Behavioural responses of the Colorado potato beetle to trichomes and leaf surface chemicals of Solanum tarijense

Wild Solanum species constitute a source of resistance to several pests and diseases of potato. Several species of wild tuber‐bearing potato have been identified as resistant to the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), including Solanum tarijense Hawkes (Solanaceae). Our objective was to determine the mechanism of resistance of S. tarijense to the Colorado potato beetle and, because the resistance is limited to the adult stage of the insect, to study the host selection behaviour on resistant plants. In the field, Colorado potato beetles demonstrated a unique behaviour when in contact with S. tarijense, abandoning the plant by falling to the ground after a few minutes. The abundant trichomes on the leaves of S. tarijense induced the falling behaviour. However, on S. tarijense feeding remained low even after the trichomes were mechanically removed. Observations demonstrated that the normal sequence of behaviour leading to feeding was interrupted before adult beetles fed on S. tarijense leaves. Feeding experiments using volatile and non‐volatile fractions of leaf surface extracts identified a phagodeterrent effect of the volatile fraction. Our results contrast with a similar evaluation of the mode of resistance of Solanum berthaultii Hawkes, a close relative of S. tarijense, on which some feeding occurred and adults did not show falling behaviour. This study presents information on S. tarijense as a new source of resistance to the Colorado potato beetle that can be used for potato breeding.     

Effects of leaf damage on oviposition choice in an invasive paropsine beetle

In 2007, an invasive paropsine beetle, Paropsisterna nr. gloriosa Blackburn, caused severe defoliation of Eucalyptus in mixed‐species foliage plantations in south‐west Ireland. At many of the plantations, Eucalyptus parvula L.A.S. Johnson & K.D. Hill was the most heavily damaged species while Eucalyptus pulverulenta Sims was generally resistant to the beetle. However, at the most heavily damaged site beetles moved to feed on E. pulvarulenta presumably during periods when suitable foliage (new leaves) of E. parvula had been severely depleted. The present study examines factors underlying shifts in oviposition from the preferred to non‐preferred host. In choice and no‐choice experiments, P. nr. gloriosa laid more eggs directly on new E. parvula foliage compared with new E. pulverulenta foliage. However, in choice experiments where new E. parvula foliage was unavailable (but old foliage available), more eggs were laid on new E. pulverulenta foliage. The potential for prior feeding damage to stimulate or deter oviposition on either host was also examined. Prior damage to new and old E. parvula leaves increased egg‐laying directly on the damaged foliage; however, prior damage to E. pulverulenta may have inhibited oviposition. The results suggest that in mixed‐species plantations, facilitation of oviposition on preferred hosts through prior feeding damage helps maintain the relative resistance of E. pulverulenta against P. nr. gloriosa, even under high beetle densities. However, the vulnerability of E. pulverulenta will increase where suitable age‐classes of preferred‐host foliage are severely depleted or unavailable.     

Suppressive effect of exposure to Chenopodium album var. centrorubrum on the feeding response of the tortoise beetle, Cassida nebulosa, to spinach

Depending upon preceding dietary exposure, the tortoise beetle, Cassida nebulosa L. (Coleoptera: Chrysomelidae), exhibits various feeding responses to spinach, Spinacia oleracea L. (Chenopodiaceae). Adult tortoise beetles reared as adults on the main host Chenopodium album var. centrorubrum Makino (CAC) (Chenopodiaceae) eat little spinach, whereas those reared on spinach accept this plant as food. In this study, we investigated the suppressive effect of exposure to CAC on the feeding response of tortoise beetles to spinach. To investigate the effect of chemical factors in CAC, we provided beetles with Gomphrena globosa L. (Amaranthaceae) as the base food and exposed them to a CAC extract, because bioassays have indicated that exposure to G. globosa did not affect the feeding response of beetles to spinach. Adults reared on G. globosa treated with CAC extract consumed significantly less than adults reared on untreated G. globosa . Thus, we determined that exposure to CAC components is responsible for the suppressive effect on the feeding response to spinach observed in the beetles. In a choice feeding test with membrane filter discs, we found that adults reared on G. globosa treated with CAC extract exhibited a greater feeding deterrent response to the spinach deterrent than adults reared on untreated G. globosa leaves. Therefore, the suppression of spinach consumption due to exposure to CAC is probably caused by the development of a response to the feeding deterrents present in spinach.     

Oviposition site selection on oats: the effect of plant architecture,plant and leaf age,tissue toughness,and hardness on cereal leaf beetle,Oulema melanopus

In sequentially planted oat stands, the cereal leaf beetle (CLB), Oulema melanopus (L.) (Coleoptera: Chrysomelidae: Lemini), is found in greater numbers, and lays more eggs, on later planted (younger) oats (Avena sativa L.) (Poaceae). Plant characteristics that could explain this ovipositional preference were examined in a series of experiments. Cage and open field whole plant preference tests confirmed the attraction of ovipositing females to younger oats. A cage effect illustrated the role of plant architecture (plant height) in CLB host selection. Two multiple‐choice and one no‐choice excised leaf experiments determined that characteristics of individual leaves associated with leaf insertion level (leaf number from base to apex) and age influence ovipositional site selection. Leaves of higher insertion level have higher nitrogen content, but fewer eggs are laid on those leaves. Two experiments examining the interaction between total leaf nitrogen and leaf insertion level showed that only leaf insertion level affected oviposition choice. Published literature suggests variation in secondary plant compounds cannot explain O. melanopus ovipositional preference among leaves. Grass leaves of higher insertion level have more extensively developed cells associated with tissue toughness and hardness. The data and supporting literature suggest tissue toughness and hardness are deterring oviposition on oat leaves of higher insertion level. However, newly eclosed larvae are able to feed on leaves usually avoided as oviposition sites. The explanation for this result may be a lack of correlation between host suitability and ovipositional preference.     

Geographic variation in oviposition choice of a leaf beetle: the relationship between host plant ranking, specificity, and motivation

The degree of adaptation of herbivorous insects to their local flora is an important component of the evolutionary processes that lead to host plant specialization in insects. In this study we investigated geographic variations in the oviposition preference of the leaf beetle Oreina elongata Suffrian (Coleoptera: Chrysomelidae: Chrysolini) in relation to differences in host plant specialization, in the field. We focused on the mechanisms of host choice and asked whether potential differences among populations are due to variations in host plant ranking and/or host plant specificity. We performed a combination of simultaneous choice and sequential no‐choice experiments with two of the major host plants of the beetle [Cirsium spinosissimum (L.) and Adenostyles alliariae (Gouan) (Asteraceae)]. The results suggested that spatial variation in host plant specialization has resulted in differences between populations in some aspects of the oviposition choice of O. elongata, while other aspects seem unaffected. We found no variation in host plant ranking among populations, as estimated in simultaneous choice tests. In contrast, the sequential no‐choice test indicated that host plant specificity was lower in a population that never encountered the highest ranked plant in the field. This finding agreed with our expectations, and we discuss our results in relation to the commonly used hierarchical threshold model. The results suggested that the mechanism for the differences in specificity is the variation among populations in the general motivation to oviposit, rather than quantitative differences in relative preference for the two hosts. We stress that it is essential to establish which of the two mechanisms is most important, as it will affect the probability of evolutionary change in host plant ranking.     

Isolation and characterization of microsatellite loci in the Alpine leaf beetle,Oreina elongata

For a study of local adaptations in the Alpine leaf beetle, Oreina elongata, we developed six microsatellite loci and screened them in 305 individuals from 13 populations. All markers were polymorphic with three to 15 alleles per locus. Average observed and expected heterozygosity values were 0.14 and 0.62, respectively. Four markers showed heterozygote deficiency and deviated significantly from Hardy–Weinberg expectations, indicating the presence of null alleles.     

Glucosinolates,flea beetle resistance,and leaf pubescence as taxonomic characters in the genus Barbarea (Brassicaceae)

Glucosinolate content of leaves and roots, diversity in leaf pubescence, and resistance to two near-isogenic lines of the flea beetle Phyllotreta nemorum with or without an R-gene, were determined for 27 accessions of 7 Barbarea taxa, i.e. B. stricta, B. orthoceras, B. intermedia, B. verna, B. vulgaris var. vulgaris, the G-type of B. vulgaris var. arcuata and the P-type of B. vulgaris var. arcuata. Four variable glucosinolate biosynthetic characters were deduced. For (formally) homophenylalanine-derived glucosinolates: (1). Presence or absence of 2-hydroxylation, and if present, R- or S-configuration of 2-hydroxylation; (2). presence or absence of p-hydroxylation; and for tryptophan-derived glucosinolates: (3). presence or absence of N-methoxyglucobrassicin; and (4). presence or absence of 1,4-dimethoxyglucobrassicin. Three phenotypes of leaf-pubescence were observed; (1). glabrous to glabrate leaves; (2). glabrous to glabrate leaves with hairs along the edge; (3). pubescent leaves. The hairs were characterized as simple by scanning electron microscopy. Full resistance to a flea beetle line (ST) was found in B. vulgaris var. vulgaris and in the G-type of var. arcuata; partial resistance was found in B. verna and B. intermedia, while the remaining taxa were fully susceptible to the ST line. All investigated Barbarea taxa were susceptible to larvae from another line containing an R-gene, indicating a similar flea beetle resistance mechanism in the three resistant species. Most Barbarea taxa could be characterized by a particular combination of the investigated characters. The most aberrant was the P-type of B. vulgaris var. arcuata, and the taxonomic status of this type should be reconsidered.     

Effect of vegetation density, height, and connectivity on the oviposition pattern of the leaf beetle Galeruca tanaceti

Vegetation structure can profoundly influence patterns of abundance, distribution, and reproduction of herbivorous insects and their susceptibility to natural enemies. The three main structural traits of herbaceous vegetation are density, height, and connectivity. This study determined the herbivore response to each of these three parameters by analysing oviposition patterns in the field and studying the underlying mechanisms in laboratory bioassays. The generalist leaf beetle, Galeruca tanaceti L. (Coleoptera: Chrysomelidae), preferentially deposits its egg clutches on non‐host plants such as grasses. Earlier studies revealed that oviposition within structurally complex vegetation reduces the risk of egg parasitism. Consequently, leaf beetle females should prefer patches with dense, tall, or connected vegetation for oviposition in order to increase their reproductive success. In the present study, we tested the following three hypotheses on the effect of stem density, height, and connectivity on oviposition: (1) Within habitats, the number of egg clutches in areas with high stem densities is disproportionately higher than in low‐density areas. The number of egg clutches on (2) tall stems or (3) in vegetation with high connectivity is higher than expected for a random distribution. In the field, stem density and height were positively correlated with egg clutch presence. Moreover, a disproportionately high presence of egg clutches was determined in patches with high stem densities. Stem height had a positive influence on oviposition, also in a laboratory two‐choice bioassay, whereas stem density and connectivity did not affect oviposition preferences in the laboratory. Therefore, stem height and, potentially, density, but not connectivity, seem to trigger oviposition site selection of the herbivore. This study made evident that certain, but not all traits of the vegetation structure can impose a strong influence on oviposition patterns of herbivorous insects. The results were finally compared with data on the movement patterns of the specialised egg parasitoid of the herbivore in comparable types of vegetation structure.     

DC-EPG assisted comparison of European spittlebugs and sharpshooters feeding behaviour on grapevine

Xylem-feeding is apparently the only requirement making an insect a competent vector of the bacterium Xylella fastidiosa, an organism responsible for the devastation of the Southern Italian olive forest and nowadays considered one of the most feared threats to agriculture and landscape in Europe, including vineyards. Here, we used the direct current-electrical penetration graph (DC-EPG) technique to compare and describe the feeding behaviour on grapevine of four xylem-feeding species considered candidate vectors of X. fastidiosa widespread in Europe, namely two spittlebugs (the meadow spittlebug Philaenus spumarius and the spittlebug Neophilaenus campestris) and two sharpshooter leafhoppers (the rhododendron leafhopper Graphocephala fennahi and the green leafhopper Cicadella viridis). We created a standard for the analysis of EPG waveforms recorded with a DC-EPG device, describing feeding activities performed by these insects from stylet insertion into the plant to withdrawal. This standard, along with freely available software, has been developed to harmonize the calculation of feeding behavioural parameters in xylem-feeders. The most relevant differences between the two vector taxa were the probing frequency and the dynamics of xylem ingestion. Sharpshooters tended to perform significantly more probes than spittlebugs. In contrast, the latter spent longer times in low-frequency xylem ingestion, characterized by scattered contractions of the cibarial dilator muscle interspersed with periods of pump inactivity. Cicadella viridis was the species displaying the highest frequency of the electrical pattern found to be associated with X. fastidiosa inoculation in spittlebugs (Xe). Feeding behavioural data presented here represent an important step forward for deepening our knowledge of xylem-sap feeding insects' interaction with both the host plants and the bacterium they transmit.     

Attachment ability of the beetle Chrysolina fastuosa on various plant surfaces

To study the role of different structures of a plant surface preventing insect attachment, a variety of plant surfaces were screened. Attachment ability of the beetle Chrysolina fastuosa Scop. (Coleoptera, Chrysomelidae) was measured on 99 surfaces among them smooth, hairy, felt-like, waxy, and glandular ones of three plant organs (stems, leaves, fruits) of 83 plant species belonging to 45 families. Insects attached successfully to smooth, hairy, and felt-like substrata. These surface types did not effect the further attachment of C. fastuosa, indicating the adhesive system remained intact after contacting these substrata. However, the beetles could not attach properly to surfaces covered with wax crystalloids or glandular hairs. In most experiments on pruinose plant substrata, no influence of the surfaces on the subsequent attachment ability of insects was observed. Only in one case (the stem of Acer negundo), was such an impairment recorded, but recovery of attachment ability was fast. Crystalloids of this plant species probably temporarily disable function of tenent setae of C. fastuosa. Four hypotheses, explaining anti-adhesive properties of plant surfaces, covered with wax crystalloids are proposed. A plant surface with glandular trichomes disabled the attachment system of the beetle for a long time. Secretions of trichomes probably glue tenent setae together making further attachment impossible.     

Host-plant discrimination and evolution of feeding preference in the Colorado potato beetle Leptinotarsa decemlineata

ABSTRACT. The behaviour of newly emerged adult Colorado potato beetles on preferred hosts follows a stereotyped pattern of sampling, feeding, grooming and rest. Reduced meal sizes on less-preferred hosts is accompanied by increased sampling and frequent interruptions in feeding. A systematic increase in pre-ingestive sampling on less-preferred foodplants indicates that beetles discriminate among closely related species within the Solanaceae. This ability may depend primarily on stimuli perceived at, and near, the leaf surface. Three geographic populations of beetles have adapted to different local host plants, but have not lost their preference for feeding on an ancestral host species. Host shifts by oligophagous insects to related plant species may evolve through selection for feeding generalists in isolated populations, and may not require genetic changes affecting the perception of a particular novel host.     

Effects of leaf toughness on mandibular wear of the leaf beetle, Plagiodera versicolora

Abstract. 1. The effects of leaf toughness on mandibular wear of the leaf beetle Plagiodera versicolora Laich. (Coleoptera: Chrysomelidae) were evaluated by feeding adult beetles young (tender) and old (tough) leaves of Salix babylonica and S. alba 'Tristis'.
2. Tough leaves erode the cutting surface of beetle jaws more so than tender leaves.
3. Beetles with worn jaws consumed leaves at a slower rate than ones with less mandibular wear.
4. Because rates of leaf consumption and egg production are positively correlated, increased mandibular wear may reduce beetle fecundity.
5. These results support the belief that leaf toughness may act as a potent defence affecting morphology, feeding behaviour, and ultimately spatial and temporal patterns of herbivores.     

水稻施用硅肥对稻纵卷叶螟幼虫取食和成虫产卵选择性的影响

采用感虫水稻品种TN1,设置3种施硅水平,即高硅(0.32 g Si/kg土壤)、低硅(0.16 g Si/kg土壤)和不施硅对照(0 g Si/kg土壤),研究了施用硅肥对稻纵卷叶螟产卵和取食选择性的影响。结果表明:稻纵卷叶螟幼虫对硅处理水稻叶片的取食选择性和成虫在硅处理水稻上的着卵量、着卵率均显著低于对照水稻。高硅处理水稻叶片中的硅含量、可溶性糖含量和碳氮比高于对照,而氮含量低于对照;低硅处理水稻叶片的碳氮比高于对照、氮含量低于对照。同时,硅处理显著降低水稻的卷叶株率和卷叶率。这些结果表明,施硅能增强稻纵卷叶螟对水稻的不选择性,从而增强水稻对稻纵卷叶螟的抗性。     

Host selection behaviour of deathwatch beetle, Xestobium rufovillosum: Oviposition preference choice assays testing old vs new oak timber, Quercus sp.

In a choice bio-assay adult female deathwatch beetles were offered two dendrochronologically dated wood blocks from oak timber to study oviposition preference behaviour. There was a clear preference for ovipositing on old wood dating from the 13th to 19th centuries rather than new wood from the 20th century. Control, same-century choice, experiments showed that beetles will oviposit on young wood and that the age of the wood does not alter the overall oviposition potential. Oviposition frequency varied with insect age. Fecundity of insects collected from an infested building was similar to that of insects maintained in culture.     

Feeding damage by larvae of the mustard leaf beetle deters conspecific females from oviposition and feeding

Herbivorous insects may be informed about the presence of competitors on the same host plant by a variety of cues. These cues can derive from either the competitor itself or the damaged plant. In the mustard leaf beetle Phaedon cochleariae (Coleoptera, Chrysomelidae), adults are known to be deterred from feeding and oviposition by the exocrine glandular secretion of conspecific co-occurring larvae. We hypothesised that the exocrine larval secretion released by feeding larvae may adsorb to the surface of Chinese cabbage leaves, and thus, convey the information about their former or actual presence. Further experiments tested the influence of leaves damaged by conspecific larvae, mechanically damaged leaves, larval frass and regurgitant on the oviposition and feeding behaviour of P. cochleariae. Finally, the effect of previous conspecific herbivory on larval development and larval host selection was assessed. Our results show that (epi)chrysomelidial, the major component of the exocrine secretion from P. cochleariae larvae, was detectable by GC-MS in surface extracts from leaves upon which larvae had fed. However, leaves exposed to volatiles of the larval secretion were not avoided by female P. cochleariae for feeding or oviposition. Thus, we conclude that secretion volatiles did not adsorb in sufficient amounts on the leaf surface to display deterrent activity towards adults. By contrast, gravid females avoided to feed and lay their eggs on leaves damaged by second-instar larvae for three days when compared to undamaged leaves. Mechanical damage of leaves and treatment of artificially damaged leaves with larval frass or regurgitant did not affect oviposition and feeding of P. cochleariae. Since no adverse effects of previous herbivory on larval development were detected, we suggest that female P. cochleariae avoid Chinese cabbage leaves damaged by feeding larvae for other reasons than escape from competition or avoidance of direct negative effects that result from consuming induced plant material.     

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.     

Plant resistance, plant traits, and host plant choice of the leaf-folding sawfly on the arroyo willow

Abstract. 1. The hypotheses that genetic variation in host plant resistance of the arroyo willow affected leaf folder ( Phyllocolpa sp.) (Hymenoptera: Tenthredinidae) density and that genetic variation in shoot length and leaf length was correlated with resistance were tested.
2. Willows grown in pots and exposed to ovipojsition by the leaf folding sawfly in cages had significantly different densities among clones, indicating variation in resistance caused by genetic differences among conspecific host plants.
3. There was a general correspondence between leaf folder density on potted cuttings and on the plants in the field that were the sources of cuttings.
4. In behavioural choice experiments, susceptible clones (with highest leaf fold densities) had the highest oviposition activity of female leaf folders compared to clones that were resistant to the leaf folder.
5. Clones differed significantly in shoot length and leaf length among clones grown in pots, among clones in the field, and between shoots with galls and shoots without galls on clones in the field.
6. Leaf folder density was significantly positively correlated with mean shoot length on field clones in 1985 and 1986, but was not correlated with leaf length, although leaf length and shoot length were correlated.
7. Leaf length variation among willow clones accounted for a significant portion of the variation in resistance of potted willows, but shoot length was unimportant.