No abstract

The cynipid Diplolepis triforma Shorthouse and Ritchie induces a fusiform, multichambered stem gall from leaf buds on Rosa acicularis Lindl. in central and western Canada. Galls at all stages of development were fixed and sectioned using botanical histological techniques to illustrate, for the first time, the unique host-modifying abilities of gall-inducing cynipids that distinguish them from other phytophagous insects. Key events in gall ontogeny, whereby D. triforma gains control and redirects the development of attacked host tissues to provide larvae with shelter and food, include proliferation of cytoplasmically dense parenchymatous cells within the strands of the procambium at the point of egg contact, appearance of nutritive cells when larvae first begin to feed, formation of new xylem and phloem extending from unaffected vascular bundles to the larval chambers, formation of several layers of nutritive cells during the period of larval feeding, and formation of sclerenchyma cells around each larval chamber. The role of these tissues in galler biology is explained.

Sections of stem galls induced by Diplolepis triforma Shorthouse and Ritchie obtained by plant histological techniques were used to examine anatomical features of immature stages of the insect found in situ. First-instar larvae feed over the entire surface of larval chambers, whereas maturing larvae feed only at one end of the chambers. Last-instar larvae undergo two phases of development: feeding occurs in the first phase, whereas in the second, or prepupal phase, feeding has ceased, the mid- and hind-guts become joined, contents of the gut are voided, and compound eyes, gonads, and internal ovipositional apparatus begin to differentiate. The prepupal phase also consists of two phases: the first occurs in the fall and winter, and the second occurs in the spring prior to pupation. The spring phase exhibits external adult features within the integument of the last-instar and moults to the pupa. Sections of some maturing larvae contained endoparasitoid larvae of the genus Orthopelma Taschenberg (Hymenoptera: Ichneumonidae).

Herbivorous insects are often exposed to broad temporal and spatial variations in microclimate conditions within their host plants and have adapted a variety of behaviors, such as avoidance or basking, to either offset or benefit from such variation. Field experiments were carried out to investigate the influence of daily and intratree variations in microclimate on the behaviors (feeding, resting, dispersal, and hiding) and associated performance of late-instar larvae of the yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer) (Hymenoptera: Tenthredinidae) within crowns of 1.25-1.5 m tall black spruce (Picea mariana [Miller] Britton Sterns Poggenburg); late instars feed on developing shoots of young spruce and are often exposed to microclimatic extremes with unknown effects on performance. Larvae fed diurnally from just after dawn (0800 h) until dusk (2000 h) and rested throughout the night, with brief periods of dispersal occurring in the morning and evening. Neither larval behavior nor abiotic conditions differed significantly between the upper and lower crowns of trees. Temperature, humidity, and solar insolation all explained >90% of variation in feeding; however, sunrise and sunset were the most likely cues influencing diurnal behavior. Most larvae (94%) fed on the bottom, shaded side of shoots, and field experiments indicated that this behavior is adaptive with respect to microclimate, probably reducing hygrothermal stress. Thus, behavioral adaptations by P. alaskensis to daily and within-shoot microclimatic variation may reduce the risk of hygrothermal stress during dispersal or feeding, while still allowing larvae to feed on the preferred and highly nutritious upper crown foliage of young spruce.

Stem galls induced by the cynipid Diplolepis triforma Shorthouse and Ritchie are frequently inhabited by several species of parasitic chalcids and ichneumonids that consume inducers or other parasitoids. Two of these parasitoids, the eurytomid Eurytoma spongiosa Bugbee and the torymid Glyphomerus stigma (Fabr.), consume gall tissues after they have consumed their insect hosts and are considered entomophytophagous. Both are koinobiont ectoparasitoids. Eurytoma spongiosa oviposits within gall chambers and attacks larvae of the inducer throughout the summer season, from the early growth phase of the galls until the galls mature. If the inducer is killed by E. spongiosa in early summer, nutritive gall cells degenerate into vacuolate parenchyma and are consumed. If the inducer is killed later in the summer, when galls begin to mature, nutritive cells persist in the chambers for about 1 week before degenerating. Glyphomerus stigma kills and consumes inducer larvae when galls are maturing. New nutritive cells appear under the influence of G. stigma and are then consumed. The ability of entomophytophagous chalcids to promote the formation of gall cells provides insight into the derivation of the gall-inducing guild.