Abstract
The effects of two herbivorous insects, Galerucella calmariensis Duftschmid and Myzus lythri L. (Coleoptera: Chrysomelidae), feeding on purple loosestrife, Lythrum salicaria L. (Myrtiflorae: Lythraceae), were measured in the presence of two insect predators, Harmonia axyridis Pallas (Coleoptera: Coccinellidae) and Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae). A greenhouse cage experiment examined the direct effects of these predators on these herbivores, and indirect effects of predation on aboveground biomass, defoliation, number of leaves, and internode length. Eight treatment combinations with G. calmariensis, M. lythri, H. axyridis and C. carnea were applied to caged L. salicaria. The experiment ended when G. calmariensis adults were observed, 11 to 13 days after release of first instar G. calmariensis. G. calmariensis larvae alone removed significant amounts of leaf tissue and reduced the number of L. salicaria leaves. Predators did not reduce levels of defoliation by G. calmariensis. C. carnea had no effect on G. calmariensis survival, but H. axyridis reduced G. calmariensis survival in the presence of M. lythri. Both predators reduced the survival of M. lythri. This short duration greenhouse study did not demonstrate that predator-prey interactions altered herbivore effects on L. salicaria.
Highlights
Purple loostrife, Lythrum salicaria L. (Myrtiflorae: Lythraceae) is an invasive weed with high fecundity that displaces native vegetation (Thompson et al 1987; Balough and Bookhout 1989; Blossey et al 2001)
In a second cage study, ten L. salicaria plants were enclosed with 45 G. pusilla eggs in individual cages for 35 days resulting in 14% defoliation (Wiebe 2001)
Over 50% defoliation was measured in L. salicaria plants with G. calmariensis only
Summary
Lythrum salicaria L. (Myrtiflorae: Lythraceae) is an invasive weed with high fecundity that displaces native vegetation (Thompson et al 1987; Balough and Bookhout 1989; Blossey et al 2001). Since 1994, more than 1.4 million individual Galerucella spp. have been released in Iowa wetlands (J.J. Obrycki, unpublished data). In a field cage study, Cortilet (1998) demonstrated that the percentage defoliation and terminal bud damage of L. salicaria increased with increasing G. calmariensis larval density. After 47 days, 50 G. calmariensis larvae caused 25% defoliation and more than 20 terminal buds per stem were damaged (Cortilet 1998). In a second cage study, ten L. salicaria plants were enclosed with 45 G. pusilla eggs in individual cages for 35 days resulting in 14% defoliation (Wiebe 2001). Katovich et al (1999) released 50 Galerucella spp. adults and larvae on caged L. salicaria plants for two months, resulting in an average of 86% defoliation
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