Interaction of field-isolated fungi with monarch butterfly larvae and Bt maize pollen in Iowa
This research explores the interactions of monarch butterfly larvae, Danaus plexippus, with the phylloplane of milkweed leaves, Bt-maize pollen and fungi. Previous research has shown that pollen from event 176 maize is hazardous to monarch larvae. Yet, it is unknown whether fungal colonization of this pollen neutralizes or magnifies this hazard. Colony forming units (CFU) of fungi and pollen density were quantified from surfaces of milkweed leaves collected during the 2002 maize season from inside and outside three maize fields for eight sampling dates. Pollen deposition significantly differed among fields, dates, and positions (inside and outside of maize fields). The CFU values significantly differed among dates, and peaks in CFU were observed both inside and outside maize fields when pollen deposition peaked. Alternaria, Aspergillus, Cladosporium, Fusarium, Gliocladium, Penicillium, Trichoderma, and Zygomycetes were the most prevalent fungi isolated and identified. Fusarium verticillioides/proliferatum was the most frequently observed species grouping among sampling dates both inside and outside maize fields. Many fungi that were isolated from milkweed phylloplanes can produce mycotoxins. In greenhouse studies, milkweed leaves were sprayed with a fungus (Fusarium sporotrichioides or Cladosporium), event 176 pollen, a combination of fungus and pollen, or water. Two weeks later, larvae that were put on leaf discs with either fungus consumed less tissue and weighed less compared to those put on the water-treated leaf discs. Fungi present with even 176 pollen on milkweed leaf discs did not significantly reduce Bt pollen toxicity to monarch larvae. In another study a suspension of event 176 pollen was sprayed onto glass slides followed by water or a fungal conidial suspension (Alternaria, Cladosporium, F. proliferatum, and F. sporotrichioides) and incubated for up to 21 days. Cry1Ab concentrations for event 176 pollen sprayed with conidial suspensions and water did not significantly differ. However, Cry1Ab concentrations differed significantly among incubation periods as degradation occurred. Fungi present on milkweed phylloplanes were shown to affect monarch larvae, but fungi were not found to significantly influence the degradation of Cry1Ab protein in event 176 pollen.