Dan T. Quiring

Influence of inter-tree variation in time of budburst of white spruce on herbivory and the behaviour and survivorship of Zeiraphera canadensis

Abstract. 1 Field studies were carried out to determine the effects of intra‐tree variation in the time of budburst of white spruce, Picea glauca Moench (Voss.), on the behaviour and survivorship of, and herbivory by, the spruce bud moth, Zeiraphera canadensis Mutt. & Free. 2 There was significant variation in the time of budburst among whorls, shoots and buds. Budburst was acropetal, with buds in the interior of the lower crown bursting first and terminal buds on terminal shoots in the upper crown bursting last. 3 Bud moths laid the greatest proportion of their eggs in the middle of the crown and egg hatch was usually best synchronized to budburst in this region. Many eggs hatched before terminal buds on terminal shoots in the upper crown had burst and thus intra‐tree variation in budburst decreased the probability that first‐instar larvae would colonize the most important plant parts for growth. 4 However, many later instars dispersed upwards and outwards in the crown and colonized the late bursting buds in the upper crown. Such dispersal reduces the effectiveness of intra‐plant variation in budburst to reduce herbivory and permits Z.canadensis to eat young nutritious buds for a longer period of time. 5 Intra‐tree variance in the date of budburst was greater than that between trees but there were no consistent differences between the intra‐tree variance of trees in half‐sib families with high or low susceptibility to Z.canadensis. Differences between trees in herbivory, bud moth density and survivorship were not related to the amount of intra‐tree variation in budburst.

A phloem-sap feeder mixes phloem and xylem sap to regulate osmotic potential

Phloem-sap feeders (Hemiptera) occasionally consume the dilute sap of xylem, a behaviour that has previously been associated with replenishing water balance following dehydration. However, a recent study reported that non-dehydrated aphids ingested xylem sap. Here, we tested the hypothesis that the consumption of xylem sap, which has a low osmolality, is a general response to osmotic stresses other than dehydration. Alate aphids were subjected to different treatments and subsequently transferred onto a plant, where electrical penetration graph (EPG) was used to estimate durations of passive phloem sap consumption and active sucking of xylem sap. The proportion of time aphids fed on xylem sap (i.e., time spent feeding on xylem sap/total time spent feeding on phloem plus xylem sap) was used as a proxy of the solute concentration of the uptake. The proportion of time alate aphids fed on xylem sap increased: (1) with the time spent imbibing an artificial diet containing a solution of sucrose, which is highly concentrated in phloem sap and is mainly responsible for the high osmotic potential of phloem sap; (2) with the osmotic potential of the artificial diet, when osmotic potential excess was not related to sucrose concentration; and (3) when aphids were deprived of primary symbionts, a condition previously shown to lead to a higher haemolymph osmotic potential. All our results converge to support the hypothesis that xylem sap consumption contributes to the regulation of the osmotic potential in phloem-sap feeders.

Interactions between size and temperature influence fecundity and longevity of a tortricid moth, Zeiraphera canadensis

Females of Zeiraphera canadensis Mut. & Free., the spruce bud moth, were reared in the laboratory at constant and alternating temperatures, and in an outdoor insectary, to (1) determine the effects of temperature, age and size on several reproductive parameters and, (2) to test the hypothesis that body size-temperature interactions influence longevity and realized fecundity. Egg maturation was linearly related to age and large moths developed eggs at a higher rate than small ones. Mcan lifetime oviposition rate reached a maximum and remained stable at temperatures ≥20° C while the mean lifetime rate of egg maturation increased linearly with temperature, indicating that higher temperatures adversely affect oviposition. The production of nonviable eggs increased with age but also with temperature, suggesting high temperature (≥25° C) reduces egg quality and/or hinders fertilization. The realized fecundity and longevity of females reared under an alternating temperature regime (mean 20° C) was significantly less than that of females reared at constant 20° C. Similar realized fecundity, longevity and mean lifetime oviposition rates for females reared at temperatures alternating between 10 and 25° C (mean 20° C) and those at constant 25° C reflected the inability of females to recover from elevated diurnal temperatures. Longevity was positively related to female body size at constant 15 and 20° C but the relationships were negative for moths exposed to diurnal temperatures equal to or exceeding 25° C. Due to the reduced longevity of large moths at high temperatures, linear regressions between size and realized fecundity were only significant at constant temperatures ≤20° C. At higher temperatures, the size-fecundity relationship became curvilinear as a result of the diminished reproductive output of large individuals. Reduced fecundity and longevity of large females at high temperatures may have been due to elevated internal temperatures of large-bodied moths. Large females in a controlled-environment chamber maintained at 25° C developed an internal temperature excess (i.e. temperature above ambient) of nearly 2° C while small-bodied females exceeded ambient by only 0.3° C. However, when held at 20° C, the temperature excess of large-bodied moths was much less than 1° C and small-bodied females did not differ from ambient. Such interactions between temperature and body size suggest that there should be stabilizing selection toward moderate-sized individuals and may explain the absence of size-related effects on fecundity and longevity previously reported for several other lepidopterans.

Advantages of a mixed diet: feeding on several foliar age classes increases the performance of a specialist insect herbivore

Two field studies were carried out to determine the influence of Abies balsamea foliage age on the preference and performance of larvae of Neodiprion abietis, a specialist Diprionid sawfly. Preference was determined by examining N. abietis defoliation on all age classes of foliage. Performance was estimated using larval survival, cocoon weights and the percentage of adults that were females. Neodiprion abietis preference for, and performance on, current-year foliage was very low, peaked on 2 or 3-year-old foliage, and declined on older foliage. Thus, sawfly feeding preference was adaptive. However, survival and cocoon weight were highest when sawflies were allowed to feed on all age classes of foliage, demonstrating that an insect specialist may perform better when feeding on several age classes of foliage from a single host plant species. These results indicate that either different larval instars have different nutritional requirements, or that food mixing provides the best diet, permitting the herbivore to obtain needed nutrients while avoiding ingestion of toxic doses of secondary metabolites. In addition, our results suggest that limited availability of varied foliage has more negative consequences for N. abietis females than for males, as the percentage of survivors that were females decreased when juvenile mortality was high. Our results emphasize the importance of considering non-linear changes in foliar quality as leaves age on herbivore preference and performance, and demonstrate how a herbivore can use this variability to maximize its fitness.

Influence of reproductive ecology on feasibility of mass trapping Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae).

(1) The degree of protandry, the average age at which females mated and the effects of size and adult nutrition on the reproductive performance of male western corn rootworms, Diabrotica virgifera virgifera, were evaluated to assess the potential of male removal as a management tactic for this pest. (2) Although some males emerged earlier, most males emerged at the same time as females. Dissections of teneral and young non-teneral females, as well as observations of newly emerged females in cornfields, showed that most females mate during the first 24 h after emergence. The synchrony between bimodal temporal distributions of mating and adult emergence, which reached peaks in the morning and the late afternoon/early evening, supported this conclusion. (3) When supplied an excess of virgin females in field cages, large males usually initiated mating at an earlier age and mated more often than small ones. However, neither longevity nor the mass of spermatophores were related to male size. Thus, small males, which sometimes mate twice daily, transfer a greater proportion of their resources at each mating. (4) Males which were provided young silks, ears of corn, leaves and tassles for the first week after emergence, after which they were only given leaves (i.e. poor food) died sooner and mated less often than similar sized males given continuous access to young silks, ears of corn, leaves and tassles (i.e. best food). The mass of spermatophores produced by males in the best food treatment was larger than those produced by males fed poor food. Spermatophore mass was positively correlated with age, especially during the first 10 days after emergence. (5) The reproductive potential of populations of D. virgifera virgifera males far exceeds the number of available virgin females in cornfields. Thus, intraspecific competition between males for mates should be considerable in this system. (6) The results of this study indicate that tactics relying exclusively on male removal do not have a large potential to manage D. virgifera virgifera populations.

Influence of host size on oviposition behaviour and fitness of Elachertus cacoeciae attacking a low-density population of spruce budworm Choristoneura fumiferana larvae.

1. Oviposition behaviour and host size − fitness relationships of a gregarious, idiobiont ectoparasitoid, Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae), were studied by implanting one fourth‐, fifth‐, and sixth‐instar spruce budworm Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae) larva per tree in a stand where the density of the wild C. fumiferana population was low.

Role of xylem consumption on osmoregulation in Macrosiphum euphorbiae (Thomas).

Aphids are phloem feeders that occasionally ingest xylem sap. The duration of xylem consumption by Macrosiphum euphorbiae (Hemiptera: Aphididae) was positively correlated with the level of dehydration of alate aphids of different ages after a period of starvation, supporting the hypothesis that aphids ingest xylem sap to replenish their water balance. However, the duration of xylem sap ingestion but not phloem sap consumption varied in unstarved alate adults of different ages. Furthermore, both alate and apterous aphids ingested xylem sap at the end of their life, when aphids were not dehydrated but when fecundity started to decrease. Fecundity was negatively correlated with the proportion of time spent ingesting xylem sap, and that over the entire reproductive life of alate and apterous aphids. The lower proportion of xylem ingested by apterous than by alate aphids during the first few days of adult life may be related to a higher symbiont density in apterous morphs. As previous studies have demonstrated a relationship between sucrose assimilation, which is directly influenced by fecundity and symbiont density, and osmoregulation, we suggest that xylem consumption may play a role in the osmoregulation of haemolymph of aphids.

WHY DOES EARLY-SEASON HERBIVORY AFFECT SUBSEQUENT BUDBURST?

Delays in budburst in the year following herbivory have previously been attributed to a defensive response to early-season herbivory or localized resource deficien- cies (resource availability hypothesis) limiting bud development. We carried out field ex- periments to test the resource availability hypothesis as well as the crown architecture hypothesis, which attributes changes in the time of budburst in the year following herbivory to changes in the proportion of different bud types. We examined predictions from these hypotheses using a defoliator, Choristoneura fumiferana; a defoliator/stem feeder, Zeira- phera canadensis; and a stem galler, Adelges abietis; on white spruce, Picea glauca. The time of budburst and final leaf size of unexploited midcrown shoots of white spruce were compared to those of adjacent exploited shoots. We also evaluated budburst in a manipulated field study, in which tree growth was reduced through root pruning and increased through fertilization. Partial defoliation by C. fumiferana and Z. canadensis did not affect budburst, but did result in smaller leaves. Almost complete defoliation of shoots by C. fumiferana and galling by A. abietis resulted in delayed budburst. Reductions in leaf size were directly related to the amount of herbivory (i.e., low or high) by C. fumiferana and Z. canadensis. Budburst of trees whose growth rate had been decreased by root pruning or increased by fertilizing occurred later and sooner, respectively, than budburst on unmanipulated trees. The time of budburst was inversely correlated with foliar nitrogen and water content and the length of unexploited shoots. These results supported the resource availability hypothesis. The in- fluence of resource availability on time of budburst was observed at the shoot, branch, and whole-tree levels. When >50% of stems were destroyed by Z. canadensis, budburst occurred earlier or at the same time in the following spring. This was due to the production of basal buds, which burst before all other buds on a shoot. The production of early-bursting basal buds more than compensated, or just compensated, for a small delay in budburst of terminal, distal-lateral, and medial-lateral buds. Thus, changes in the time of budburst in the year following herbivory were due to changes in crown architecture as well as to changes in resource availability. We predict that increased intra-tree heterogeneity in the time of bud- burst should make P. glauca more, rather than less, susceptible to two of the herbivores studied because of the presence of a highly nutritious food source, newly burst buds, for a longer period of time. We suggest that the influence of previous herbivory on the time of budburst in the next growing season is a commonly overlooked factor that may have a large influence on herbivore abundance and distribution.

Temporal patterns of balsam fir sawfly defoliation and growth loss in young balsam fir

A 2-year field study was carried out with the balsam fir sawfly, Neodiprion abietis (Harris), on balsam fir, Abies balsamea (L.) Mill., to determine: (1) the temporal patterns of defoliation and foliage weight loss associated with early- and late-larval instars; (2) the losses in branch and tree stem volume growth as a result of defoliation by larvae; and (3) to determine the extent to which individual branches are autonomous, and their utility for predicting stem growth losses due to defoliation. In addition, this study also permitted us to determine the relationships between defoliation or foliage weight and branch and tree stem volume growth. Defoliation, foliage weight loss and larval development occurred more quickly in the first year due to warmer temperatures, but final levels of defoliation and foliage weight loss were similar in both years. Cumulative defoliation levels increased from <5% at the beginning of the study to 35 and 63%, respectively, in fall 1999 and 2000. Defoliation was greatest on 1-year-old foliage in both years and most defoliation and foliage weight loss occurred after the initiation of third-instar. Based upon individual year–volume increment relationships for each branch and tree, mean volume increment loss in 1999 and 2000, respectively, was 20 and 42% for observation branches and 12 and 35% for tree stems. Similar growth loss estimates of 12 and 46%, respectively, for 1999 and 2000, were obtained when comparing the mean specific volume increment for control and observation branches. Foliage weight after sawfly attack was related to branch and stem volume growth, but defoliation was only related to branch volume loss. Although branch and tree stem growth were strongly related, growth losses in a branch were not related to those in a tree stem. Branch removal and measurement is a practical alternative to cutting trees to estimate stem volume growth in balsam fir.

Body size of spruce-galling adelgids is positively related to realized fecundity in nature

Characterization and quantification of reproductive capacity is essential for understanding insect population dynamics. Female body size has been shown to account for much of the variation in potential and realized fecundity (i.e. potential and actual number of eggs laid during lifetime, respectively) for many insect species in the laboratory or in field cages (e.g. Beckwith, 1976; Wratten, 1977; Dixon & Dharma, 1980; Hinton, 1981; Spurgeonet al., 1995). It has become generally accepted that fecundity increases with increasing body size (Gilbert, 1984; Freeman & Geoghagen, 1987; Shine, 1988; Honek, 1993), with few exceptions (Leather, 1988; Klingenberg & Spence, 1997); most studies evaluating the effects of various factors on insect fitness, such as individual behaviour (e.g. Quiring & McNeil, 1987), assume a positive relationship. Relationships between female body size and fecundity observed under controlled conditions may not occur in nature, where abiotic and biotic factors fluctuate. Realized fecundity has been shown to be positively related to female body size for some species reared under constant (e.g. Preziosi et al., 1996) and fluctuating (e.g. Tammaru et al., 1996) temperatures in controlled environments. However, Carroll & Quiring (1993) showed that the female body size–fecundity relationship of spruce bud moth, Zeiraphera canadensis Mut. & Free., which was linear at constant temperatures ø 20 °C, was parabolic when the insects were exposed to diurnal temperatures ù 25 °C, due to reduced longevity of large individuals. Similarly, estimates of lifetime fecundity in the water strider Aquarius remigis Say (Hemiptera: Gerridae) were positively related to body size, but in 1 of 2 years levelled off at larger sizes, due to decreased longevity of larger individuals (Preziosi & Fairbairn, 1997). Despite the importance of the assumption that realized fecundity of females is positively related to size in nature,