Scale insects: An emergent threat to Southern California salt marshes

Background/Question/Methods

Herbivores can strongly reduce marsh plant growth in Atlantic marshes, with potentially huge cascading effects on important ecosystem functions. While these dramatic grazing effects are rare in Pacific marshes, it is unclear if this is a result of a bias in research effort. A potential threat to Southern California marshes is the scale insect, Haliaspis spartinae – a feeding specialist on Pacific cordgrass, Spartina foliosa. Although this scale insect is expanding into new marshes and it has been implicated in the failure of restored marshes to achieve functional equivalence to nearby natural marshes, tests of the consequences of scale infestations on cordgrass are lacking.  We conducted a scale removal experiment in south San Diego Bay for 19 weeks.  During weekly visits, we 1) measured several cordgrass traits, 2) counted scales on each leaf on each experimental stem, and 3) maintained treatments.

Results/Conclusions

Haliaspis infestations reduced Spartina growth by 36%, survivorship by 70%, maximum quantum yield by 10%, and seed production by 59%. Between May and October, we identified two large dispersal periods. Weekly increases in scale populations were much greater on +Scale plants suggesting either that 1) dispersal is highly localized to the parent’s stem or 2) juveniles recruit preferentially to stems containing adults. If dispersal is local, then commonly used revegetation strategies that transplant stems between marshes may inflate the dispersal potential of this species. Thus, the identification of the causes and consequences of threats to these protected habitats will be critical to future conservation and management efforts.