Three physiological variables, haematocrit, haemoglobin concentration and ventilation frequency, were measured to test how fathead minnows Pimephales promelas and small and large yellow perch Perca flavescens responded to three different dissolved oxygen concentrations. All fish were monitored continuously for any indications of stress in response to these manipulations. Within and between species, smaller individuals were the most tolerant of hypoxic environments. A species effect, however, did contribute to this observation, with fathead minnows being more tolerant of hypoxic environments than similar-sized yellow perch. In aquatic ecosystems where smaller fishes are more tolerant to hypoxia than their larger predators, hypoxic environments may have the potential to act as a refuge from predators.

Bark beetles (Coleoptera: Scolytidae) are commonly associated with live host trees that are stressed, a relationship that has been attributed to lower host defenses or greater nutritional quality of these trees. However, most bark beetle species commonly inhabit freshly dead trees where induced host defenses are absent. In this study, we investigate the role of tree vigor at the time of death for pine engraver bark beetles, Ips pini (Say), breeding in freshly dead jack pine, Pinus banksiana Lamb. As indices of tree vigor, we considered tree size, phloem thickness, and several measures of recent growth rate (last year's growth increment, mean annual increment and basal area increment in the past 5 and 10 years, and periodic growth ratio). We examined the relationship between these indices in three stands, aged 60, 77, and 126 years, and found that phloem thickness, previously shown to have a strong positive effect on bark beetle reproduction, was only weakly associated with tree growth rate and inconsistently related to tree size among the three stands. To examine the effects of tree vigor on pine engraver reproduction, we felled 20 trees of various sizes from the 77-year-old stand, and experimentally established breeding males and females in 25-cm-long sections. Offspring were collected and characteristics of breeding galleries were measured. Using stepwise regression, we consistently found that indices associated with tree growth rate best explained beetle reproductive performance, as they were positively related to parental male and female establishment on logs, female reproductive success, length of egg galleries, proportion of eggs resulting in emerged offspring, and negatively related to the length of the post-egg gallery. Surprisingly, phloem thickness had no unique effect on pine engraver reproduction, except for a weak negative effect on the establishment success of parental females. The strong effect of tree vigor observed in this study suggests that substantial mortality of vigorous trees, such as caused by windthrow, can contribute to significant increases in bark beetle populations that could trigger outbreaks in living trees.

Abstract.  1. Defence against parasites and pathogens can be essential, yet not all hosts respond similarly to parasitic challenge. Environmental conditions are thought to explain variation in host responses to parasites.2. Lestes forcipatus damselflies emerging later in the season have shown higher resistance to the mite, Arrenurus planus, than hosts emerging earlier. This study was undertaken to determine whether variation in environmental temperatures characteristic of early vs. late emergence times, degree or costs of mite parasitism, and/or size of newly emerged adults could explain seasonal variation in defence and resistance to ectoparasitic mites.3. In this study damselflies from early vs. late emergence groups differed in size at emergence and mite intensity. In general, early hosts were larger and had more mites than later hosts. However only experimental temperatures experienced by damselflies at emergence influenced defence and resistance against mites and not host size or degree of parasitism.4. More specifically, hosts from early and late emergence groups did not differ in defence and resistance when held at the same temperatures in incubators. Housing at a high temperature, indicative of later in the season, was associated with higher defence and resistance for damselflies from both early and late emergence groups.5. These results indicate that daily temperatures in relation to emergence timing can account for seasonal increases in resistance for this temperate insect. Seasonal increases in resistance may be expected for other temperate insect–parasite associations and should have important implications for the phenology of parasites and for seasonal variation in parasite‐mediated selection.

Abstract.  1. Greater immune function is associated with the high‐density melanic phase of polyphenic insects, appearing to compensate for density‐dependent increases in susceptibility to parasites and/or pathogens. Other types of discrete variation in cuticular colour occur in insects (which may or may not be associated with melanin pigmentation), but whether this variation is predictive of immune ability has not been investigated. 2. In the mountain stone weta Hemideina maori, a black morph and yellow banded morph occur. These morphs are not seasonally polyphenic and have discrete haplotype genetic markers. Black individuals are typically found at lower local densities than yellow individuals, contrary to relations between cuticular melanism and density seen in polyphenic insects. 3. Yellow males and females had greater melanotic encapsulation responses upon immune challenge than did black males and females, but these differences were not associated with differences in temperature selection between morphs. Morph differences in melanotic encapsulation responses were somewhat related to differences between morphs in haemocyte concentrations. 4. These results indicate that a common form of immune expression is not heightened with dark coloration in the mountain stone weta. Thus, earlier findings of greater immunity associated with darker cuticles in phase polyphenic insects cannot be extended to insects with other forms of discrete colour variation. These findings will help in elucidating causes and consequences of such colour polymorphism, which is widespread in several insect orders.