title: Pesticide mixtures, endocrine disruption, and amphibian declines: are we underestimating the impact?
abstract: Amphibian populations are declining globally at an alarming rate. Pesticides are among a number of proposed causes for these declines. Although a sizable database examining effects of pesticides on amphibians exists, the vast majority of these studies focus on toxicological effects (lethality, external malformations, etc.) at relatively high doses (parts per million). Very few studies focus on effects such as endocrine disruption at low concentrations. Further, most studies examine exposures to single chemicals only. The present study examined nine pesticides (four herbicides, two fungicides, and three insecticides) used on cornfields in the midwestern United States. Effects of each pesticide alone (0.1 ppb) or in combination were examined. In addition, we also examined atrazine and S-metolachlor combined (0.1 or 10 ppb each) and the commercial formulation Bicep II Magnum, which contains both of these herbicides. These two pesticides were examined in combination because they are persistent throughout the year in the wild. We examined larval growth and development, sex differentiation, and immune function in leopard frogs (Rana pipiens). In a follow-up study, we also examined the effects of the nine-compound mixture on plasma corticosterone levels in male African clawed frogs (Xenopus laevis). Although some of the pesticides individually inhibited larval growth and development, the pesticide mixtures had much greater effects. Larval growth and development were retarded, but most significantly, pesticide mixtures negated or reversed the typically positive correlation between time to metamorphosis and size at metamorphosis observed in controls: exposed larvae that took longer to metamorphose were smaller than their counterparts that metamorphosed earlier. The nine-pesticide mixture also induced damage to the thymus, resulting in immunosuppression and contraction of flavobacterial meningitis. The study in X. laevis revealed that these adverse effects may be due to an increase in plasma levels of the stress hormone corticosterone. Although it cannot be determined whether all the pesticides in the mixture contribute to these adverse effects or whether some pesticides are effectors, some are enhancers, and some are neutral, the present study revealed that estimating ecological risk and the impact of pesticides on amphibians using studies that examine only single pesticides at high concentrations may lead to gross underestimations of the role of pesticides in amphibian declines.
authors.affiliation: Laboratory for Integrative Studies in Amphibian Biology, Department of Integrative Biology, Museum of Vertebrate Zoology, Group in Endocrinology, University of California, Berkeley, California 94720-3140, USA. tyrone@berkeley.edu
authors.forename: Tyrone B
authors.initials: TB
authors.lastname: Hayes
authors.forename: Paola
authors.initials: P
authors.lastname: Case
authors.forename: Sarah
authors.initials: S
authors.lastname: Chui
authors.forename: Duc
authors.initials: D
authors.lastname: Chung
authors.forename: Cathryn
authors.initials: C
authors.lastname: Haeffele
authors.forename: Kelly
authors.initials: K
authors.lastname: Haston
authors.forename: Melissa
authors.initials: M
authors.lastname: Lee
authors.forename: Vien Phoung
authors.initials: VP
authors.lastname: Mai
authors.forename: Youssra
authors.initials: Y
authors.lastname: Marjuoa
authors.forename: John
authors.initials: J
authors.lastname: Parker
authors.forename: Mable
authors.initials: M
authors.lastname: Tsui
mesh_terms: D000663:Amphibians; D000818:Animals; D049628:Body Size / Q000187:drug effects; D003345:Corticosterone / Q000097:blood; D017753:Ecosystem; D052244:Endocrine Disruptors / Q000493:pharmacokinetics / Q000633:toxicity*; D004781:Environmental Exposure; D005260:Female; D005417:Flavobacterium / Q000472:pathogenicity; D020387:Food Chain; D006066:Gonads / Q000166:cytology / Q000187:drug effects; D008297:Male; D008675:Metamorphosis, Biological / Q000187:drug effects; D010575:Pesticides / Q000493:pharmacokinetics / Q000633:toxicity*; D011894:Rana pipiens; D018570:Risk Assessment; D012733:Sex Differentiation; D046468:Sexual Development / Q000187:drug effects; D013950:Thymus Gland / Q000166:cytology / Q000187:drug effects / Q000276:immunology; D013997:Time Factors; D014982:Xenopus laevis
chemical_list: D052244:Endocrine Disruptors; D010575:Pesticides; D003345:Corticosterone