Risk to Pollinators from the Use of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 254

(Clements and Bale 1988 ; Tashiro and Kuhr 1978 ). However, the potential for use
of these data to provide a worst-case estimate of CPY exposure to ground nesting
bees through soil is limited, because it is unclear how much of this mortality was
due to contact with soil, versus ingestion of CPY on plant roots. Different behav-
ioral and physiological differences between soil pests and ground nesting pollina-
tors might influence uptake of CPY from soil.
Several factors would likely reduce the risk of CPY to ground nesting bees. The
tendency of CPY to adsorb to soil surfaces reduces bioavailability of CPY in most
soil environments (Racke 1993 ), but the time for toxicity to drop below levels that
cause mortality or sublethal effects on ground nesting pollinators is unknown. The
dearth of studies on nesting of bees within agriculture fields (Julier and Roulston
2009 ; Kim et al. 2006 ; Williams et al. 2010 ; Wuellner 1999 ) suggests that most spe-
cies nest outside cultivated fields where the risk of exposure is low. Many of these
are solitary bees that forage specifically on non-agricultural plants and nest in non-
agricultural soil (Willmer 2011 ). Nonetheless there are some important pollinators
including squash bees ( Peponapis sp.) that nest within crops (Julier and Roulston
2009 ), where the potential for exposure to CPY is much higher. Although CPY is
not registered for use on squash or cucurbit crops pollinated by squash bees, there
may be other bees that do nest in crops treated with CPY.
Exposure might also vary depending on the architecture of cells within the nest.
Although cells of some nests are unlined excavations into the soil, those of other
species are lined with a cellulose- or wax-like material (Michener 2007 ) that possi-
bly provides a barrier from direct contact with soil. Even without a lining, the poten-
tial for transfer of residues from soil to the insect during entry and exit from a nest is
much lower than when the insect is digging through the soil. Immature life stages are
expected to be less exposed than adults. For example, bumble bees usually nest
underground, but larvae develop within their own cocoons. Eggs and larvae cannot
actually contact the soil at all until late development and adult emergence (Michener
2007 ), and their dietary exposure is limited to the levels that can be successfully
brought to the hive by adult foragers. Contact with freshly contaminated soil is not
likely to be a major contribution to aggregate CPY exposure for ground-nesting bees,
but it has not been characterized or quantified and remains an area of uncertainty.
No studies were found, in which soil was collected from mason bee ( Osmia spp.)
nests from within areas exposed to CPY. Exposure via soil in this group is limited
by the rapid degradation and limited bioavailability of CYP on soil. As with other
nesting bees, CPY exposure of immature life stages is limited to concentrations that
are collected by adults, and because larvae do not emerge until long after the nest is
built, CPY residues on this soil would be negligible.

5.3 Strengths and Uncertainties

The current assessment delineated potential exposure pathways of CPY to pollina-
tors in detail. Sufficient exposure and effects data relevant to honey bees were avail-
able and permitted a satisfactory characterization of the risk of CPY to them.