Predicted fates of ground-nesting bees in soil heated by wildfire: Thermal tolerances of life stages and a survey of nesting depths

Authors: Cane, James; NEFF, JOHN - Retired Non ARS Employee

Submitted to: Biological Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2011
Publication Date: 8/11/2011
Citation: Cane, J.H., Neff, J.L. 2011. Predicted fates of ground-nesting bees in soil heated by wildfire: Thermal tolerances of life stages and a survey of nesting depths. Biological Conservation. 144: 2631-6.

Interpretive Summary: The frequency, intensity, and extent of wildfire has been shifting dramatically in the Western United States over the past 30 years. The results have generally been detrimental to plant communities of Western range lands and forests, although wildflowers sometimes respond with greater bloom. The post – fire fates of native bees that pollinate those wildflowers has not been known, but it's critical for understanding their prospects for pollination. Most of these bees are not social and nest underground. We experimentally demonstrated that the four life stages of solitary bees survived soil heating of 122F but die at 130F. Only the top 2 inches of soil heats this much under fires where bees nest. Only 9% of 460 species of ground nesting bees nest this shallowly and are thus vulnerable to wildfire’s heat. Most wild bees will survive wildfire and will need bloom, either recovering or seeded, to feed the next season’s generation.

Technical Abstract: Periodic wildfire defines plant community composition and dynamics in many of the world’s semi-arid biomes, whose climates and floras also favor wild bee diversity. Most solitary bee species in these biomes nest underground (so-called “mining” bees). To evaluate their susceptibility to fire, we tested heat tolerances of bees’ four life stages using an experimental design that mimicked heat conduction dynamics of natural soils. All life stages survived heating of up to 45° C for 27 min, but none survived heating at 54° C for nine mins; the prepupal stage survived best. At near-lethal temperatures, more prolonged heating caused more mortality. These data were related to published studies of heat penetration depths in soil during fire and an exhaustive compilation of published nesting depths reported for mining bees. We conclude that only a small minority (9%) of the shallowest-nesting mining bees are likely to die from soil heating by wildfire. Among these, ground-nesting megachilids (Osmia, Megachile) will be most vulnerable, owing to their shallow horizontal nests. Because mining bees prevail in most biomes of the temperate zone, any vegetation rehabilitation efforts should therefore expect that bee communities will largely survive the immediate effects of wildfire, and therefore will need pollen and nectar resources in the year(s) after fire.