Edith Ladurner

Foraging and Nesting Behavior of Osmia lignaria (Hymenoptera: Megachilidae) in the Presence of Fungicides: Cage Studies

Abstract During orchard pollination studies in California, we observed dramatic changes in nesting and foraging behavior of Osmia lignaria Say (Hymenoptera: Megachilidae) after sprays with tank mixtures containing fungicides. A characteristic pattern of postspray events observed includes erratic behavior and interrupted foraging and nesting activity for several days. In an effort to determine whether fungicidal sprays were disruptive to bee foraging and thus to pollination, we exposed O. lignaria females nesting in field cages planted with lacy scorpionweed, Phacelia tanacetifolia Benth (Hydrophyllaceae), to selected spray mixtures normally encountered in California orchard production systems: iprodione (Rovral), propiconazole (Orbit), benomyl (Benlate), and captan (Captan 50 WP); the surfactant Dyne-Amic, alone and mixed with Rovral; and the tank mixture IDB (Rovral + Dyne-Amic + the foliar fertilizer Bayfolan Plus). An additional cage sprayed with an equal volume of water acted as control, and a cage sprayed with the insecticide dimethoate as a toxic standard. For each female O. lignaria, we recorded time spent inside the nest depositing pollen-nectar loads, foraging time, cell production rate, and survival. All females in the dimethoate treatment died postspray + 1 d. Before death, some of these females behaved similarly to our previous orchard observations. A high proportion of females in the IDB cage were inactive for a few hours before resuming normal foraging and nesting activity. No lethal or behavioral effects were found for any of the other compounds or mixtures tested. Our results indicate that the fungicide applications that we tested are compatible with the use of O. lignaria as an orchard pollinator.

Osmia cornuta (Hymenoptera Megachilidae) densities required for apple pollination: a cage study

SUMMARY To estimate Osmia cornuta densities required for pollination of ‘Braeburn’ apple, trees were caged and supplied with different bee densities in 2000 and 2001. The following pollination treatments were compared: (1) one O. cornuta pair per five trees: 5 adjacent trees, 2 ‘Braeburn’, 1 ‘Granny Smith’ (pollenizer) and 2 ‘Braeburn’, caged with 1 female and 1 male O. cornuta; (2) one O. cornuta pair per tree: 5 trees as above, caged with 5 female and 5 male O. cornuta; (3) open pollination: 5 trees as above, uncaged; (4) no pollinators: 5 trees as above, caged without pollinating insects. Fruitlet-set (fruitlets retained on the tree before June drop), misshapen fruit-percentages (percent fruits with at least one empty carpel), and seed-set (number of seeds per fruit) were measured. Cages with no pollinators had the lowest fruitlet-set (2000: 11.5%; 2001: 7.7%) and seed-set (2000: 1.6; 2001: 1.9), and the highest proportion of misshapen fruits (2000: 96.8%; 2001: 96.3%) in both years. Values obtained in cages with one O. cornuta pair per five trees were not significantly different from those obtained in open-pollinated trees. Cages with one O. cornuta pair per tree had fruitlet-set (2000: 37.9%; 2001: 20.4%) similar to cages with one pair per five trees (2000: 33.0%; 2001: 18.6%) or to open-pollinated trees (2000: 31.0%; 2001: 21.7%), but had significantly higher seed-set (2000: 7.5 vs. 5.7 and 6.0, respectively; 2001: 5.6 vs. 4.3 and 4.0, respectively) and lower misshapen fruit incidence (2000: 27.5% vs. 54.0% and 56.5%, respectively; 2001: 54.8% vs. 71.5% and 76.1%, respectively) in both years. One O. cornuta pair per five trees can provide commercially acceptable fruitlet-set, whereas one O. cornuta pair per tree can ensure maximum seed-set and thus high-quality yields on ‘Braeburn.’