The slow growth/high mortality hypothesis predicts that herbivorous insects feeding on suboptimal host plants are subjected to higher predation mortality owing to the longer time spent in the vulnerable juvenile stages compared with conspecifics feeding on optimal plants. We tested this hypothesis for the willow-feeding leaf beetle Galerucella lineola raised on one suitable (Salix viminalis) and one unsuitable (S. dasyclados) willow species as well as on plants from an interspecific cross between the two species. Cohorts of larvae raised on caged plants (protected from enemies) and uncaged plants (exposed to enemies) were monitored daily throughout larval development in two consecutive years. Larvae raised on S. viminalis developed faster, grew larger and survived better than those raised on S. dasyclados. The suitability of the hybrid plants was intermediate to that of their parents. Our results strongly support the slow growth/high mortality hypothesis. In both years, total predation during the larval period was higher on S. dasyclados than on S. viminalis. Furthermore, the daily predation rate (i.e. the proportion of larvae preyed upon per day) was higher on S. dasyclados than on S. viminalis. When hybrid plants were included in the analysis total predation was positively correlated with both larval development time and daily predation rate. We suggest that high predation on beetles on low-quality plants is the combined result of their longer development time and elevated daily predation rate. The results are discussed in relation to the evolution of host plant selection and the paradox of sublethal plant defenses.

1. Free‐living insects are often thought of as more vulnerable to environmental hazards than concealed insects, such as galling or mining insects. The possibility that larvae of the free‐living leaf beetle Galerucella lineola seek out existing plant structures and thereby become partly concealed was explored. 2. Neonate larvae of G. lineola frequently feed in rolled‐in margins of young leaves of their host plant, Salix viminalis. In addition to nutritional benefits from feeding on young leaves, larvae may also gain protection against adverse weather conditions and general predators by feeding in the leaf rolls. Field and laboratory experiments were conducted to test these hypotheses. 3. Artificial shelters were constructed and cohorts of neonate larvae were placed on experimental plants. In all experiments, larvae preferred to feed in shelters, even when shelters were constructed on mature leaves. 4. In one of the experiments, fewer larvae disappeared when shelters were provided. In a predator exclusion experiment, however, no differences in predator‐inflicted mortality on G. lineola were found between shelter‐containing shoots and control shoots. 5. A laboratory experiment showed increased protection from desiccation when shelters were present; growth rate was higher for larvae feeding on plants with shelters. 6. Thus, free‐living insects may not always be as exposed to environmental hazards as is often assumed. In particular, young larvae may take advantage of preformed structures on their host plant and feed in a concealed microhabitat. Because mortality, in general, is high during early instars, shelter‐seeking behaviour may increase survival significantly. The existence of preformed shelters may therefore be a plant characteristic that should be considered when exploring the environmental risks associated with the free‐living habit.

The possible roles of plant quality (vigor) and natural enemies in the development of a localized outbreak of the leaf beetle Galerucella lineola (Coleoptera: Chrysomelidae) in a stand of Salix cinerea were investigated. Caged and uncaged larvae on six bushes in the outbreak area were compared with caged and uncaged larvae on six bushes in an adjacent nonoutbreak area in terms of performance. In 1997, when the studies were performed, the natural density of the insect (beetles plus eggs) was six times higher in the outbreak area compared with the nonoutbreak area. Even though the vigor (measured as shoot length) of bushes in the outbreak was 72% higher than that of bushes in the nonoutbreak area, we found no difference between areas in the performance (survival, developmental time, pupal weight) of caged larvae or in the willingness of caged females to lay eggs. Among larvae exposed to natural enemies, the disappearance rate was significantly higher in the nonoutbreak area. The density of generalist predators was significantly higher in the nonoutbreak than in the outbreak area. We conclude that differences in plant quality, despite the observed difference in plant vigor, could not explain the observed difference in beetle density between areas. Lower predation pressure in the outbreak area could, however, not be excluded as a possible reason for the higher density of leaf beetles in this area.

Summary. The harvestman Mitopus morio (Phalangidae) is a generalist predator. It is known to prey on larvae of the chrysomelid leaf beetle Oreina cacaliae defended by plant acquired pyrrolizidine alkaloids (PAs). Tracer feeding experiments were performed to determine how harvestmen tolerate protoxic PAs. Minced meat containing either [ 14 C]senecionine or [ 14 C]senecionine N-oxide was fed to M. morio and subsequently feces and bodies were analyzed. Labeled alkaloid N-oxide remained stable and was eliminated almost unaltered with the feces; only 10% was recovered as tertiary PA. In contrast, approximately 80% of labeled tertiary alkaloid (senecionine) ingested with the diet was N-oxidized and eliminated; the remaining 20% consisted of unchanged senecionine and a polar metabolite of unknown structure. Harvestmen process their diet by excreting digestive juice, indicated by bleaching of the meat color. Analysis of the processed diet revealed some N-oxidation of [14 C]senecionine, suggesting the gut as the site of Noxidation. Analysis of the bodies of harvestmen 80 hours after the tracer feeding pulse revealed only trace amounts of the polar metabolite. Neither senecionine nor its N-oxide could be detected in the body extracts. The results are discussed in relation to the strategies of PA adapted insects to avoid accumulation of tertiary PAs in living tissues.

Pure species and F1 hybrid families of Salix viminalis and S. dasyclados were tested for resistance to four single uredinium isolates of Melampsora rust in laboratory experiments using excised leaves. Rust isolates were derived from: S. viminalis, S. dasyclados, a S. viminalis x triandra hybrid, and S. daphnoides. Incidence of infection, number of uredinia per leaf, and numbers of spores per uredinium were measured. As expected, the isolate from S. daphnoides did not infect any of the willow species or hybrids tested. For the other three rust isolates that were tested, the parent from which the isolate was derived was susceptible, the other parent was resistant, and hybrids were intermediate in resistance for incidence and uredinia per leaf. These patterns indicate additive inheritance of 'these resistance traits in hybrids. Numbers of spores per uredinium were similar on the hybrids and the susceptible parent for one rust isolate, suggesting dominant inheritance of this trait in the hybrids.