Defensive role of leaf trichomes in resistance to herbivorous insects in Datura stramonium

This study assessed the role of leaf trichome density as a component of resistance to herbivores, in six populations of Datura stramonium. Phenotypic selection on plant resistance was estimated for each population. A common garden experiment was carried out to determine if population differences in leaf trichome density are genetically based. Among population differences in leaf trichome density, relative resistance and fitness were found. Leaf trichome density was strongly positively correlated to resistance across populations. In 5 out of 6 populations, trichome density was related to resistance, and positive directional selection on resistance to herbivores was detected in three populations. Differences among populations in mean leaf trichome density in the common garden suggest genetic differentiation for this character in Datura stramonium. The results are considered in the light of the adaptive role of leaf trichomes as a component of defence to herbivores, and variable selection among populations.     

Reiterative and interruptive signaling in induced plant resistance to chewing insects

Our understanding of induced resistance against herbivores has grown immeasurably during the last several decades. Based upon the emerging literature, we argue that induced resistance represents a continuum of phenotypes that is determined by the plant’s ability to integrate multiple suites of signals of plant and herbivore origin. We present a model that illustrates the range of signals arising from early detection through herbivore feeding, and then through subsequent plant generations.     

Variation in leaf trichomes and nutrients of Wigandia urens (Hydrophyllaceae) and its implications for herbivory

Leaf trichome variation was studied in a population of Wigandia urens (Hydrophyllaceae) in relation to water availability, diversity of herbivorous insects, and grazing rates. Plants of W. urens have glandular and urticant (stinging hairs) trichomes, and it is possible to distinguish two types of leaves: smooth leaves with only glandular trichomes and bristly leaves with both types of trichomes. Density of urticant trichomes was negatively correlated with weekly mean precipitation. Fourteen species of insects in five different orders feed on leaves of W. urens throughout the year. Grazing rates varied according to type and age of leaves. Grazing rates were lower in smooth than bristly young leaves but the rates were similar for mature leaves. Secondary metabolites, screened using qualitative methods, showed similar classes of compounds in both types of leaves. Bristly leaves had significantly higher concentrations of nitrogen, phosphorous and water and these nutritional differences were well correlated with the differences in grazing rates between bristly and smooth leaves.     

Apparent transgenerational effects of host plant in the leaf beetle Ophraella notulata (Coleoptera: Chrysomelidae)

The natural host of Ophraella notulata is Iva frutescens (Asteraceae); its close relative feeds on a related plant, Ambrosia artemisiifolia. We reared beetles on both plants, obtained progeny from the four possible crosses (two sexes X two parental hosts), and reared the progeny on both plant species. Survival to the imaginal stage of progeny reared on Iva varied with both maternal and paternal host. Hatchling feeding response to both plants showed a maternal host X paternal host interaction. Consumption of Ambrosia by adult beetles was, counter to expectation, higher for progeny of Iva-reared males than Ambrosia-reared males. Oviposition response, although based on too few data to be definitive, was peculiar: parental host did not affect oviposition on Ambrosia; on Iva daughters of Iva-reared males laid significantly more eggs than did daughters of Ambrosia-reared males, but only if they had been reared on Iva; those reared on Ambrosia displayed the reverse pattern. We discuss the possibility that nongenetic paternal transmission of host plant effects may explain these results, but offer a somewhat uncomfortable hypothesis of selection as a preferable explanation. An important outcome of the experiment is that it provided no evidence of maternal effects of host plant on offspring feeding or oviposition.     

Comparative leaf anatomical studies of some Sterculia L. species (Sterculiaceae)

A comparative study was undertaken on the leaves of 12 Sterculia species in order to assess anatomical variations which may be useful in species identification and to evaluate their significance in the taxonomy of the genus. All species have glandular and non-glandular trichomes, anomocytic stomata, multiple and mucilaginous epidermis, mucilaginous cavities and druses in mesophyll tissues. A small amount of variation was observed in the outline of transverse sections of midribs and petioles, the presence of central vascular bundles, and the type of trichomes. Some characters, such as the vertically divided adaxial epidermis in S. macrophylla , are unique to certain species and are therefore useful in diagnostic characters, while S. coccinea and S. elongata appear to be similar in the structure of the petiole and midrib. The results, however, do not show any groupings of species and do not support the observations of others on groups derived from wood anatomy.     

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.     

Systematic leaf anatomy of Caladeniinae (Orchidaceae)

Leaf anatomy of 25 species in 15 genera of Caladeniinae (Diurideae, Orchidaceae), excluding Caladenia, was investigated to determine diagnostic characters to be used in forthcoming, broad-based cladistic analyses of the subtribe and to assess interspecific and intergeneric relationshipS. Of the characters examined, those that show the most variation among the study taxa are presence and types of trichomes, cuticular sculpturing, anticlinal walls of epidermal cells, heterogeneity of chlorenchyma, distribution and length: width ratios of stomata. Anatomical evidence supports the generic concept of Leptoceras Lindley but contradicts that of Drakonorchis Hopper & A.P. Brown. Paracaleana is not sufficiently distinct from Caleana to warrant generic status. Lyperanthus serratus and L.suaveolens are hyperstomatic, a rare condition in Orchidaceae. On the basis of these and other characters, Lyperanthus, as currently circumscribed, is polyphyletic. From leaf structure Caladeniinae as now conceived is polyphyletic and comprises seven groups: (1) Caladenia, Leptoceras, Elythranthera, Eriochilus, Glossodia, Aporostylis; (2) Adenochilus, Rimacola; (3) Arthrochilus, Chiloglottis, Spiculaea, Leporella; (4) Caleana (including Paracaleana); (5) Bumettia; (6) Lyperanthus suaveolens and L.serratus; (7) Lyperanthus nigricans.     

Effect of defoliation on leaf growth,sexual expression and reproductive success of Cnidoscolus aconitifolius (Euphorbiaceae)

Several studies on herbivory have shown that plants may have different responses to foliar damage. Most studies have been done using hermaphroditic species, here we present, using a defoliation experiment, different types of response to foliar damage by the monoecious Cnidosculus acontifolius. We evaluated four treatments: natural herbivory (control), and 25%, 50% and 100% artificial defoliation. The results suggest that this species is capable of showing the following compensatory responses: (a) undercompensation in total production of flowers and in fruit set (damage >25%); (b) full compensation in seed weight (all levels of damage); and (c) overcompensation in leaf growth and in the proportion of female flower production (damage 25%). The results show that C. aconitifolius can exhibit a very fast and plastic response depending on the intensity of damage and the time since defoliation. The increase in foliar growth suggests a long-term tolerance response to hervibory, whereas the change in the proportion of female flowers produced does not appear to have a long-term effect due to the decrease of fruit production in all levels of damage. We suggest that the response in foliar growth and the change in sexual expression by C. aconitifolius, are associated with the high resource availability conditions prevalent in the population studied.     

寄主挥发物、叶色和表皮毛在美洲斑潜蝇寄主选择中的作用

在室内条件下 ,初步研究了寄主挥发物、叶色和表皮毛在美洲斑潜蝇寄主选择中的作用。在嗅觉仪试验中 ,寄主叶片挥发物对美洲斑潜蝇雌成虫没有明显的引诱作用 ;在叶色反应试验中 ,美洲斑潜蝇雌成虫在叶子圆片上停留的时间明显大于在滤纸圆片上停留的时间 (p<0 .0 1) ,其在有叶片区域分布的数量明显多于空白对照 (p <0 .0 1) ;在表皮毛试验中 ,美洲斑潜蝇在无毛叶片上的产卵量明显大于在有毛叶片上的产卵量 (p <0 .0 1)。上述结果表明 ,在对寄主的定向和定位过程中 ,美洲斑潜蝇的视觉起着重要的作用 ,而嗅觉不起作用 ;叶片表皮毛有抑制产卵的作用     

Effect of plant variety, plant age and photoperiod on glandular pubescence and host-plant resistance to potato moth (Phthorimaea operculella) in Lycopersicon spp.

The effect of plant age and daylength on glandular pubescence was determined for two lines of tomato derived from Lycopersicon hirsutum (BTN 979 and LA 1777A) and a variety of L. esculentum (N 91‐1‐1‐1‐1). Densities of type I, IV, VI and VII glandular trichomes were lowest in N 91‐1‐1‐1‐1 and, over all varieties, were more dense on plants aged greater than 6 wk. Daylength interacted with variety to significantly affect densities of type VII trichomes only. Host‐plant resistance to Phthorimaea operculella was determined in preliminary tests using insects cultured from founders from a potato crop and in confirmatory tests using (less readily available) insects recovered from foliage of a tomato crop. Mortality of ex‐potato neonates on LA 1777 A and BTN 979 foliage was higher 18 h after placement than for N 91‐1‐1‐1‐1, with no effect of day length or plant age. Mortality for ex‐tomato neonates followed a similar trend. Ten days later, two‐thirds of ex‐tomato larvae had established mines on N 91‐1‐1‐1‐1 but fewer (16.7%) were live on other varieties. Stepwise multiple regression using variety as the sole factor was significant (P<0.001) in accounting for 61.4% of the variation in ex‐tomato larval survival but addition of other factors to the regression model was not significant. BTN 979 supported fewer, smaller adults to develop than did N 91‐1‐1‐1‐1, whilst no adults developed on LA 1777 A. In a non‐choice test using ex‐potato adults, significantly more eggs were laid on N 91‐1‐1‐1‐1 than on L. hirsutum varieties and 9‐wk‐old plants were preferred over plants three weeks older or younger. The same variety and plant age trends were evident in a free‐choice test using ex‐tomato adults.     

Predators mediate host plant resistance to a phytophagous pest in cabbage with glossy leaf wax

This study examined the role of generalist predators in producing higher mortality ofPlutella xylostella L. (Plutellidae) larvae on glossy vs. normal-wax cabbage,Brassica oleracea var.capitata L. To test this, survival and feeding ofP. xylostella were measured on individually caged glossy and normal-wax plants with and without each of three generalist predators,Chrysoperla carnea (Stephens) (Chrysopidae),Orius insidiosus (Say) (Anthocoridae), andHippodamia convergens Guerin-Meneville (Coccinellidae). In the greenhouse, predators always significantly reduced survival ofP. xylostella larvae on glossy plants, but never on normal-wax plants. In the field, predators significantly reducedP. xylostella survival on glossy plants, but onlyC. carnea was effective on normal-wax plants. In similar experiments with excised leaves,O. insidiosus andC. carnea were more effective predators on the glossy leaves, whileH. convergens was equally effective on both kinds of leaves. Patterns for feeding were similar, but significance levels differed from those forP. xylostella survival. The greater effectiveness of predators on glossy plants is apparently due to the reported improved mobility of these animals on glossy leaf surfaces. The data also suggest that reduced mining byP. xylostella exposes the larvae to more predation on glossy plants and contributes some to the resistance. Regardless of the mechanism, resistance toP. xylostella on glossyB. oleracea appears to depend on the action of generalist predators for its full expression. This dependence on predation must be considered in the development and deployment of glossy insect-resistantB. oleracea.     

Egg size of skippers (Lepidoptera: Hesperiidae) in relation to their host specificity and to leaf toughness of host plants

The adaptive significance of egg size of skippers (Lepidoptera; Hesperiidae) in Japan was evaluated in relation to the leaf toughness of their major host grasses. The hesperids that fed on tougher grasses laid larger eggs. Hesperids that laid larger eggs were larger in body size, but lower in fecundity. They also had a wider host range. Thus, despite the lower fecundity, hesperids may benefit from large eggs by having a wider host range of larvae. Grass feeders had wider range of host plants than broadleaf feeders.     

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.     

Relative availability of nitrogen in host plants of invertebrate herbivores: three possible nutritional and physiological definitions

Summary It has been postulated that free amino acids in the diet of herbivores are a more readily available source of nitrogen than are amino acids in intact proteins. When plants are subjected to any of a number of stress factors the concentration of free amino acids in their tissues may increase relative to the protein content. It has been further postulated that such changes in available nitrogen can increase the survivorship of very young herbivores feeding on the tissues of stressed plants. In this paper, the concept of readily available nitrogen is given more specific definitions in the effort to uncover an underlying nutritional or physiological mechanism responsible for such changes in larval survivorship. The first hypothesis considered is that amino acids are more readily available than proteins in the sense that, as they occur in plant tissue, they are a more nutritious source of needed nitrogenous compounds. The second hypothesis considered is that amino acids are absorbed into the insect gut free from interference by proteinase inhibitors, and thus are more chemically available than are proteins. Finally, free amino acids, because of their solubility and greater mobility, may be more available to herbivores because of physical factors restricting the movement and digestion of proteins from plant tissue into the insect gut. The applicability of each hypothesis to certain types of insects feeding on particular plant tissues is discussed.The investigation reported in this paper (No. 86-7-224) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director     

Foliage acceptability to browsing ruminants in relation to seasonal changes in the leaf chemistry of woody plants in a South African savanna

Summary We investigated seasonal changes in food selection by hand-reared kudus and impalas in savanna vegetation in northern Transvaal, South Africa. The acceptability of the leaves of woody plants to these animals was compared with leaf concentrations of nutrients, fibre components and old leaf phenophases. No consistently significant correlation was found between acceptability and any single chemical factor. Based on an a priori palatability classification, discriminant function analysis separated relatively palatable species from unpalatable species in terms of a linear combination of protein and condensed tannin concentrations. The high acceptability of certain otherwise unpalatable species during the new leaf phenophase was related to elevation of protein levels relative to condensed tannin contents. Species were added to the diet during the dry season approximately in the order of their relative protein-condensed tannin difference.     

The effects of foliar pubescence and nutrient enrichment on arthropod communities of Metrosideros polymorpha (Myrtaceae)

Abstract.  1. Nutrient resource availability and host-plant foliar pubescence both influence arthropod food webs, but multifactor studies are needed to understand their interdependence and relative importance. Arthropods were sampled by clipping foliage from Metrosideros polymorpha (Myrtaceae) trees of pubescent, glabrous, and intermediate leaf forms on fertilised and unfertilised plots.
2. Fertilisation decreased leaf mass per area (LMA) but did not change the relative mass of pubescence within leaf morphological classes.
3. Fertilisation increased densities of individuals in four taxonomic orders, densities of individuals and species of all trophic levels, and the biomass of Collembola and Homoptera. Herbivore relative diversity (Shannon H ') also increased with fertilisation, but detritivore diversity declined due to increasing dominance of Salina celebensis (Schaeffer) (Collembola).
4. Detritivore density, driven again by S. celebensis , increased with decreasing leaf pubescence, but Heteroptera and Acari were most abundant on the intermediate pubescence class, and Psocoptera density and biomass increased with increasing pubescence. Trophic-level species density did not change with leaf morphological class, but relative diversity of all arthropods and of detritivores increased with increasing pubescence.
5. Both resource availability and leaf pubescence affected Metrosideros arthropod communities. However, the pervasive positive influence of fertilisation did not translate to compositional shifts, and there were no interactions with leaf morphological class. In contrast, the effects of leaf pubescence on arthropod density, biomass, and diversity were more restricted taxonomically, and non-parametric manova and redundancy analyses demonstrated significant differentiation in community composition on the pubescent morphology.     

The colonization of native phytophagous insects in North America by exotic parasitoids

Classical biological control could have a major environmental cost if introduced natural enemies colonize and disrupt native systems. Although quantifying these impacts is difficult for systems already colonized by natural enemies, the a priori condition for such impacts can be evaluated based on the extent to which exotics have acquired native hosts. We use native host records for exotic parasitoids introduced into North America for biological control to document the number of exotic species that have been recorded from at least one native insect species. We also evaluate the ability of six biological and ecological variables to predict whether or not a parasitoid will move onto natives. Sixteen percent of 313 parasitoid species introduced against holometabolous pests are known from natives. Further, the likelihood that a parasitoid had colonized native hosts was largely unpredictable with respect to the independent variables. We conclude that given the quality of the data available either now or in the foreseeable future, coupled with inherent stochasticity in host shifts by parasitoids, there are no rules of thumb to assist biological control workers in evaluating if an introduced parasitoid will colonize native insect communities. Received: 2 July 1997 / Accepted: 3 August 1997     

Predator avoidance in phytophagous mites: response to present danger depends on alternative host quality

We studied whether volatiles released by putative host plants affect the antipredator response of an herbivorous mite, Tetranychus urticae, when the patch was invaded by Phytoseiulus persimilis. Tetranychus urticae laid a lower number of eggs on tomato leaves than on lima bean leaves, suggesting that lima bean is a preferred host food source for T. urticae. In addition, T. urticae preferred lima bean plant volatiles to tomato plant volatiles in a Y-tube olfactometer test. To investigate the antipredator response of T. urticae, we examined the migration of T. urticae from a lima bean leaf disc to a neighbouring leaf disc (either a tomato or lima bean leaf disc) when ten predators were introduced into the original lima bean disc. A Parafilm bridge allowed for migration between the leaf discs. No migrations occurred between leaf discs when there were no predators introduced to the original leaf disc. However, when predators were introduced migrations did occur. When the neighbouring leaf disc was upwind of the original disc, the migration rate of the mite from original lima bean leaf disc to a neighbouring tomato leaf disc was significantly lower than that to a neighbouring lima bean leaf disc. By contrast, when the neighbouring leaf disc was downwind of the original leaf disc, there was no difference in the migration rates between lima bean leaf discs and tomato leaf discs. The number of T. urticae killed by P. persimilis for each treatment was not different, and this clearly shows that the danger was the same in all treatments regardless of the decision made by T. urticae. From these results, we conclude that T. urticae change their antipredator response by evaluating the difference in host plant volatiles in the patch they inhabit.