Soil tillage is the most energy consuming of all agricultural field operations (Chauhan et al., 2012). In addition to monetary and environmental cost of fossil fuels, the time and labor required for tillage are also significant factors for farmers to consider. Herbicides reduce the need for tillage (Gianessi and Reigner, 2007), but can lead to herbicide resistance in weeds and unintended effects on beneficial organisms (Liebman, 2001; Powles and Yu, 2010).
There has been recent interest in alternative weed control practices for organic growers who rely heavily on mechanical cultivation, timely hand weeding, or both. Organic and conventional growers may value an alternative to mechanical cultivation since tillage can increase soil erosion, degrades soil structure and microbial communities, and releases sequestered carbon dioxide back into the atmosphere (Gianessi and Reigner, 2007; Hobbs et al., 2008). Concerns that the overuse of tillage or herbicides may have negative impacts on human health and ecological integrity, plus increased time and labor costs for growers, indicate a need for weed control practices that mitigate these effects while still maximizing yield (Liebman, 2001).
Mulching can control weeds in fruit and vegetable crops with the added benefit that it limits the need for tillage and exposure of beneficial insects to potentially harmful pesticides. A wide variety of mulch materials have been studied, including plastic film, wood fiber, paper, and living or dead plant material. Many novel mulch materials remain to be investigated with regard to their ability to suppress weeds and improve crop yields. The effects of novel mulches on crop pollinators should also be studied to ensure their overall positive effect on crop management.
Although crop pollinators in general may benefit from reductions in herbicide use and tillage operations, pollinators that use agricultural fields for nesting deserve consideration. One such pollinator is the squash bee, a major pollinator of squash, pumpkins, and gourds (Cucurbita sp.). It is a solitary, ground-nesting species that produces one generation per year (Williams et al., 2009). The females make one or more nests per season where eggs are deposited and provisioned solely with squash pollen. Males and females forage on nectar and pollen, and females tend to nest directly under the squash plants from which they forage (Roulston and Goodell, 2011). Female squash flowers must be visited a minimum of seven times for optimum seed set and fruit development, depending on the crop cultivar and bee species (Julier and Roulston, 2009; Walters and Taylor, 2006). Each flower is open for 1 d, from about sunrise until noon (Nepi et al., 1996; Tepedino, 1981).
Farms with consistent management practices provide good study environments for oligolectic (specialist) bee species since current generations of bees have developed from the resources that were available to the previous generation (Roulston and Goodell, 2011). For example, weed management strategies used within squash fields could directly or indirectly impact the population of squash bees present. A decrease in the abundance of squash bees may limit pollen transfer needed for fruit production since squash has a relatively high pollination demand (Julier and Roulston, 2009; Walters and Taylor, 2006). Insufficient wild pollinators could also raise production costs for growers if honeybee (Apis mellifera) colonies need to be purchased during the relatively long squash-flowering season.
Squash bees are expected to show high sensitivity to insecticide application and tillage because they nest within squash fields and have no noncrop host plants in most of their range (Hurd et al., 1974). Squash bees place the greatest density of their brood cells 16 to 30 cm under the soil surface (Mathewson, 1968), which overlaps with some tillage depths. The survival and abundance of native pollinators depends in part upon the availability of food and nesting resources within foraging range. Herbicide application may indirectly harm bees by reducing floral abundance and foraging resources on farms (Holzschuh et al., 2008). Although squash bees collect pollen exclusively from squash, it has been known to collect nectar from other floral hosts (K. Goodell, unpublished data).
The use of mulch for weed control could alleviate some incidental risks posed to pollinators by herbicides and tillage, but could also alter important aspects of pollinator habitat with negative consequences for squash bee populations and crop pollination. Mulch applied to the soil surface acts as a physical barrier to weed emergence and could similarly prevent bee nesting. Squash bees prefer irrigated soils that are low in clay content (Roulston and Goodell, 2011). The way in which mulch alters the soil microclimate may positively or negatively influence nest site preference or larval development (Roulston and Goodell, 2011).
Mulches may also affect resource allocation patterns of plants that influence fruit development and the production of floral resources, such as nectar and pollen. Nectar quantity and composition vary widely due to environmental factors such as temperature, soil moisture, and humidity (Cruden et al., 1983; Fahn, 1979; Wyatt et al., 1992). In addition, mulches that immobilize nitrogen because of a high carbon to nitrogen ratio may influence the quality (e.g., protein and sugar content) of the floral resources produced and thus pollinator visitation and crop pollination.
The objective of this research was to determine the effect of mulch materials on floral resource production, pollination, and fruit set of zucchini, as well as the potential for nesting by its specialist pollinator, the squash bee. Specifically, we tested the following hypotheses: 1) particulate mulch materials will decrease soil temperature fluctuation and increase soil moisture levels in comparison with black plastic and bare soil; 2) particulate mulches will be suitable areas for squash bees to nest, whereas black plastic sheet mulch will prohibit squash bees from nesting; 3) mulches that increase soil moisture will increase the quantity and quality of nectar and pollen produced; and 4) mulches that reduce pollen and nectar abundance and quality will reduce pollination of flowers and be associated with aborted or misshapen, poor quality fruit. We tested these hypotheses by investigating mulch effects on soil moisture and temperature, crop plant growth, floral resource production, pollination and fruit set, and squash bee nesting.
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