Host Plant Acceptance Research Articles

Interspecific Interactions Affect Pests Differently

Spider mites, Tetranychus urticae Koch (Acari: Tetranychidae) and aphids, Myzus persicae (Sulzer) (Pterygota: Aphididae) share many host-plants, similar abiotic conditions and are world-wide distributed therefore, they often occur simultaneously in crops. However, the effects of interspecific interactions on the biology of these pests were poorly investigated. To test if they perform differently under intra- versus inter-specific interactions, host-plant acceptance, fecundity, survival, the total number of individuals and the rate of increase in the number of individuals were studied doing non-choice bioassays under laboratory conditions with leaf discs of tomato (Solanum lycopersicum L. ‘Ailsa Craig’), pak choi (Brassica rapa L. var. chinensis ‘Black Behi’) and bean (Phaseolus vulgaris L. ‘Saxa’). Alone, the pests differently accepted the host-plants. The acceptance of pak choi by spider mites was lower under interspecific interactions and higher on tomato for aphids. In general, spider mites’ performance decreased when aphids were present; the fecundity, the number of individuals and the rate of increase being significantly lower on pak choi and bean. In contrast, aphids produced more offspring in the presence of spider mites, leading to a higher rate of increase in aphids individuals on tomato and pak choi. Thus, pest’ responses to interspecific interactions is species-specific.

Disrupting Buchnera aphidicola, the endosymbiotic bacteria of Myzus persicae, delays host plant acceptance

Myzus persicae Sulzer, like almost all aphids, associates with the endosymbiotic bacterium, Buchnera aphidicola. Although the accepted function of B. aphidicola is to complete the aphid diet with nutrients such as essential amino acids and vitamins, there is evidence that the bacteria may participate in the plant–insect interaction. Moreover, bacterial proteins with potential effector action on the metabolism of the host plant have been identified in the saliva of M. persicae. However, the possible involvement of B. aphidicola in relation to host plant acceptance by aphids needs further investigation. The aim of this study was to evaluate the effect that the disruption of the B. aphidicola–M. persicae symbiosis has on aphid feeding behaviour and on the expression of aphid salivary genes. The antibiotic rifampicin was administrated to adult aphids through artificial diets to disrupt the bacterial primary endosymbionts. Comparisons were made with control aphids, feeding from diet without rifampicin, as well as normal aphids fed on radish plants. Differences were found in the feeding behaviour of aposymbiotic aphids, which had delayed host acceptance and problems during stylet penetration into host plants. It was also found that B. aphidicola disruption down-regulated the expression of the Mp63 salivary protein gene. Together, these results indicate that B. aphidicola plays a role in plant–aphid interactions. The validity of the use of artificial diets in plant–aphid studies is also discussed.

Effects of three novel resistant black raspberry selections on Amphorophora agathonica feeding behavior and performance

Host plant resistance is a practical and cost-effective approach for growers to manage insect pests. Recently, three new sources of resistance in black raspberry (Rubus occidentalis; selections ORUS 3778-1, ORUS 3817-1, and ORUS 4109-1) against the large raspberry aphid, Amphorophora agathonica, were identified. We studied stages of host plant acceptance: host plant attraction, parturition (deposition of nymphs), nymph survival, and adult feeding behavior (using the electrical penetration graph [EPG]) to identify the location of the plant resistance. Aphids were more attracted to the susceptible cultivar ‘Munger’ (control) than the resistant selections. Parturition occurred on the resistant selections, but fewer nymphs were deposited on resistant lines relative to the susceptible control. Nymphs survived only an average of 3.3–3.6 days on resistant selections, whereas 94 % were still alive after 11 days on ‘Munger.’ There were differences in feeding behavior between the susceptible control and the resistant selections, but no differences between the three resistant selections. The tissues responsible for resistance appear to be the mesophyll and phloem sieve elements. Aphids had a reduced probability of salivation into the phloem sieve elements of resistant selections, and only one aphid each on ORUS 3778-1 and ORUS 4109-1 successfully ingested from the phloem. Because feeding behavior of A. agathonica did not differ between resistant selections, independent confirmation that resistance is conferred by unique genes should be obtained before pyramiding these sources together.

Plant response to touch affects the behaviour of aphids and ladybirds

Touching between leaves of the same plant and/or by neighbouring plants is one of the most common mechanical stimuli to which an individual plant has to respond on a daily basis. The possible ecological implications of a plant’s response to touch on plant–insect interactions have not been explicitly investigated. We examined whether plant response to 1 min daily touching over a period of 6 days affects host plant acceptance by the bird cherry-oat aphid Rhopalosiphum padi L. on maize and by the black bean aphid Aphis fabae Scop. on bean, as well as olfactory preference of an aphid predator, seven-spotted ladybird Coccinella septempunctata L. Maize plants responded to touch with significant reduction in plant height, total plant biomass, leaf weight, leaf surface, shoot/root ratio and specific leaf area (SLA), while bean plants responded with reduced stem height and reduced SLA. Both aphid species showed significantly reduced acceptance of touched plants compared with untouched plants. The two aphid species and male and female ladybirds preferred volatiles from untouched plants over those from touched plants. Volatiles in the headspace of touched and untouched plants were collected and identified. Stepwise discriminant analyses identified (E)-nerolidol and (E)-β-caryophyllene in maize and 6-methyl-5-hepten-2-one and an unidentified sesquiterpene in bean as the best discriminating compounds in the volatile profiles of touched plants. Our study suggests that touch-induced changes in plants can potentially affect host plant selection by aphids and habitat searching by ladybirds. Thus, touch-induced changes in plants may have significant effects at higher trophic levels.

Response of Solanum tuberosum to Myzus persicae infestation at different stages of foliage maturity

Young leaves of the potato Solanum tuberosum L. cultivar Kardal contain resistance factors to the green peach aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) and normal probing behavior is impeded. However, M. persicae can survive and reproduce on mature and senescent leaves of the cv. Kardal plant without problems. We compared the settling of M. persicae on young and old leaves and analyzed the impact of aphids settling on the plant in terms of gene expression. Settling, as measured by aphid numbers staying on young or old leaves, showed that after 21 h significantly fewer aphids were found on the young leaves. At earlier time points there were no difference between young and old leaves, suggesting that the young leaf resistance factors are not located at the surface level but deeper in the tissue. Gene expression was measured in plants at 96 h postinfestation, which is at a late stage in the interaction and in compatible interactions this is long enough for host plant acceptance to occur. In old leaves of cv. Kardal (compatible interaction), M. persicae infestation elicited a higher number of differentially regulated genes than in young leaves. The plant response to aphid infestation included a larger number of genes induced than repressed, and the proportion of induced versus repressed genes was larger in young than in old leaves. Several genes changing expression seem to be involved in changing the metabolic state of the leaf from source to sink.

Transgenic Bt Rice Does Not Challenge Host Preference of the Target Pest of Rice Leaffolder, Cnaphalocrocis medinalis (Lepidoptera: Pyralidae)

BackgroundTransgenic Bt rice line T2A-1 expresses a synthesized cry2A gene that shows high resistance to Lepidoptera pests, including Cnaphalocrocis medinalis (Guenée) (Lepidoptera: Pyralidae). Plant volatile orientation cues and the physical characteristics of the leaf surface play key roles in host location or host-plant acceptance of phytophagous insects. These volatile compounds and physical traits may become altered in Bt rice and it is not known whether this influences the behavior of C. medinalis when searching for oviposition sites.ResultsThe results of electronic nose analysis showed that the Radar map of Bt rice cultivars was analogous to the non- Bt rice cultivars at each growing stage. PCA analysis was able to partly discriminate between some of the Bt vs. non-Bt rice sensors, but could not to separate Bt cultivars from non-Bt cultivars. The total ion chromatogram between Bt and non-Bt rice cultivars at the seedling, booting and tillering stages were similar and 25 main compounds were identified by GC-MS. For most compounds, there was no significant difference in compound quantities between Bt and non-Bt rice cultivars at equivalent growth stages. The densities of the tubercle papicles and the trichomes on the upper and lower surfaces were statistically equal in Bt and non-Bt rice. The target pest, C. medinalis, was attracted to host rice plants, but it could not distinguish between the transgenic and the isogenic rice lines.ConclusionsThere were no significant differences between the Bt rice line, T2A-1 and the non-Bt rice for volatiles produced or in its physical characteristics and there were no negative impacts on C. medinalis oviposition behavior. These results add to the mounting evidence that Bt rice has no negative impact on the target insect oviposition behavior.

Ecological and genetic factors influencing the transition between host‐use strategies in sympatric Heliconius butterflies

Shifts in host-plant use by phytophagous insects have played a central role in their diversification. Evolving host-use strategies will reflect a trade-off between selection pressures. The ecological niche of herbivorous insects is partitioned along several dimensions, and if populations remain in contact, recombination will break down associations between relevant loci. As such, genetic architecture can profoundly affect the coordinated divergence of traits and subsequently the ability to exploit novel habitats. The closely related species Heliconius cydno and H.melpomene differ in mimetic colour pattern, habitat and host-plant use. We investigate the selection pressures and genetic basis underlying host-use differences in these two species. Host-plant surveys reveal that H.melpomene specializes on a single species of Passiflora. This is also true for the majority of other Heliconius species in secondary growth forest at our study site, as expected under a model of interspecific competition. In contrast, H.cydno, which uses closed-forest habitats where both Heliconius and Passiflora are less common, appears not to be restricted by competition and uses a broad selection of the available Passiflora. However, other selection pressures are likely involved, and field experiments reveal that early larval survival of both butterfly species is highest on Passiflora menispermifolia, but most markedly so for H.melpomene, the specialist on that host. Finally, we demonstrate an association between host-plant acceptance and colour pattern amongst interspecific hybrids, suggesting that major loci underlying these important ecological traits are physically linked in the genome. Together, our results reveal ecological and genetic associations between shifts in habitat, host use and mimetic colour pattern that have likely facilitated both speciation and coexistence.

Does phloem-based resistance to aphid feeding affect host-plant acceptance for reproduction? Parturition of the pea aphid,Acyrthosiphon pisum, on two near-isogenic lines ofMedicago truncatula

Probing behaviour (prior to parturition) and parturition of two clones (PS01 and N116) of the pea aphid, Acyrthosiphon pisum on two genotypes (near-isogenic lines (NILs)) (Q174_5.13 and Q174_9.10) of Medicago truncatula were investigated using electrical penetration graph (EPG) coupled with simultaneous visual monitoring for parturition. Line Q174_5.13 has been reported to show a phloem-based resistance to feeding in the clone PS01 but to be susceptible to the clone N116, whereas Q174_9.10 has shown to be susceptible to both aphid clones. The time taken to first parturition by clone PS01 was similar on Q174_5.13 and Q174_9.10. Prior to parturition, no aphids on Q174_5.13 contacted phloem, but 5% of the aphids on Q174_9.10 showed phloem salivation (recognized by EPG pattern E1). No phloem contact was observed with aphid clone N116 on either NILs of Medicago before first parturition occurred, and the time taken to first larviposition was similar on Q174_5.13 and Q174_9.10. The results indicate that the initiation of parturition of the clone PS01 and N116 on both NILs does not require the phloem contact and seems unchanged by a phloem-based resistance mechanism to feeding on Medicago. This finding suggests that host recognition and decisions about parturition occur before phloem contact or ingestion, and act independently on R-gene-mediated resistance.

Chemical Ecology of the Colorado Potato Beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), and Potential for Alternative Control Methods.

The Colorado potato beetle (CPB) has been a major insect pest to potato farming for over 150 years and various control methods have been established to reduce its impact on potato fields. Crop rotation and pesticide use are currently the most widely used approaches, although alternative methods are being developed. Here we review the role of various volatile and nonvolatile chemicals involved in behavior changes of CPB that may have potential for their control. First, we describe all volatile and nonvolatile chemicals involved in host plant localization and acceptance by CPB beetles, including glycoalcaloids and host plant volatiles used as kairomones. In the second section, we present the chemical signals used by CPB in intraspecific communication, including sex and aggregation pheromones. Some of these chemicals are used by natural enemies of CPBs to locate their prey and are presented in the third section. The last section of this review is devoted a discussion of the potential of some natural chemicals in biological control of CPB and to approaches that already reached efficient field applications.

EPG technique as a tool to reveal host plant acceptance by xylem sap‐feeding insects

AbstractThe discovery of a new exotic insect herbivore triggers responses from biosecurity agencies, one of which is the decision of whether or not to attempt eradication. Rapid determination of the host range of the new invader is necessary, but when sap‐sucking insects are first collected from plants, the lack of visible signs of feeding damage makes it difficult to determine their host status. We investigated the Electrical Penetration Graph (EPG‐DC) technique as tool to assess host range of a xylem sap‐feeding invader, using Carystoterpa fingens (Hemiptera: Aphrophoridae), a New Zealand endemic xylem feeder, as a model insect. Real‐time probing and feeding events over a 12‐h recording period were compared for adult C. fingens on 18 plant species. Hebe azure, a known host, was designated the ‘reference plant species’ against which events on all other plants were statistically compared. EPG waveforms were categorized on their amplitude, frequency, voltage and electrical origin, and six parameters (time taken to first probe, time to first xylem ingestion from first probe, total probing time, number of xylem‐ingesting events, duration of the longest xylem‐ingesting event and total xylem ingestion time) were measured. The total xylem ingestion period (i.e. the actual feeding period) on each plant species expressed as a percentage of total probing time was considered the best parameter for comparing the host status of plants with H. azure. Although the EPG data overestimated the actual host range of C. fingens, we consider that they provided a reasonable first guide to the potential host status of the unknown plants, and so might usefully be used to rapidly assess whether a plant from which a new invader was collected was a host, or whether the association was merely incidental.

Two sugar isomers influence host plant acceptance by a cereal caterpillar pest

Plant sugars are often considered as primary feeding stimuli, conditioning host plant acceptance by herbivorous insects. Of the nine sugars identified from methanolic extracts of seven grass species, only turanose, a sucrose isomer, was negatively correlated with the survival and growth of the noctuid larva of cereal stemborer, Busseola fusca. Sucrose was the most abundant sugar, although it did not vary significantly in concentration among the plant species studied. Using Styrofoam™ cylinders impregnated with increasing concentrations of turanose or sucrose, the two sugars had opposing effects: turanose appeared phagodeterrent while sucrose was phagostimulatory. Electrophysiological studies indicated that B. fusca larvae were able to detect both sugars via their styloconic sensilla located on the mouthparts. The findings indicate that, whereas sucrose is a feeding stimulant and positively influences food choice by B. fusca larvae, turanose negatively contributes to larval food choice. The balance in concentrations of both sugars, however, somehow influences the overall host plant choice made by the larvae. This can partly explain host plant suitability and choice by this caterpillar pest in the field.

Settling behavior of the glassy‐winged sharpshooter, Homalodisca vitripennis, vector of Xylella fastidiosa, on southern highbush blueberry cultivars

AbstractThe xylem‐limited bacterium Xylella fastidiosa Wells, Raju, Hung, Wiseberg, Mandelico‐Paul & Brenner causes bacterial leaf scorch, a new disease of southern highbush blueberry [Vaccinium corymbosum L. (Ericaceae) interspecific hybrids] in the southeastern USA. The bacterium is transmitted by sharpshooter leafhoppers (Hemiptera: Cicadellidae), with the glassy‐winged sharpshooter (GWSS), Homalodisca vitripennis (Germar), being the most common potential vector in blueberry plantings. Considerable differences are observed in disease intensity among blueberry cultivars in the field, but it remains unknown whether these differences are due to variations in cultivar susceptibility to the bacterium, differences in attractiveness to its vector(s), or both. The settling and feeding behavior of GWSS was evaluated on three southern highbush blueberry cultivars (FL 86‐19, Star, and Emerald) in the greenhouse; in previous field studies, these cultivars were found to be affected by bacterial leaf scorch at high, moderate, and low levels, respectively. Potted plants were caged together with field‐collected GWSS adults for a period of 5 days, with either all three cultivars caged simultaneously (choice test) or individually (no‐choice test). In the choice tests, no differences in cultivar attractiveness were observed during the first 2 days of study. However, from the 3rd day, significantly more adults settled on Emerald (52.7%) than on Star (25.5%) and FL 86‐19 (17.7%). In contrast, in no‐choice tests, the overall host plant acceptance for the three cultivars was similar, and 89.3–92.1% of adults settled on these plants. A significantly greater number of adults settled on the stems (79.3%) vs. the leaves (11.2%). Moreover, they preferred the middle portion of the shoot (56.9%) compared with the upper (7.8%) and the lower (14.7%) portions. In a separate study on FL 86‐19, greater numbers of GWSS nymphs settled on the upper (actively growing) portion of the shoot (82.3%), whereas only 15.8 and 1.5% settled on the middle and lower portions, respectively.

An electrophysiological analysis of the effect of phagostimulant mixtures on the responses of a deterrent-sensitive cell of gypsy moth larvae, Lymantria dispar (L.)

Gypsy moth larvae, Lymantria dispar (L.), are polyphagous feeders. The medial styloconic sensillum of this species bears a taste receptor cell that responds to alkaloids and another that responds to the sugar alcohol, inositol. The lateral styloconic sensillum bears a taste receptor cell that is sensitive to the sugar, sucrose. We tested the effect of two phagostimulants, namely sucrose and inositol, on the response of the deterrent-sensitive cell and found that both phagostimulants suppressed its response, equally, while their combination was significantly more effective. We also tested the effect of two alkaloids (i.e., strychnine and caffeine), which deter feeding in this species, on the response of the inositol- and sucrose-sensitive cells. Although both of these deterrents had no effect in suppressing the response of the sucrose-sensitive cell, they both had an effect in suppressing the inositol-sensitive cell. We also found that sucrose suppressed the response of the inositol-sensitive cell, whereas inositol had no significant effect on the response of the sucrose-sensitive cell. In this paper, we examined the effect of mixtures of these compounds to determine the nature of their interaction. In the context of host–plant interactions and, for example, host recognition, whereby host plant acceptability depends on the total sensory impression acquired from responses to multiple plant components rather than the presence or absence of single stimulant or deterrent compounds, this study could have a direct bearing in the development of natural compounds (i.e., alkaloids) for pest control and crop protection. It will also contribute to our understanding of the neural basis of the feeding behavior of this insect.

Prefeeding and Acceptance Behavior of an Oligophagous Beetle is Dependent on Plant Suitability and Rearing History

Host prefeeding and acceptance behavior of herbivorous insects may strongly differ depending upon the plant species and previous experience. We examined prefeeding behaviors and acceptance of host and non-host plants by the beetle Phaedon cochleariae in populations with different rearing histories using a camera. Suitable host plants evoked short test biting followed by long feeding durations indicating that these first contacts provide sufficient information to stimulate feeding. In contrast, plants of intermediate quality caused prolonged test biting but feeding duration was rather low. When offered unsuitable plants, beetles showed only rare leaf contacts and test biting, demonstrating that test biting behavior is a useful measure of plant suitability. Rearing history had a minor influence on host plant exploitation behavior.

Host plant resistance among tomato accessions to the spider mite Tetranychus evansi in Kenya

The spider mite Tetranychus evansi has a broad range of host plants. Control of T. evansi has been a big challenge to tomato farmers due to its fast rate of reproduction, development of resistance to chemical pesticides and its ability to use weeds as alternative hosts when the tomato plants are not available. The aim of the current study was to determine the host plant acceptance and the relative contributions of trichomes in the control of the red spider mite by comparing the survival, development and oviposition rates of the red spider mite on eight tomato accessions. Leaflets from eight tomato varieties were assayed with the spider mites to determine the egg laying capacity and developmental time of the spider mites on the tomato accessions as well as the trichome densities. Densities of trichome types I, IV, V and VI varied among the tomato accessions. Variation in types I, IV and VI accounted for most of the variation in mite responses. The varieties with high densities of types IV and VI had the highest fecundity and mite development did not go beyond the larval stage. The developmental time varied significantly among the tomato accessions. The results indicated that the higher the density of trichome type I the lower the adult survival. The findings indicated possible resistance of some of the tested tomato accessions against T. evansi which is partially associated with trichomes types and density.

Plant volatile-induced aphid resistance in barley cultivars is related to cultivar age

Recent studies have shown that volatile chemical interaction between certain barley (Hordeum vulgare) cultivars can cause reduced host plant acceptance by the aphid Rhopalosiphum padi, and that certain cultivars can induce this effect while others can respond. In this study, we tested whether inducing and responding capabilities are linked to year of release in Swedish two-rowed spring barley. Eighteen cultivars released between 1897 and 1992 were tested in randomly selected subsets with pairwise combinations of volatile emitters and receivers. Significantly reduced aphid acceptance as a result of exposure to volatiles from plants of a different cultivar were found in 24% of the cultivar combinations. In general, older cultivars had a higher degree of aphid resistance after barley volatile treatment than did younger cultivars. The inducing effect of the emitter was also related to date of emitter cultivar release but the time relationship was reversed. Combinations with a younger volatile emitter and an older volatile receiver gave the strongest reduction in aphid acceptance of treated plants. Linear relationships between microsatellite diversity of emitting cultivars and their efficiency as inducers indicated that younger cultivars might have a more unique odour, whereas older cultivars may be more sensitive to induction.

Performance of eighteen tetranychid mite species (Acari: Tetranychidae) on borage and strawberry

Borage (Borago officinalis L.) is anticipated as a banker plant in strawberry greenhouses to adopt integrated aphid management, because borage has a stimulatory influence on parasitoid wasps attacking aphids. However, before actually using borage, we had to answer the following question: will borage become a hotbed for spider mites? To investigate this, host plant suitability and acceptance by 18 tetranychid mite species were tested on borage and strawberry leaves. The mites were two Panonychus species, two Eotetranychus species and 14 Tetranychus species, including the green and red forms of T. urticae that were counted as separate species. Borage appeared an unsuitable host, because females of all 18 mite species did not lay eggs, females of most species could not survive for more than 5 days, and larvae of all 18 mite species did not develop to nymphal stages on borage. Conversely, strawberry was a suitable host for 13 of the 18 spider mite species: females laid eggs and about 90% of larvae successfully reached adulthood. Thus, these13 species have the potential to become pests of strawberry. Offering female spider mites the choice between interconnected borage and strawberry leaf discs, (1) most females moved from borage to strawberry except for the two Panonychus species and E. asiaticus, and (2) few females moved from strawberry to either borage or strawberry. Consequently, it is highly unlikely that borage will become a hotbed for any of the spider mite species.

The ecological differentiation of asexual lineages of cotton aphids: alate behaviour, sensory physiology, and differential host associations

The cotton aphid (Aphis gossypii Glover) is a cosmopolitan pest species consisting of various host plant-associated lineages that appear to have different patterns of insecticide resistance, electrophoretic banding, and reproductive output across alternative host species. Debate continues as to whether these differences represent intraspecific variation or indicate a complex of cryptic or sibling species. We studied the host plant-related ecology of two asexual lineages of A. gossypii associated with the families Malvaceae and Cucurbitaceae, respectively, in particular the host plant finding and acceptance processes of the alate forms. The two lineages proved to be differentiated across host plants in terms of: (1) their landing and settling behaviour, nymphal production rate, and subsequent colony establishment (i.e. nymphal survival and development) and (2) their sensory abilities in relation to host plant-derived odours. Furthermore, genetic structuring according to host plant type was detected using the gene for elongation factor 1-α. The magnitude of the genetic differentiation across lineages was low, perhaps indicating a relatively recent divergence. The results demonstrate: (1) the presence of (at least) two cryptic species within the current definition of A. gossypii, as anticipated in previous studies, and (2) the practicalities of using a multidisciplinary approach to understanding the 'limits' of asexual species. If sexuals can be generated from each of these lineages, this conclusion will be open to further testing.

The role of thaumatin II in cucumber resistance against Tetranychus urticae Koch: laboratory and greenhouse evaluation

The role of thaumatin II in cucumber resistance against Tetranychus urticae Koch: laboratory and greenhouse evaluation The role of thaumatin II in the resistance of transgenic cucumber plants of the T6 generation against the two-spotted spider mite (TSSM) (Tetranychus urticae C. L. Koch 1836, Acari: Tetranychidae) was determined in greenhouse experiments evaluating: (1) mite behaviour towards cucumber lines (T224 09, T212 01, T210 06) differing in the level of thaumatin II expression, (2) mite reproductive capacity, (3) mite colonisation ability, and (4) mite leaf damage. Additionally, using Western blot, the presence of thaumatin II in leaves of transgenic cucumber plants of the T6 generation was analysed before and after TSSM feeding. Even plants with a relatively high leaf thaumatin II level (T224 09 and T212 01) were equally well accepted by TSSM in a ‘free-choice’ test, which excludes thaumatin II as an important factor in the process of host plant acceptance. However, a reduced fecundity of mite females and a lower, than in the control, number of mites found in the period of permanent feeding on thaumatin-rich plants, indicate a putative role of thaumatin II in the plant-mite interactions. The lack of a clear relationship between the level of thaumatin II and plant resistance to TSSM may be a sign that thaumatin II is not a direct factor involved in antibiosis. In response to TSSM, the thaumatin II content decreases, increases, or remains unchanged in mite-infested leaves, which excludes thaumatin II as being directly involved in induced defence.

Arbuscular mycorrhizal symbiosis increases host plant acceptance and population growth rates of the two-spotted spider mite Tetranychus urticae

Most terrestrial plants live in symbiosis with arbuscular mycorrhizal (AM) fungi. Studies on the direct interaction between plants and mycorrhizal fungi are numerous whereas studies on the indirect interaction between such fungi and herbivores feeding on aboveground plant parts are scarce. We studied the impact of AM symbiosis on host plant choice and life history of an acarine surface piercing-sucking herbivore, the polyphagous two-spotted spider mite Tetranychus urticae. Experiments were performed on detached leaflets taken from common bean plants (Phaseolus vulgaris) colonized or not colonized by the AM fungus Glomus mosseae. T. urticae females were subjected to choice tests between leaves from mycorrhizal and non-mycorrhizal plants. Juvenile survival and development, adult female survival, oviposition rate and offspring sex ratio were measured in order to estimate the population growth parameters of T. urticae on either substrate. Moreover, we analyzed the macro- and micronutrient concentration of the aboveground plant parts. Adult T. urticae females preferentially resided and oviposited on mycorrhizal versus non-mycorrhizal leaflets. AM symbiosis significantly decreased embryonic development time and increased the overall oviposition rate as well as the proportion of female offspring produced during peak oviposition. Altogether, the improved life history parameters resulted in significant changes in net reproductive rate, intrinsic rate of increase, doubling time and finite rate of increase. Aboveground parts of colonized plants showed higher concentrations of P and K whereas Mn and Zn were both found at lower levels. This is the first study documenting the effect of AM symbiosis on the population growth rates of a herbivore, tracking the changes in life history characteristics throughout the life cycle. We discuss the AM-plant-herbivore interaction in relation to plant quality, herbivore feeding type and site and the evolutionary implications in a multi-trophic context.