Susan Mopper

Chronic Herbivory: Impacts on Architecture and Sex Expression of Pinyon Pine

Pinyon pine, Pinus edulis (Engelm.), in Northern Arizona is exposed to recurring high levels of herbivory by the moth Dioryctria albovitella (Hust.). During a 3-year period, infested trees experienced on average a 30 percent reduction in annual shoot production. This herbivory affects tree architecture, growth rate, reproductive output, and sexual expression. Less infested trees produce 47 percent more trunk wood, 43 percent more branch wood, and are monoecious. Architectural changes in infested trees can result infunctionally male plants due to a complete loss of normal female cone-bearing ability. When herbivores are experimentally removed, normal growth and reproduction patterns resume. These strong herbivore impacts should represent a potent selection pressure in the evolution of host traits.

Adaptive genetic structure in phytophagous insect populations

Genetic variation in insect populations is frequently structured into discrete groups, or demes, that form in response to stochastic forces or natural selection. Because host-plant populations are often highly heterogeneous, phytophagous insects may form demes that are adapted to the unique traits of individual plants. Recent field experiments indicate that selection pressures imposed by host-plants can promote rapid adaptive evolution in natural insect populations at very fine spatial scales. Adaptive deme formation may be more common among endophagous insects, which feed and reside within plant tissue, than for externally feeding insects, because internal feeders experience stronger plant-mediated selection pressures.

Genetic Structure and Local Adaptation in Natural Insect Populations

There are essentially two options open to a reviewer of an edited compilation of articles on a common theme written by different authors. One is to briefly summarize the contents of each chapter sequentially, leaving the reader to do his or her own integration of the material covered. The other is to attempt to place the compilation in context of the current status of the field, and, perhaps, not go into too much detail of what each chapter contains. Both options have their pros and cons, and the choice of one over the other reflects not much more than the reviewers subjective preference. Having said that, let me make my bias clear and state that I shall follow the latter approach in reviewing this book, which is a compilation of 16 chapters on various genetic aspects of local adaptation in insects, including both reviews and original results, written by many different authors, several of whom have made major contributions to the field. The extremely high level of host specialization exhibited by phytophagous insects has interested ecologists and evolutionary biologists for well over three decades, in part because, all else being equal, it should intuitively be a more adaptive strategy to be a generalist, thus being freed from having ones evolutionary fate inextricably tied up with that of ones host. Thus the focus of much attention has been upon those factors, ecological and genetic, t h a t may facilitate the evolution of extreme specialization in phytophagous insects, leading potentially to host-race formation and sympatric speciation. A major paradigm in thinking on the evolution of specialization has been that of trade-offs in fitness on different hosts. In this view, insect genotypes that have relatively high fitness on a particular host species tend to have poor fitness on other hosts, a situation that may promote the formation of host races. This view, however, has been under criticism of late and it has become clear that in the absence of other ecological factors promoting host-

Effects of Salinity on Endogenous Aba, Iaa, Ja, and Sa in Iris hexagona

Phytohormones play critical roles in regulating plant responses to stress. We investigated the effects of salinity on abscisic acid (ABA), indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonic acid (JA) in leaves, stalks, fruits, and seeds of Iris hexagona, a native wetland species. Using gas chromatography–mass spectroscopy with selected ion monitoring, our experiments demonstrated significant and different shortand long-term changes in iris phytohormones. ABA and JA generally increased and IAA and SA declined in response to salinity. We conclude that these phytohormones may have separate and interactive effects on how plants respond and adapt to stress in natural environments.

A Meta-Analysis of Adaptive Deme Formation in Phytophagous Insect Populations

The adaptive deme formation (ADF) hypothesis predicts that herbivorous insects become locally adapted to their host plants over time. Since its inception, approximately 17 independent studies have tested ADF, and they are divided in support and rejection of the hypothesis. This field of insect evolutionary ecology has a contentious history, and the contradictory studies obscure our understanding of the general evolutionary importance of adaptive deme formation in phytophagous insects. We conducted a meta‐analysis in an attempt to clarify this issue. Meta‐analysis is a statistical method for quantitatively comparing and synthesizing the results of different studies in a way that is more objective than a traditional literature review. Our analysis indicates that local adaptation is an important phenomenon in diverse insect systems. Contrary to predictions of the original hypothesis, there was no evidence that insect dispersal ability, and ostensibly gene flow, was associated with local adaptive differentiation. There was some indication that breeding (parthenogenetic, haplodiploid, diplodiploid) and feeding (exophagous, endophagous) modes may influence the evolution of locally adapted demes. Our analysis supports the theory of adaptive deme formation and provides guidance for future research directions.

LOCAL ADAPTATION AND AGENTS OF SELECTION IN A MOBILE INSECT

The deme‐formation hypothesis states that selection can produce adaptive genetic variation within and among phytophagous insect populations. We conducted three field experiments and tested this prediction by transferring eggs and measuring performance of a mobile leafmining insect, Stilbosis quadricustatella. In Experiment 1, we compared the rate of mine initiation of leafminers transferred to natal and novel sites. In Experiment 2, we compared mine‐initiation rate of leafminers transferred to natal and novel host‐plant species. In Experiment 3, we compared the mine‐initiation rate, mine‐completion rate, and sources of mortality of miners transferred to neighboring natal and novel Quercus geminata trees. In the first, second, and third experiments, leafminer larvae initiated significantly more mines at the natal site, on the natal plant species, and on the natal Q. geminata tree, evidence for adaptive differentiation. Furthermore, plant‐mediated mortality was significantly lower among miners transferred to natal Q. geminata trees. This result supports a key assumption of the deme‐formation hypothesis: insects adapt to the defensive phenotypes of individual trees. However, natural‐enemy mortality was significantly higher among miners transferred to natal trees, essentially reversing the plant effect. Therefore, rates of successful mine completion were similar on natal (19%) and novel (17%) trees. This experiment suggests that host plants and natural enemies may represent opposing forces of selection. Leafminers adapted to individual trees may realize a selective advantage only when natural‐enemy densities are low.

Differential Herbivory in an Oak Population: The Role of Plant Phenology and Insect Performance

Individual trees in a North Florida population of Quercus geminata, (sand live oak) vary widely in rates of herbivory by a leaf-mining moth (Stilbosis quadricusta- tella). Some oaks are always heavily infested while neighboring trees remain lightly at- tacked. We conducted field experiments in 1991 and 1992 to determine if leafminer per- formance on heavily and lightly attacked trees contributed to this pattern. We also compared the leaf production phenology, leaf area, and stem length of heavily and lightly attacked trees to determine if tree phenotypic traits influenced herbivory. Our results indicate that: (1) Lightly attacked trees produced leaves earlier in the spring than heavily attacked trees, and the difference among trees in leaf production phenology explained 61% of the variation in leafminer densities (P = 0.008). (2) Lightly attacked trees produced significantly smaller leaves than heavily attacked trees, but leaf size explained only 13% of the variation in herbivory (P = 0.048). (3) There were no differences in larval performance on heavily and lightly attacked trees in either 1991 or 1992, with the exception of egg mortality, which was significantly higher on lightly attacked trees in 1991 (P = 0.04). (4) Host plants caused greater leafminer mortality than natural enemies, but neither differed significantly between heavily and lightly attacked trees. Our transfer experiments and observations of wild leafminers revealed little difference in leafminer performance on heavily and lightly attacked Quercus geminata trees. However, tree phenotypic traits differed, especially the timing of spring foliation, which occurred significantly earlier among trees with low rates of herbivory. These results suggest that leafminer ovipositional preference, rather than performance, could be an important deter- minant of differential herbivory. There may be an unknown selective advantage to pref- erential oviposition on trees with delayed spring leaf production.

Increased moth herbivory associated with environmental stress of pinyon pine at local and regional levels

Abstract Using 6 years of observational and experimental data, we examined the hypothesis that water and nutrient stress increase the susceptibility of pinyon pine (Pinus edulis) to the stem- and cone-boring moth (Dioryctria albovittella). At two geographic levels, a local scale of 550 km2 and a regional scale of 10,000 km2, moth herbivory was strongly correlated with an edaphic stress gradient. At a local scale, from the cinder soils of Sunset Crater to nearby sandy-loam soils, nine of ten soil macro- and micronutrients, and soil water content were lowest in cinder-dominated soils. Herbivore damage was six times greater on trees growing in the most water and nutrient deficient site at Sunset Crater compared to sites with well-developed soils. Percentage silt-clay content of soil, which was highly positively correlated with soil nutrient and soil moisture at a local scale, accounted for 56% of the variation in herbivory at a regional scale among 22 sites. Within and across sites, increased stem resin flow was positively associated with reduced moth attack. On the basis of moth distribution across a stress gradient, we predicted that pinyons growing in highly stressful environments would show increased resistance to herbivores if supplemented with water and/or nutrients. We conducted a 6-year experiment at a high-stress site where individual trees received water only, fertilizer only, and water + fertilizer. Relative to control trees, stem growth and resin flow increased in all three treatments, but only significantly in the water + fertilizer treatment. Although there was no significant difference in herbivore damage among these three treatments, there was an overall reduction in herbivore damage on all treatment trees combined, compared to control trees. This experiment suggests that release from stress leads to increased resistance to insect attack and is consistent with our observational data. While other studies have predicted that short-term stress will result in herbivore outbreaks, our studies extend this prediction to chronically stressed host populations. Finally, while flush-feeders are not predicted to respond positively to stressed host plants, we found a positive association between herbivore attack and stressed pinyon populations.

Sex and Drugs and Herbivores: Sex-Biased Herbivory in Arroyo Willow (Salix Lasiolepis)

We document through correlative and experimental analyses sex—biased herbivory in the arroyo willow, Salix lasiolepis. Male willows typically support higher densities of five species of tenthredinid sawflies than do female willows. Significant intersexual variation sawfly densities exists in naturally occurring willow clones and in clones grown in a common garden. Arroyo willow is sexually dimorphic in leafing phenology and in aspects of leaf chemistry. Male willows have significantly longer vegetative shoots than do females at the time of sawfly oviposition (P = .01). Males also have significantly higher foliar concentrations of nitrogen, phosphorus, and potassium than do females. Female willows have a marginally significant (P = .093) higher concentration of total phenols. Of the dimorphic traits, only variation in leaf phenology is a consistent predictor of variation in sawfly densities among willow clones. See full-text article at JSTOR

SPATIOTEMPORAL VARIATION IN LEAFMINER POPULATION STRUCTURE AND ADAPTATION TO INDIVIDUAL OAK TREES

Stilbosis quadricustatella leafminers are microlepidopteran specialists of sand-live oak (Quercus geminata). These tiny moths produce one generation per year and have a parasitic life-cycle and long larval stage that develops entirely within a single oak leaf. Differences in host-plant age, phenotype, and phenology generate a coarse-grained, spatially heterogeneous environment for the leafminer population. Previous reciprocal trans- fers of leafminer eggs among mature oaks revealed that S. quadricustatella are locally adapted to individual oak trees. In this paper we use genetic markers and an extinction- recolonization experiment to explore further variation in leafminer population structure. Allozyme loci indicate significant interdemic genetic structure among recent colonists of new host trees, which weakens in the 10th generation and disappears by the 40th generation. In contrast, adaptive demic structure is evident by the 10th generation and is strong in the 40th generation, despite the potential for substantial intertree dispersal. We propose that host heterogeneity combined with leafminer fidelity to natal trees promotes divergent se- lection and rapid demic evolution on individual oaks, despite potentially high gene flow between the leafminers inhabiting them.