Infection of susceptible/tolerant barley genotypes with Barley yellow dwarf virus alters the host plant preference of Rhopalosiphum padi clones depending upon their ability to transmit BYDV

Discovering mechanisms of plant–virus–vector interactions is fundamental to understand their ecology and evolution and to apply this knowledge in plant protection. To study the influence of varying Barley yellow dwarf virus (BYDV) transmission efficiencies on host plant preference of Rhopalosiphum padi (L.) clones, we performed host choice experiments with two barley cultivars (BYDV susceptible cv. ‘Rubina’ and BYDV tolerant cv. ‘Vixen’) including healthy and virus-infected plants. For the susceptible barley cultivar ‘Rubina’, aphid clone R07 (high transmission efficiency) preferred BYDV-infected over healthy host plants after 24 h, while clones D10 (medium transmission efficiency) and R09 (low transmission efficiency) preferred neither host. In contrast, BYDV infection of ‘Vixen’ did not affect the plant’s appeal for aphid clone R07. Host plant access, indicated by ingestion and observed by electrical penetration graph technique for a period of 2 h, was facilitated on BYDV-infected cv. ‘Rubina’ for the clones R07 and D10, whereas an opposite effect was observed for the clone R09. For R07 and R09, the difference was not visible after a period of 5 h. As observed earlier for BYDV-infected wheat, enhanced emission of volatile organic compounds associated with virus-induced attraction was detected for BYDV-infected cv. ‘Rubina.’ It is concluded that host plant preference is possibly linked with a high BYDV transmission efficiency as observed for the clone R07, leading to a fitness advantage of this clone as indicated by early increased ingestion. This advantage is not present on BYDV-tolerant genotypes most likely due to the absence of infection symptoms.

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 of Medicago 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.

Estimating the effect of poplar resistance on the performance of the woolly poplar aphid, Phloeomyzus passerinii, in various experimental conditions

The woolly poplar aphid, Phloeomyzus passerinii (Signoret) (Sternorrhyncha: Aphididae), causes the most damage to poplars in southern Europe and the Near East as outbreaks can lead to massive mortality in mature stands. Previous studies, conducted on poplar cuttings or young trees, showed that resistance varied depending on the poplar genotype. However, these results did not indicate how aphid populations were affected by tree resistance and were only partially confirmed by field observations. This study investigated the effect on aphid settlement and performance of poplar genotypes belonging to Populus × canadensis Moench, Populus × interamericana Brockh., Populus nigra L., Populus deltoides Bartram ex Marshall, and Populus trichocarpa Torr. & A. Gray ex Hook. using a single aphid clone. The study was conducted under three experimental conditions: (i) unrooted cuttings, (ii) potted cuttings, and (iii) mature trees. The aphid settlement rate, the larval development rate, and the fecundity were affected by the poplar genotype, highlighting the antixenotic and antibiotic properties involved in tree resistance to P. passerinii. The laboratory results for aphid performance and settlement rate on unrooted cuttings were transposable to both potted cuttings and mature trees. Laboratory studies could, therefore, provide useful information for identifying the parameters involved in resistance mechanisms.

Temperature modulation of photoperiodism and the timing of late-season changes in life history for an aphid, Acyrthosiphon pisum

Where winters are severe, aphids reproduce parthenogenetically and viviparously in summer, switch to sexual reproduction in late summer, and produce winter-hardy eggs by the end of the season. The role of day length and temperature in initiating seasonal changes from parthenogenetic to sexual reproduction by pea aphids, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), are described and the selection pressures that affect the timing of this transition are investigated. Over four seasons, a pea aphid clone was sampled from field cages through late summer in southern Manitoba, Canada, and reared in the laboratory to determine the phenotypes of progeny produced as the season progressed. The timing of transitions from one phenotype to another under natural day length and temperature, and the critical day lengths that caused the transitions, coincided with expectations from laboratory studies of photoperiodic responses. Males and mating females appeared later when the weather in August was warm than when it was cool. The timing of seasonal changes was adapted to minimize the physiological time to the end of the season, which maximized the number of asexual summer generations. Ambient temperature modulated the response to day length and fine-tuned the timing of sexual reproduction to adapt for annual variation in autumn weather.

Preinfestations of tomato plants by whiteflies (Bemisia tabaci) or aphids (Macrosiphum euphorbiae) induce variable resistance or susceptibility responses

In addition to constitutive plant resistance against pests or pathogens, plants can activate protective mechanisms upon contact with an invader or a chemical elicitor. Studies on induced plant resistance to herbivores, especially piercing-sucking insects, are less abundant than those devoted to pathogens. Several experiments under controlled conditions have been conducted to demonstrate that infestations byMacrosiphum euphorbiaeinduce plant resistance toBemisia tabaciin susceptible tomato plants. After three days of exposure to 20 apterous adult aphids, the plants acquired a transiently induced resistance toB. tabaciwhen aphid removal occurred one or 18 hours prior toB. tabaciinfestation; the effect disappeared when four days passed between aphid and whitefly infestations. The resistance observed was both locally and systemically induced. Other assays were performed to evaluate the effect of preinfestation with ten adults ofB. tabaciduring 48 h on the tomato responses to two different clones (Sp and Nt) ofM. euphorbiae. The numbers of nymph and adult aphids were counted after the same time interval as the pre-reproductive period and 20 (Sp clone) or 22 (Nt clone) days after adult aphid removal. The tomato responses induced by whitefly feeding depend on the aphid clone. For the Sp clone, the number of aphid nymphs ten days after adult removal was significantly higher on whitefly preinfested plants than on uninfested plants. However, no significant differences were observed when the aphid clone Nt was tested. The duration of plant response to a previous infestation byB. tabaciis apparently limited.

Influence of temperature on pea aphidAcyrthosiphon pisum(Hemiptera: Aphididae) resistance to natural enemy attack

The ability to resist or avoid natural enemy attack is a critically important insect life history trait, yet little is understood of how these traits may be affected by temperature. This study investigated how different genotypes of the pea aphidAcyrthosiphon pisumHarris, a pest of leguminous crops, varied in resistance to three different natural enemies (a fungal pathogen, two species of parasitoid wasp and a coccinellid beetle), and whether expression of resistance was influenced by temperature. Substantial clonal variation in resistance to the three natural enemies was found. Temperature influenced the number of aphids succumbing to the fungal pathogenErynia neoaphidisRemaudière & Hennebert, with resistance increasing at higher temperatures (18 vs. 28°C). A temperature difference of 5°C (18 vs. 23°C) did not affect the ability ofA. pisumto resist attack by the parasitoidsAphidius erviHaliday andA. eadyiStarý, González & Hall. Escape behaviour from foraging coccinellid beetles (Hippodamia convergensGuerin-Meneville) was not directly influenced by aphid clone or temperature (16 vs. 21°C). However, there were significant interactions between clone and temperature (while most clones did not respond to temperature, one was less likely to escape at 16°C), and between aphid clone and ladybird presence (some clones showed greater changes in escape behaviour in response to the presence of foraging coccinellids than others). Therefore, while larger temperature differences may alter interactions betweenAcyrthosiphon pisumand an entomopathogen, there is little evidence to suggest that smaller changes in temperature will alter pea aphid–natural enemy interactions.