Eastern Bumblebee (B. impatiens) has a long population cycle that extends into the period of food scarcity described. Data collected by Malfi & Roulston.
The impact of seasonal food scarcity on bumblebee (Bombus impatiens) colony success; are late-season bumblebees at elevated risk?
Study Objective and Methods: My goal in this study was to evaluate how apparent food scarcity, characteristic of the study region in mid-summer (see above), affects colonies of a bumblebee species that is active across a seemingly risky period of drought. In the study region, Bombus impatiens is the only common species that remains active into the late summer. Available information suggests that additional native species with this same phenological pattern were once present in the region. Although we do not have adequate historical records to confirm the extent of loss of late-season bees from the study area, current research does indicate that late-season bees are particularly vulnerable to decline.
To assess how colony growth and reproduction are affected by this temporal resource dynamic, I conducted two large field experiments in 2012 and 2013 in which I monitored 8 and 24 Bombus impatiens colonies over their lifetimes. In the first season, half of the colonies (n = 4) were given food supplements (pollen and artificial nectar) and half were given no supplements. In 2013, I expanded upon this experiment by using 24 colonies. In this second season, colonies were subjected to one of four food treatments (n = 6 per treatment); colonies were given food supplements (i) never, (ii) all season, (iii) the first six weeks of the season (June 1- July 15), or (iv) in the second six weeks of the season (July 15 - August 31). For the duration of each experiment, weekly night-time counts of workers, males, and new queens were conducted for each colony. In each season, comprehensive surveys of two large meadows were carried out weekly to obtain estimates of local floral resource abundance. Each experiment was designed so that a comparison of food treatment groups will reveal whether (and when) ambient levels of resources limited colony growth and reproductive output; the second experiment, being larger, allowed for the testing of additional hypotheses as well. Through the 2013 data, I will also assess how colony size influences colony survival, resource acquisition, and reproduction.
Analysis of this data is currently underway - stay tuned for more details!
Field Experiment 2013