Life history of Galerucella nymphaeae and implications of reproductive diapause for rearing univoltine chrysomelids

Galerucella nymphaeae L., a chrysomelid that feeds on Lythrum spp., water lily and water chestnut, is closely related to two European species that were recently introduced into North America for biological control of L. salicaria, purple loosestrife. To develop a paradigm for continuously rearing these and other univoltine chrysomelids, we conducted field and laboratory studies on G. nymphaeae's development, reproduction and diapause. A high incidence of reproduction without diapause occurred when fourth instars, pupae, and adults (held as pairs) experienced very long daylengths (LD 18:6h), i.e. longer than those G. nymphaeae encounters in nature. Under similar photoperiodic conditions, adults maintained in groups showed a significantly higher rate of reproductive diapause than those held as pairs. Females laid three to seven egg masses/week, and the size of egg masses varied between nine and nineteen eggs. During their reproductive lifetimes, individual females showed a highly significant propensity to lay a consistent number of eggs/egg mass. After diapausing under short daylengths and low temperature (LD 10: 14h, 5 degrees C), adults transferred to long days (LD 18:6h at 21 degrees C) had high rates of diapause termination and postdiapause oviposition. In contrast, those transferred to short daylengths (LD 10:14h at 21 degrees C) had low rates of reproduction. Laboratory-derived heat-degree models accurately predicted egg and pupal, but not larval, development in the field. In nature, most females in the summer generation entered reproductive diapause without ovipositing; a small proportion of females that emerged relatively early (by mid June) oviposited before entering diapause. The overwintering population consists of adults from the first-generation and a small number from the second generation.