Testing the enemy release hypothesis in a native insect species with an expanding range

The enemy release hypothesis (ERH) predicts that the spread of (invasive) species will be facilitated by release from their enemies as they occupy new areas. However, the ERH is rarely tested on native (non-invasive, long established) species with expanding or shifting ranges. I tested the ERH for a native damselfly (Enallagma clausum) whose range has recently expanded in western Canada, with respect to its water mite and gregarine parasites. Parasitism levels (prevalence and intensity) were also compared betweenE. clausumand a closely related species,Enallagma boreale, which has long been established in the study region and whose range is not shifting. A total of 1,150 damselflies were collected at three ‘old’ sites forE. clausumin Saskatchewan, and three ‘new’ sites in Alberta. A little more than a quarter of the damselflies collected were parasitized with, on average, 18 water mite individuals, and 20% were parasitized by, on average, 10 gregarine individuals. I assessed whether the differences between levels of infection (prevalence and intensity) were due to site type or host species. The ERH was not supported:Enallagma clausumhas higher or the same levels of parasitism in new sites than old sites. However,E. borealeseems to be benefitting from the recent range expansion of a native, closely related species through ecological release from its parasites because the parasites may be choosing to infest the novel, potentially naïve, host instead of the well-established host.

Parasitism of damselflies (Enallagma boreale) by gregarines: sex biases and relations to adult survivorship

We studied host damselflies Enallagma boreale (Odonata: Coenagrionidae) and their gregarine parasites (Apicomplexa: Eugregarinidae) to elucidate the causes and consequences of any sex biases in parasitism of adult hosts. Larvae of both sexes were highly infected, but there was no difference between male and female larvae in either prevalence or intensity of gregarine infections. Newly emerged adults had few or no parasites, thereby setting the stage for investigating accumulation of parasites by adults. Adult females had a higher prevalence and intensity of infection by gregarines than did males, but only on 1 (of 2) days when the potential confounding factor of host age was controlled for. Both adult males and females showed a positive correlation between longevity under conditions of food stress and the number of gregarines they initially carried. This finding may be explained if the food ingested with the infective cysts is more beneficial than the parasites are harmful, and it also has implications for investigating sex biases in numbers of trophically transmitted parasites of such insects.

Female morphs of the damselfly Enallagma boreale Selys (Odonata: Coenagrionidae): a benefit for androchromatypes

Females of the coenagrionid damselfly Enallagma boreale Selys occur as three distinct colour morphs at a study site in eastern Ontario, Canada; > 65% are blue (termed androchromatypes) like the conspecific male, ca. 30% are green, and < 5% have both blue and green on their abdomen. I provide experimental evidence which suggests that one advantage of androchromatypism is avoidance of harassment from heterospecific males of the congeneric damselfly Enallagma ebrium (Hagen).

The effect of temperature and photoperiod on larval development in Odonata

Enallagma boreale (Selys), a summer species, and Leucorrhinia glacialis (Hagen) and Libellula quadrimaculata (Linné), both spring species, were reared at various temperatures and photoperiods.E. boreale developed fastest at all temperatures, supporting my hypothesis that summer species have higher thermal coefficients for growth than spring species, but refuting my hypothesis that spring species grow faster at low temperatures.Photoperiod affected the development rate of L. glacialis and L. quadrimaculata at low temperatures, but did not affect E. boreal at any temperature, supporting my hypothesis that spring species are most likely to use photoperiod in regulating development. The striking growth-rate responses of the spring species to photoperiod at low temperatures suggest photoperiod is important in regulating development in temperate regions.

Survival and mortality of coenagrionid nymphs (Odonata: Zygoptera) from the ice of an aestival pond

Live coenagrionid nymphs of three species, Coenagrion angulatum Walker, C. resolution (Hagen), and Enallagma boreale Selys, were recovered from the ice of an aestival pond. The percentage of such nymphs surviving decreased progressively throughout the winter, but some remained viable by the time of the spring thaw. It is suggested that the level of mortality was unusually high as a result of lower than normal temperatures in the ice.