Rachael Long

Insecticide Use in Hybrid Onion Seed Production Affects Pre- and Postpollination Processes

ABSTRACT Research on threats to pollination service in agro-ecosystems has focused primarily on the negative impacts of land use change and agricultural practices such as insecticide use on pollinator populations. Insecticide use could also affect the pollination process, through nonlethal impacts on pollinator attraction and postpollination processes such as pollen viability or pollen tube growth. Hybrid onion seed (Allium cepa L., Alliaceae) is an important pollinator-dependent crop that has suffered yield declines in California, concurrent with increased insecticide use. Field studies suggest that insecticide use reduces pollination service in this system. We conducted a field experiment manipulating insecticide use to examine the impacts of insecticides on 1) pollinator attraction, 2) pollen/stigma interactions, and 3) seed set and seed quality. Select insecticides had negative impacts on pollinator attraction and pollen/stigma interactions, with certain products dramatically reducing pollen germination and pollen tube growth. Decreased pollen germination was not associated with reduced seed set; however, reduced pollinator attraction was associated with lower seed set and seed quality, for one of the two female lines examined. Our results highlight the importance of pesticide effects on the pollination process. Overuse may lead to yield reductions through impacts on pollinator behavior and postpollination processes. Overall, in hybrid onion seed production, moderation in insecticide use is advised when controlling onion thrips, Thrips tabaci, on commercial fields.

Indirect Effects of Field Management on Pollination Service and Seed Set in Hybrid Onion Seed Production.

ABSTRACT Pollination in crops, as in native ecosystems, is a stepwise process that can be disrupted at any stage. Healthy pollinator populations are critical for adequate visitation, but pollination still might fail if crop management interferes with the attraction and retention of pollinators. Farmers must balance the direct benefits of applying insecticide and managing irrigation rates against their potential to indirectly interfere with the pollination process. We investigated these issues in hybrid onion seed production, where previous research has shown that high insecticide use reduces pollinator attraction. We conducted field surveys of soil moisture, nectar production, pollinator visitation, pollen—stigma interactions, and seed set at multiple commercial fields across 2 yr.We then examined how management actions, such as irrigation rate (approximated by soil moisture), or insecticide use could affect the pollination process. Onions produced maximum nectar at intermediate soil moisture, and high nectar production attracted more pollinators. Insecticide use weakly affected pollinator visitation, but when applied close to bloom reduced pollen germination and pollen tube growth. Ultimately, neither soil moisture nor insecticide use directly affected seed set, but the high correlation between pollinator visitation and seed set suggests that crop management will ultimately affect yields via indirect effects on the pollination process.

Effect of Liquid and Dry PAM Applications on Sediment Concentration in Surface Runoff from Furrow-irrigated Fields

The effect of both liquid (oil-based (37% PAM) and water-based (25% PAM) formulations) and dry PAM (granules and tablets) on sediment concentration of surface runoff during furrow irrigation was investigated on loam and clay loam soils. Liquid PAM was highly effective in reducing the sediment load. Granules/tablets placed 45 m or 90 m from the head of the furrow were also effective, but, dry material placed at the head of the furrow were covered up with sediment and had no effect on sediment concentration. Sediment concentrations at the start of surface runoff were higher for the granules/tablets compared to the liquid PAM applications, but similar sediment concentrations occurred several hours after the start of runoff. PAM had no effect of infiltration.