Insect Herbivore Populations Research Articles

THE GENETIC STRUCTURE OF HOST PLANT ADAPTATION IN A SPATIAL PATCHWORK: DEMOGRAPHIC VARIABILITY AMONG RECIPROCALLY TRANSPLANTED PEA APHID CLONES.

Populations of insect herbivores that feed on several host plant species may experience different selective forces on each host. When the hosts cooccur in a local area, herbivore populations can provide useful models for the study of evolutionary mechanisms in patchy environments. A first step in such a study involves determination of the genetic structure of host adaptation in the region: how is genetic variation for host use structured within and between subpopulations of herbivores on each host? The structure of genetic variation for host use reveals patterns of local adaptation, probable selective consequences of migration between hosts, and the potential for further evolution. To estimate the population structure of host adaptation in a patchwork, 7-11 pea aphid clones were collected at the beginning of the summer from each of two alfalfa and two red clover fields within a very localized area (about 15-20 km2 ). Using a reciprocal transplant in the field, replicates of these 35 clones were allowed to develop individually on each of the two crops. A complete life table was made for each replicate. Individual fitness was calculated from the life tables as the expected rate of population increase; longevity, age at first reproduction, and total fecundity were also measured for each clonal replicate. Currently, experimental estimates of genetic variation in complete life tables are virtually nonexistent for natural populations, even for single environments (Charlesworth, 1987); field studies are even less common. Because clones from each of two source crops were tested reciprocally on both hosts, variation in relative genotypic fitness on alfalfa and clover could be partitioned among clones within source crops, between fields of the same crop, and between source crops (alfalfa or red clover), providing a view of population structure. Significant clonal variation in relative performance on alfalfa and red clover was found: clones tended to have higher fitness on the crop from which they had been collected (the "home" crop) than they did on the "away" crop, suggesting local adaptation in response to patchy patterns of selection. Clonal variability within collections from the two crops suggests the potential for changes in the genetic constitution of these aphid populations within established fields as a result of clonal selection during the summer season. Significantly negative genetic correlations across crops were found for fitness and its major components. The possibility that these negative cross-environment correlations could act as evolutionary constraints on adaptation to the patchwork is considered.

Behavioural Determination of Diet Breadth in Insect Herbivores: The Effect of Leaf Age on Choice of Host Species by Beetles Feeding on Passiflora Vines

Use of more than one host plant species by an insect herbivore population can be achieved through variation among the insects or variation among the plants, or both. The evolutionary consequences of the three are different. Variation for host preference was not found among populations of two beetle herbivores, Monomacra sp. and Disonycha quinquelineata (Chrysomelidae: Alticinae). For these two beetle species, it is shown that leaf age can be more important than plant species (within the genus Passiflora) in determining which leaves an individual beetle will eat. Both species preferred young leaves. This results in multiple host use even in the absence of variation for host preference among individual herbivores. Choice of host plant by most oligophagous and polyphagous herbivores is probably based on an assessment of two components of potential hosts: the species of host and the quality of individual or tissue within each host species. Studies of diet breadth and the evolution thereof should account for the range of plant tissues consumed as well as the number of taxa.

Water stress, leaf nutrients and defoliation: a model of dieback of rural eucalypts

AbstractWater stress, concentrations of foliar nutrients and damage of foliage by insects were studied over an eleven month period, for eight dieback eucalypts and eight closely matched healthy trees growing in close association on two grazing properties near Brisbane. Four healthy eucalypts in a neighbouring State Forest were also studied. The study region had suffered severe climatic stress between 1972 and 1976 when dieback was first observed by residents and when high populations of defoliating insects were observed. Dieback trees had a higher proportion of foliage damaged by insects and higher concentrations of foliar nutrients than did matched healthy trees. The study period was one of low climatic stress during which dieback and healthy trees followed similar seasonal patterns of predawn xylem pressure potential. Dieback trees developed lower daytime minimum xylem pressure potentials than did matched healthy trees, and differed in their stomatal responses. A model of initiation and development of rural eucalypt dieback is proposed. This may have general application to many non‐specific rural diebacks in which heavy insect damage is implicated. The balance between rural eucalypts and their insect herbivores is precarious. Any factor capable of causing extensive defoliation, or an increase in foliar nitrogen, or an increase in populations of insect herbivores may upset this balance. A positive feedback loop may be activated, whereby the production of nitrogen rich epicormic foliage enhances a build up of insect populations. Repeated insect defoliation leads to tree dieback. Evidence in the literature supporting the model is reviewed and aspects requiring further research are outlined.

Insect Herbivore Relationship to the State of the Host Plant: Biotic Regulation of Ecosystem Nutrient Cycling through Ecological Succession

This paper provides a conceptual framework for increasing our understanding of the relationships between plant resistance to insect herbivores and insect herbivore influences on ecosystem nutrient cycling and succession. For a given plant species, adequate nutrient/light availability favors establishment and productivity; small insect herbivore populations regulated by plant biochemistry stimulate primary productivity and short-term nutrient cycling. As biomass and competition for nutrients and light increase during succession, plants become stressed as they approach their tolerance limits. Earlier successional plant species initially dominating the plant community have higher nutrient/light requirements and become stressed sooner than later successional plant species. Abundance and stress make the earlier plant species more apparent and susceptible, than later plant species, to insect herbivores. Insect herbivore populations increasing rapidly on stressed hosts tend to regulate long-term nutrient cycling patterns by accelerating changes in competitive relationships between plant species with different nutrient/light requirements.

Some consequences for a parasitic herbivore, the milkweed longhorn beetle, Tetraopes tetrophthalmus, of a host-plant shift from Asdepias syriaca to A. verticillata.

The impact of differences in host plants on individuals and populations of insect herbivores was investigated using the milkweed longhorn beetle, Tetraopes tetrophthalmus (Forster), larvae of which feed parasitically on host rhizomes. One host, Asclepias syriaca L., was larger in stem and rhizome diameter and grew in cooler soil than the other host, A. verticillata L. The major effects on beetles were retarded phenology at the cooler site and reduced size on the smaller host. Reduced size of beetles was correlated with several important individual attributes: reduced length of life, number of ovarioles, egg size, and a reduced probability of mating with large beetles. The population consequences of these characteristics, largely inferred from these observations, were little or no outbreeding in the small population on A. verticillata.

Effects of an Organophosphorus Insecticide on the Phytoplankton, Zooplankton, and Insect Populations of Fresh‐Water Ponds

Dursban, an organophosphorus insecticide, was applied on June 3, June 18, and July 1 to eight shallow (24 cm) experimental ponds near Bakersfield, California. Four ponds were treated at 0.028 kg/ha (=0.025 lb./acre), four were treated at 0.28 kg/ha, and four were kept as controls. Phytoplankton, zooplankton, and insect populations were sampled on 21 dates between May 20 and August 16. Insect larvae and nymphs were more reduced in numbers than were insect adults, reflecting in part the ability of the latter to emigrate and immigrate. Twenty—four—hour posttreatment samples for the second and third, but not first, treatments showed greater reductions of predaceous Notonectidae, Dytiscidae, Coenagrionidae, larval Hydrophilidae) than of "herbivorous" (Corixidae, Baetidae, adult Hydrophilidae) insect populations. Predaceous insect populations generally recovered to control—pond levels more slowly than herbivorous insect populations. Five weeks after the last insecticide treatment, predaceous insects averaged only 45% and 9% as abundant in low dose and high dose ponds, respectively, as they were in control ponds (P < 0.02); corresponding figures for herbivorous insects were 108% and 206% (P < 0.10). Initially, Cyclops vernalis and Moina micrura were the only crustacean zooplankters present, and both experienced high mortality due to treatments. Recovery in low dose ponds was variable, requiring 1—3 weeks, and in high dose ponds occurred only 3—6 weeks after the final treatment. Populations of Diaptomus pallidus seemed unaffected by the lower rate of treatment, but became large only after Cyclops populations had been reduced by the insecticide or other factors. Ceriodaphnia sp. became abundant in late July or early August in all four control ponds but not in any treated ponds. Herbivorous rotifers, principally in the genera Brachhionus, Polyarthra, Hexarthra, Filinia, and Tripleuchlanis, increased dramatically, often five— to twentyfold, within 1—3 days after Moina and Cyclops populations were decimated by Dursban, but became abundant in control ponds only during the brief absence of Moina in early July. All planktonic rotifers, except Polyarthra, were more abundant in treated than in contro ponds; all benthic—littoral rotifers (e.g., Lecane, Monostyle, Tripleuchlanis, Platyias, Lepadella, Testudinella) were more abundant in control than in treated ponds. The predaceous rotifer Asplanchna brightwelli was 35 times more abundant in treated than in control ponds, feeding primarily on herbivorous rotifers, sometimes on Cyclops, Diaptomus, Moina, and large algae, and rarely on small mayfly (Baetidae) nymphs and chironomid larvae. Asplanchna populations were composed of three morphotypes, ampulliform, cruciform, and campanuliform, the last of which tended to be highly cannibalistic. The "wings" of humps of the cruciform morphotype apparently represent a mechanism for minimizing such cannibalism. The reduction of herbivorous crustaceans by Dursban and the restraint of herbivorous rotifers by Asplanchna predation permitted the rapid increase of phytoplankton populations in treated ponds and, even 6 weeks after the last treatment, the phytoplankton was two and 16 times more abundant in low dose and high dose ponds, respectively, than in control ponds. Blooms of bluegreen algae (Anabaena, Anabaenopsis) developed in three high dose and one low dose pond, and a bloom of the diatom Synedra developed in one low dose pond. The long—spined alga Schroederia setigera seemed favored by high Moina and Diaptomuspopulations and low Cyclops populations. The significance of these results for insect—control programs is discussed.

Effects of Population Density on Growth Rates of Animal Populations

The growth rate of a population at time t is defined as the change in numbers per unit population at that time period (rt = dn/Ntd). The rate usually changes with time as the population increases or decreases. The objective of this study was determine whether the growth rate of animal populations is or is not a function of the population density. In most mathematical models of populations whose changes resemble those of actual populations, the growth rate is a decreasing function of density, meaning that as the population increases the growth rate decreases and vice versa. Records of actual populations can be used to test the relation between growth rate and density by calculating correlation coefficients for these two characteristics. The methods of doing this are described and the limitations on the data are explained. The records of 111 different populations representing 71 species were analyzed by these methods. Of the 71 species, 7 were eliminated from further analysis because their census records were not significantly different from a series of random numbers, 47 had correlation coefficients that were negative and significantly different from zero, 16 had coefficients not significantly different from zero (all but one of these estimated coefficients were negative), and one, the world's human population, had a positive coefficient significantly different from zero. There were no differences between taxonomic groups (insects other invertebrates, fish, birds, mammals). The primary conclusion is that in most animal species a population's growth rate is a decreasing function of density. The explains the relative stability of animal populations, which never continue to increase at rates their fertility would allow, and rarely decrease to extinction. Tentative conclusions are presented regarding the processes regulating population numbers. Populations of herbivorous insects at low and moderate levels are regulated by disease, parasites, and predators causing mortality that is an increasing function of density. Favorable conditions may allow an increase in insect numbers so rapid that the population temporarily escapes regulation by its enemies. Nonterritorial species of vertebrates are normally controlled by predation and, when that fails, by competition; in both cases juvenile individuals are most affected. In territorial vertebrates competition for suitable territories determines the size of the breeding population. Populations of vertebrate species (expecting man) are in general regulated by the production of adult individuals being a decreasing function of population density.