Host marking behavior in phytophagous insects and parasitoids

Oviposition behavior in phytophagous insects and entomophagous parasitoids is often modified by the presence of conspecific brood (eggs and larvae). Often, females avoid laying eggs on or in hosts bearing brood, a behavior that acts to reduce the level of competition suffered by their offspring. Avoidance of occupied hosts is typically mediated by cues and/or signals associated with brood. In this article, we review the role of Marking Pheromones (MPs) as signals of brood presence in both phytophagous and entomophagous insects. We place information about the function and evolution of MPs in the context of recent theory in the field of animal communication. We highlight the dynamics of host-marking systems and discuss how effects of MPs vary according to factors such as female experience and egg load. We also examine variation in the form and function of MP communication across a variety of insect taxa. While studies of MP communication in phytophagous insects have focused on the underlying behavioral mechanisms and chemistry of MP communication, studies in entomophagous insects have focused on the functional aspects of MPs and their role in decision-making in insects. We argue that an approach that incorporates the important contributions of both of these somewhat independent, but complementary areas of research will lead to a more complete understanding of MPs in insects. Finally, we suggest that MP systems are model systems for the study of animal signaling and its evolution.     

15N-enrichment of plant tissue to mark phytophagous insects, associated parasitoids, and flower-visiting entomophaga

New techniques are presented on the use of ¹⁵N to mark insects. ¹⁵N, a stable isotope of nitrogen, was enriched above natural abundance in plant and insect tissues. Two laboratory studies demonstrated that enriched ¹⁵N-concentrations could be tracked from plant to insect using mass spectrometry. In the first study, adult Cotesia plutellae (Kurdjimov) (Hymenoptera: Braconidae) and Hippodamia convergens Guérin-Méneville (Coleoptera: Coccinellidae) were allowed to feed at the flowers of rapid-cycling Chinese cabbage plants that had been fertilized with ¹⁵N-enriched potassium nitrate (KNO₃-¹⁵NO₃). Both insect groups were found to have significantly elevated ¹⁵N levels after visiting the flowers of the ¹⁵N-enriched plants for 48 hours. In the second study, ¹⁵N-enriched bean plant (Phaseolus vulgaris L.) tissue was incorporated into an insect diet and fed to navel orangeworms, Amyelois transitella (Walker) (Lepidoptera: Pyralidae). When the navel orangeworm larvae were 4th instars, they were removed from the diet and exposed to the parasitoid, Goniozus legneri Gordh (Hymenoptera: Bethylidae). Results indicated that the enriched ¹⁵N-concentration of the bean plants was transferred to the navel orangeworms and, subsequently, to the parasitoids. This work may provide useful techniques to help establish whether agriculturally important entomophaga visiting ¹⁵N-enriched flowers or parasitizing enriched sentinel larvae in the field can be effectively marked with ¹⁵N.     

On optimal oviposition behavior in phytophagous insects

A model is developed that predicts when an insect should oviposit on a potential larval host plant when it is encountered. Optimal behavior depends upon the suitability of this plant for larval development and on the probability of finding a more suitable host in the available time. This behavior is modeled for conditions in which the host either does or does not fluctuate in density.     

Effects of hostplant age on phytophagous insects

In Colorado beetle females fed with physiologically aged potato leaves, an inhibition or even a standstill of reproduction occurs, followed by diapause.In such beetles, the volume of the corpora allata is reduced and the medial cerebral neurosecretory cells do not transport their neurosecretory material. In feeding tests, it is shown that the beetle can detect the age of its hostplant.

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Zusammenfassung Wenn weiblichen Kartoffelkäfern zwei Arten bevorzugter Wirtspflanzen zur Verfügung stehen, ändert sich ihr Wirtswahlverhalten mit dem Ansteigen der Temperatur, während das nicht der Fall ist, wenn abgelehnte mit angenommenen Pflanzen verglichen werden.Die Käfer können zwischen alten und jungen Blättern bevorzugter Wirtspflanzen unterscheiden.Bei Kartoffelkäfern, die mit physiologisch gealtertem Laub gefüttert werden, tritt eine Hemmung oder sogar ein völliger Stillstand der Reproduktion ein, worauf eine Diapause folgt. Es scheint, daß die Corpora allata beeinflußt werden und aufhören zu arbeiten; die Käfer zeigen histologisch die charakteristischen Anzeichen diapausierender Käfer genauso wie hungernde Käfer: einen völligen Stillstand des Transportes von Neurosekret und eine dichte Anhäufung neurosekretorischen Materials innerhalb der Zellen. Nachfolgende Nahrungsaufnahme der hungernden Käfer bewirkt den Transport neurosekretorischen Materials entlang der Neurosekretbahnen und die Dispersion desselben innerhalb des Zellplasmas.Es scheint also, daß die jahreszeitlichen Veränderungen im Zustand der Wirtspflanze dem neuro-endokrinen System angezeigt werden und für die Nahrungswahl sowie für die Synchronisation von Bedeutung sind.

     

Mechanisms for regulating oxygen toxicity in phytophagous insects

The antioxidant enzymatic defense of insects for the regulation of oxygen toxicity was investigated. Insect species examined were lepidopterous larvae of the cabbage looper (Trichoplusia ni), southern armyworm (Spodoptera eridania), and black swallowtail (Papilio polyxenes). These phytophagous species are subject to both endogenous and exogenous sources of oxidative stress from toxic oxygen radicals, hydrogen peroxide (H2O2) and lipid peroxides (LOOH). In general, the constitutive levels of the enzymes superoxide dismutase (SOD), catalase (CAT), glutathione transferase (GT), and its peroxidase activity (GTpx), and glutathione reductase (GR), correlate well with natural feeding habits of these insects and their relative susceptibility to prooxidant plant allelochemicals, quercetin (a flavonoid), and xanthotoxin (a photoactive furanocoumarin). Induction of SOD activity which rapidly destroys superoxide radicals, appears to be the main response to dietary prooxidant exposure. A unique observation includes high constitutive activity of CAT and a broader subcellular distribution in all three insects than observed in most mammalian species. These attributes of CAT appear to be important in the prevention of excessive accumulation of cytotoxic H2O2. Unlike mammalian species, insects possess very low levels of a GPOX-like activity toward H2O2. Irrefutable proof that this activity is due to a selenium-dependent GPOX found in mammals, is lacking at this time. However, the activity of selenium-independent GTpx is unusually high in insects, suggesting that GTpx and not GPOX plays a prominent role in scavenging deleterious LOOHs. The GSSG generated from the GPOX and GTpx reactions may be reduced to GSH by GR activity. A key role of SOD in protecting insects from prooxidant toxicity was evident when its inhibition resulted in enhanced toxicity towards prooxidants. The role of antioxidant compounds in protecting these insects from toxic forms of oxygen has not been explored in depth. A major finding, however, is that these insects are lutein accumulators. Lutein is a dihydroxy (diol) derivative of beta-carotene, and it is a good quencher of activated forms of oxygen and free radicals. Levels of lutein are highest in P. polyxenes which specializes in feeding on prooxidant-containing plants.     

The influence of dispersal and diet breadth on patterns of genetic isolation by distance in phytophagous insects

To determine the effects of dispersal ability and diet breadth on population-genetic structure, we reviewed the allozyme literature and estimated genetic isolation by distance (IBD) for 43 species/host races of phytophagous insects. Subsequently, we tested two opposing hypotheses regarding the influence of dispersal ability on IBD: that IBD slopes do not vary with mobility, but that intercepts increase with mobility, and, alternatively, that IBD slopes vary with dispersal ability. We found that from tens of kilometers to more than 1,000 km, IBD is weak in sedentary and highly mobile species but pronounced in moderately mobile species. We attribute the weak IBD in strong dispersers to the homogenizing effects of gene flow, whereas in sedentary species, limited gene flow allows nearly all populations to diverge. In intermediate dispersers, genetic homogeneity is achieved at small spatial scales, but limited dispersal promotes genetic divergence over long distances. We also tested the hypothesis that IBD increases with decreasing diet breadth. We discovered no such pattern, casting doubt on the supposition that specialization promotes speciation by influencing population-genetic subdivision. Finally, we found that the number of populations is a more important consideration than the number of polymorphic loci in studies of IBD.     

Plant nutrient supply determines competition between phytophagous insects

Indirect competition is often mediated by plant responses to herbivore feeding damage and is common among phytophagous insect species. Plant-mediated responses may be altered by abiotic conditions such as nutrient supply, which can affect plant growth, morphology, and the concentration of primary and secondary metabolites. Nutrient supply can be manipulated by the type and amount of fertilizer applied to a plant. Brassica oleracea plants were grown in several types of fertilizer, including those commonly used in sustainable and conventional agricultural systems. The occurrence of indirect competition between two phytophagous species from different feeding guilds (a phloem-feeder and leaf-chewer) was assessed. The leaf-chewer reduced aphid populations on plants growing in most fertilizer treatments, but not on those in the ammonium nitrate fertilizer treatment, which caused the highest concentration of foliar nitrogen. The potential consequences of our findings are discussed for phytophagous species in conventional and sustainable agricultural systems.     

Interactions between phytophagous insects and their Opuntia hosts

Abstract.

• 1 The cactophagous insect community on opuntias is analysed to show the number of insect species in different taxa. An extension of this analysis gives the average species complement on large and small opuntias.
• 2 A highly significant positive correlation is found between the total number of phytophagous insect species on individual Opuntia species and a measure of the overall ‘architecture’ of their host plants.
• 3 The specificity of the phytophagous insects on opuntias is briefly considered and the community as a whole analysed by guilds. The co-evolution of the Opuntia-feeding insects and their hosts has culminated in a community of specialist insects to the exclusion of nearly all generalist phytophages.
• 4 The life history strategies of the Opuntia-feeding insects are reviewed. Common to all developmental stages are morphological and behavioural adaptations that reduce the risk of attack by natural enemies. This is clearly the consequence of living on structurally simple host plants where there is little place to hide.
• 5 The possible influence of insect herbivores on Opuntia evolution is discussed.
• 6 An understanding of the interactions between the phytophagous insect community and opuntias has clear implications for the biological control of alien Opuntia weeds.

     

Plant lectins as defense proteins against phytophagous insects

One of the most important direct defense responses in plants against the attack by phytophagous insects is the production of insecticidal peptides or proteins. One particular class of entomotoxic proteins present in many plant species is the group of carbohydrate-binding proteins or lectins. During the last decade a lot of progress was made in the study of a few lectins that are expressed in response to herbivory by phytophagous insects and the insecticidal properties of plant lectins in general. This review gives an overview of lectins with high potential for the use in pest control strategies based on their activity towards pest insects. In addition, potential target sites for lectins inside the insect and the mode of action are discussed. In addition, the effect of plant lectins on non-target organisms such as beneficial insects as well as on human/animal consumers is discussed. It can be concluded that some insecticidal lectins are useful tools that can contribute to the development of integrated pest management strategies with minimal effect(s) on non-target organisms.     

A test of host-associated differentiation across the 'parasite continuum' in the tri-trophic interaction among yuccas, bogus yucca moths, and parasitoids

Parasitic taxa span an antagonistic continuum, with some parasites inflicting no fitness costs to some that kill the host after feeding. Host-associated differentiation is postulated as a major process facilitating speciation in many parasitic taxa. Here, I examined the importance of host-associated differentiation in a parasitoid wasp that develops on yucca moths in the genus Prodoxus. Prodoxus are specialists on Yucca , and moth speciation is closely tied to differences in microhabitat use within a plant and among host plant species. Parasitoids in the genus Eusandalum have been reared from Prodoxus species distributed across Yucca . Estimates of host-use patterns obtained through rearings of adult wasps were combined with surveys of mitochondrial DNA cytochrome oxidase I sequence data and amplified fragment length polymorphism markers to determine if populations of Eusandalum were genetically structured based on host use. Eusandalum populations were genetically structured based on geographical distance rather than moth host species, microhabitats within plants, or Yucca species. The results are contrary to the patterns observed in the host genus Prodoxus . Although parasitoids exhibit parasite-like characteristics, these results suggest that Eusandalum may be best viewed as a predator. Female wasps are able to utilize any moth species present at a given locality, and there is little likelihood that host specialization may facilitate population subdivision and speciation.     

Gregariousness and repellent defences in the survival of phytophagous insects

Group living has both costs and benefits for plant‐feeding insects, but defence against predators is the most widely acknowledged benefit. Gregarious folivores typically have warning coloration and elaborate anti‐predator defences. Do these defences protect these species from predation? To see if protection from predators generally results from gregariousness, I compared the shapes of published survivorship curves of externally feeding, gregarious and solitary Lepidoptera and Symphyta. Gregarious species are less likely than solitary species to die in the larval stages. However, solitary species that have anti‐predator defences do not have higher larval survival compared to gregarious species. This result, along with evidence from experimental manipulations of group size, suggests that repellent defences per se do not increase survival of gregarious larvae. Group behaviour is undoubtedly important in affecting the higher larval survival of gregarious species, but we currently cannot determine whether predator learning, dilution of risk, or rapid development contribute most to increasing survival.     

Interactions between chilean matorral shrubs and phytophagous insects

Summary Lithraea caustica (Mol.) H. et Arn. and Colliguaya odorifera Mol. are two sclerophyllous shrubs of the scrub vegetation in Central Chile. These two species commonly grow in close association on equator-facing slopes. Leaves of L. caustica are severely predated upon by phytophagous insects while those of C. odorifera are rarely attacked. Leaf growth dynamics, leaf loss, and leaf predation by insects were correlated through the year with leaf phenolic compounds. Leaf structural properties (cuticle thickness, density, sclerenchymatic fibers, leaf thickness) were similar between the two species, while leaf phenol content differed significantly. The higher phenol content in C. odorifera may influence leaf palatability, and thus account for the observed lower leaf area loss in that species due to insect damage.     

Diversity of detoxification pathways of ingested ecdysteroids among phytophagous insects

The metabolic pathways of ingested ecdysteroids have been investigated in three insect species, the aphid Myzus persicae and two Lepidoptera, Plodia interpunctella and Ostrinia nubilalis. M. persicae produces mainly a 22-glucoside conjugate, whereas P. interpunctella eliminates a mixture of 20E and its 3-oxo and 3-epi derivatives, both in free form and as conjugates with various fatty acids. O. nubilalis only produces fatty acyl ester conjugates. These data point out the great diversity of detoxification mechanisms used by phytophagous insects in order to overcome the potential harmful effects of ecdysteroids present in their food.     

A general model for host plant selection in phytophagous insects

We develop a general theoretical framework for exploring the host plant selection behaviour of herbivorous insects. This model can be used to address a number of questions, including the evolution of specialists, generalists, preference hierarchies, and learning. We use our model to: (i) demonstrate the consequences of the extent to which the reproductive success of a foraging female is limited by the rate at which they find host plants (host limitation) or the number of eggs they carry (egg limitation); (ii) emphasize the different consequences of variation in behaviour before and after landing on (locating) a host (termed pre- and post-alighting, respectively); (iii) show that, in contrast to previous predictions, learning can be favoured in post-alighting behaviour--in particular, individuals can be selected to concentrate oviposition on an abundant low-quality host, whilst ignoring a rare higher-quality host; (iv) emphasize the importance of interactions between mechanisms in favouring specialization or learning.     

Food-deprived host-feeding parasitoids kill more pest insects

Some parasitoids not only parasitize their hosts but also destructively feed on them which could result in host mortality. We hypothesized that host-feeding parasitoids that are food-deprived before being released for biological control would destructively feed on more hosts, and an optimal duration of food deprivation of parasitoids would enhance host feeding without negatively affecting parasitism. We investigated the effects of food-deprivation durations (0, 3, 6, 10, 24 h and 20% honey solution offered), before being released, on the host mortality and parasitism by a destructive host-feeding parasitoid of whitefly nymphs, Encarsia sophia (Girault & Dodd) (Hymenoptera: Aphelinidae). In another experiment, we compared host feeding, parasitization and longevity throughout the lifespan between food-deprived and non-food-deprived parasitoids. Using fourth-instar Bemisia tabaci B biotype as the host, the responses of E. sophia differed significantly with the durations of food deprivation before they were exposed to their hosts. E. sophia adults food-deprived for 6 h killed more hosts through feeding than those that were food-deprived for 0, 3, 10 and 24 h or those that were fed with honey solution. Moreover, parasitoids that were food-deprived for 6 h parasitized more hosts than those held for other durations of food deprivation. Similar results on host-feeding were found when third-instar whitefly nymphs were used. We confirmed that E. sophia food-deprived for 6 h not only killed more whiteflies through host feeding, but also lived significantly longer and parasitized more hosts than the non-food-deprived ones throughout their lifespan. Our results could be extended to improve the effectiveness of augmentative biological control of pests through improved host feeding and parasitizing capacity by starving the parasitoids for a certain period of time before they will be used.     

Activity of gut invertase in phytophagous, saprophagous and carnivorous insects

The effect of hydrogen-ion concentration, temperature, substrate concentration, enzyme concentration and incubation period on invertase activity of Dactylosternum hydrophilioides, Catharsius molossus and Pheropsophus occipitalis has been studied. The optimum invertase activity was observed at pH 6.5, 6.8 and 6.0 and at temperatures 35, 38 and 37 degrees C in D. hydrophiliodes, C. molossus and P. occipitalis, respectively. With the increase in substrate (sucrose) concentration, an increase in invertase activity was recorded. For invertase activity estimation, 4% sucrose concentration was found suitable in these insects where an increase in enzyme concentration increases the invertase activity. The invertase activity also increases with lengthening of the incubation period but after some time it remains constant in the phytophagous, saprophagous and carnivorous insects. The activity of invertase in phytophagous, saprophagous and carnivorous insects shows their independent nature as regards the pH and temperature.     

Resistance of Pteridium aquilinum to attack by non-adapted phytophagous insects

Fresh bracken Pteridium aquilinum L. Kühn or crude bracken extracts deterred 9 non-adapted phytophagous insect species from feeding or settling. Feeding deterrent activity to 5 out of 7 insect species was present in crude bracken extracts at all times over the growing season and was significantly highest in May. Sequential isolation and purification of deterrents indicated that the sesquiterpene pterosin F (6-chlorethyl-2,5,7-trimethyl-indan-1-one) occurred in sufficiently high concentrations in late May/early June bracken frond (24–28 mg/kg fresh wt) to be partially responsible for the deterrent effects of crude bracken extracts to two insect species. A number of other fractions had feeding deterrent activity, but these were not characterized or quantified.     

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.     

Phylogenetic indices for measuring the diet breadths of phytophagous insects

Prevailing methods of measuring diet breadth of phytophagous insects are not consistent between studies and generally rely on counts of a variety of higher plant taxa (e.g. genera, families, orders). Results derived from them can be inconsistent if different taxonomic levels are used between studies. In any case, such indices do not include information from the whole branching structure of the host plant phylogeny, and do not address the fact that higher taxa are not necessarily phylogenetically equivalent. Here we present novel phylogeny-based methods which address these shortcomings. Although a previously proposed index (the Phylogenetic Diversity index) may be employed, it cannot be used to measure diets of strictly monophagous insects (i.e. those which utilise a single host species). We therefore introduce a modification of this index (the Root Phylogenetic Diversity index) which may be applied to all diets. In addition, we propose a Clade Dispersion index as a branch-length-independent measure of the degree to which hosts are scattered across the host phylogeny. We describe how these indices could be employed in studies of insect diet breadth and discuss potential problems which may be encountered in their use. Received: 16 November 1998 / Accepted: 10 February 1999     

Host specificity in phytophagous insects: selection pressure from generalist predators

The narrow host range of insect herbivores is noted, and some of the reasons why specificity has evolved are reviewed. Works indicating the need for new approaches are pointed out including the possibility that generalist predators provide a suitable pressure. Experiments to test the hypothesis that generalists are more vulnerable than specialists to predators are described. They involved a vespid wasp and over thirty species pairs of caterpillars, matched for size and density. Overall, generalists were taken more readily than specialists: some but not all reasons could be detected. The results are discussed in ecological and evolutionary terms.