Marie-Hélène Sauge

Induced resistance by Myzus persicae in the peach cultivar ‘Rubira’

The effect of a previous infestation by the green peach aphid Myzus persicae (Sulzer) on the settling behaviour and reproduction of the same aphid species was investigated in the resistant peach cultivar Rubira, and compared with that observed in the susceptible control cultivar GF305. A previous infestation of 48 h triggered induced resistance in Rubira. There were significantly fewer aphids settling on preinfested than on uninfested plants, indicating an increased rejection of Rubira as a host plant. The level of induced resistance in preinfested plants was positively related to the duration of the first infestation. In GF305, previous infestation had no detrimental effect on aphid settlement and even slightly enhanced larviposition by adult females. The aphid probing behaviour after a 48‐h preinfestation was also monitored for 8 h with the electrical peneration graph (EPG) technique. On preinfested GF305, most EPG parameters indicated an enhanced host plant acceptance. On preinfested GF305, aphids produced less sieve element salivation and more continuous sap ingestion than on uninfested GF305, indicating that the previous aphids provoked changes in plant properties beneficial to the test aphids. In Rubira, a major induced factor of resistance was thought to be expressed in the sieve element as phloem sap ingestion was 4‐fold shorter on preinfested than on uninfested plants. The time taken by the aphid stylets to reach a sieve element was also significantly increased on preinfested Rubira, suggesting the induction of resistance factors outside the phloem. The originality of the Rubira/M. persicae interaction is discussed in the perspective of a better understanding of plant induced responses to aphids.

Volatile organic compound emissions induced by the aphid Myzus persicae differ among resistant and susceptible peach cultivars and a wild relative

Little is known on aphid-induced emissions of volatile organic compounds (VOCs) from trees and particularly on their intraspecific variability in association with resistance traits. We compared VOC emissions from five peach cultivars (Prunus persica (L.) Batsch) and a wild relative (Prunus davidiana (Carrière) Franch) that differ in their level (susceptible/resistant) and type (antixenosis, antibiosis) of resistance to the green peach aphid, Myzus persicae (Sulzer). Additionally, the kinetics of VOC induction in response to aphids was compared with that by mechanical wounding. Qualitative and overall quantitative differences among peach genotypes were found in VOC emissions that were mainly composed of methyl-salicylate, farnesenes, (E)-β-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene. Irrespective of the type of resistance, all resistant genotypes had increased VOC emissions upon aphid attack, while in susceptible genotypes emissions remained low. Emission increases were highest in the genotypes that express increased aphid resistance during second infestations, which had also the highest proportions of methyl-salicylate in their emissions. VOC induction by aphids proceeded slowly with a delay of several hours. Artificial wounding of leaves did not result in emissions of aphid-induced VOCs but caused an immediate burst of green leaf volatiles and benzaldehyde. We conclude that VOC induction in resistant peach cultivars is part of a general defence syndrome that is being avoided or suppressed by M. persicae in the susceptible genotypes. The induction likely involves an aphid-specific elicitor and (methyl)-salicylate in the subsequent signalling and regulation processes that should include gene activation due to the marked delay in the emission response. The results are compared with those of the literature and discussed in view of their ecological and environmental significance.

Probing behaviour of the green peach aphid Myzus persicae on resistant Prunus genotypes

Electrical penetration graphs of Myzus persicae (Sulzer) (Homoptera: Aphididae) feeding behaviour on four resistant and two susceptible genotypes of peach (Prunus persica L. Batsch) and related species showed that resistance was mainly linked to (i) reduced duration of phloem sap uptake, (ii) reduced percentage of pattern E1 (salivary secretion into sieve elements) followed by pattern E2 (sap ingestion) and (iii) increased number of shifts from E1 to E2 and back. These results suggest the unsuitability of phloem sap, and thus repetitive failures to initiate sustained ingestion. Extensive comparisons of the EPGs also revealed more specific trends. Aphids on the most susceptible cultivar GF305 produced significantly longer potential drops than on other peach genotypes. On the resistant Rubira, aphids generated more penetrations before the first E occurred, indicating the possible presence of a resistance factor before the phloem was reached. The clone P1908 of the wild species Prunus davidiana displayed traits of both susceptibility (less but longer probes) and resistance. In particular, aphids produced more E1, suggesting difficulties in preparing sieve elements before feeding. The aphid probing process could be correlated with aphid settling behaviour and bionomics, as previously reported, and gave evidence for the existence of different mechanisms underlying resistance in the tested genotypes against M. persicae.

Settling behaviour and reproductive potential of the green peach aphid Myzus persicae on peach varieties and a related wild Prunus

Discovery in the late seventies of resistance to the green peach aphid Myzus persicae (Sulzer) (Homoptera: Aphididae) in Prunus species was based on screening in the field or in greenhouses with natural aphid populations. Here, we assess the impact of these wild and domesticated peach trees on the behaviour, development, reproductive performance and demography of cloned aphids under controlled light and temperature. Four peach varieties, i.e., Rubira, Weeping Flower Peach, Summergrand and Malo konare and the clone P1908 of the related species Prunus davidiana were tested against the highly susceptible cultivar GF305. Besides a variability in the performance of aphids among experiments, our results showed that (i) distinct mechanisms were involved in the sources of resistance studied and (ii) the ranking of the genotypes on their resistance/susceptibility status remained roughly stable throughout the experiments. Observations on the settling behaviour of first instar nymphs demonstrated antixenosis components in the resistance conferred by Rubira and Weeping Flower Peach. Nymphs began to leave the plants after a short exposure (19–21 h) and no aphid was left after 4 days. Nymphal mortality remained rather low (16%) compared to the repellent effect on aphids of both genotypes. Nymphs disappeared from Weeping Flower Peach significantly earlier than from Rubira. Summergrand, Malo konare and P. davidiana clone P1908 were accepted as host plants by aphids. On P. davidiana, decreased fecundity and intrinsic rate of natural increase (rm= 0.20, averaged on all experiments) were clear expression of antibiosis. In addition, the mean length of the mature embryos within the gonads of the females on the day of adult moult was negatively correlated with the total number of embryos, providing evidence that aphids on this genotype lacked sufficient ressources to be directed both towards production and growth of embryos. Compared to the most susceptible cultivar GF305 (rm= 0.36), Summergrand (rm= 0.26) and Malo konare (rm= 0.28) had, to a lesser extent, a negative impact on nymph production and rate of increase.

How does winter pruning affect peach tree–Myzus persicae interactions?

Winter tree pruning is a cultural practice known to modify vegetative growth, which is likely to affect the development of pests. However, it has been poorly addressed as a cultural control method for diminishing the population levels of the green peach aphid, Myzus persicae (Sulzer) (Homoptera: Aphididae), in peach [Prunus persica (L.) Batsch (Rosaceae)] orchards. In this study, we conducted a 2‐year, on‐station experiment to evaluate how winter pruning affects peach–M. persicae interactions, by examining tree vegetative growth, aphid population dynamics, and crop yield and fruit quality. We collected data under an insect‐proof shelter on adult peach trees submitted to various levels of pruning and artificially infested with aphids. Our results showed that pruning enhanced shoot growth due to the proportion of growing shoots, which increased exponentially (10–60%), whereas the growth rate of growing shoots was not affected. The degree of infestation of peach trees increased with increasing pruning intensity. This effect was mainly due to the increase of the proportion of growing shoots, on which aphids developed better than on rosettes. In turn, the higher the aphid infestation, the higher the aphid‐induced shoot‐tip damage, leaf curling, and leaf fall that disturbed the growth of growing shoots. However, aphids did not considerably reduce fruit quality at harvest. They did not affect fresh fruit weight, and the refractometric index (indicator of sugar content) was reduced by only 3–4%. The relevance of winter pruning as a cultural method for pest control in orchards conducted under integrated fruit production guidelines is discussed.

Nitrogen fertilization effects on Myzus persicae aphid dynamics on peach: vegetative growth allocation or chemical defence?

Plant nitrogen (N) fertilization is a common cropping practice that is expected to serve as a pest management tool. Its effects on the dynamics of the aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) were examined on young peach [Prunus persica (L.) Batsch (Rosaceae)] trees grown under five N treatments, ranging from N shortage to supra‐optimal supply for growth. Aphid population increased over time at the three intermediate N levels. It remained stable at the lowest N level and decreased at the highest N level. Four weeks after the start of infestation, the number of aphids displayed a parabolic response to N level. The relationships between N status and parameters of plant vegetative growth (stem diameter) or biomass allocation (lateral‐total leaf area and root‐shoot ratio) were consistent with responses proposed by models of adaptive plasticity in resource allocation patterns. However, the variation in plant growth predicted aphid population dynamics only partially. Whereas aphid number was positively correlated with plant N status and vegetative growth up to the intermediate N level, it was negatively correlated with plant N status above this level, but not with vegetative growth. The concentrations of primary and secondary (plant defence‐related) metabolites in the plant shoots were modified by N treatments: amino acids (main nutritional resource of aphids) and prunasin increased, whereas chlorogenic acid decreased with increasing N availability. Constitutive changes in plant chemistry in response to N fertilization could not directly explain the reduced aphid performance for the highest N level. Nevertheless, the indirect effect of N on the induction of plant defence compounds by aphid feeding warrants further investigation. The study focuses on the feasibility of handling N fertilization to control M. persicae in orchards, but findings may also be relevant for our understanding of the physiological relationships between the host’s nutritional status and the requirements of the insect.

Nitrogen and water supplies affect peach tree–green peach aphid interactions: the key role played by vegetative growth

The availabilities of water and nitrogen resources are among the main abiotic factors modifying plant physiology and growth. Consequently, via bottom‐up processes, they also can have an impact on the performance of herbivorous insects. However, the extent to which the modification of plant growth is responsible for such an impact remains unclear. We conducted a factorial experiment quantifying Myzus persicae aphid abundance and the vegetative growth of Prunus persica peach tree shoots under contrasting levels of nitrogen and water supplies. We used a hierarchical analysis of multiple regression models to determine whether the consequences of the availability of nitrogen and water on aphid abundance could be the result of a modification of plant growth. Maximum aphid abundance was achieved under nitrogen and water comfort conditions. The best model explaining variance in aphid abundance took into account vegetative growth and water supply, as well as their interaction. The results of the present study suggest that a higher nitrogen supply increases aphid abundance by fostering plant growth. Additionally, the positive response of aphid abundance to vegetative growth is lower in the case of water restriction because, under such conditions, aphids cannot take full advantage of tree vigour. Such a result provides new insights into aphid control in agriculture, as well as on the possible effects of climate change.

Drought-stress and plant resistance affect herbivore performance and proteome: the case of the green peach aphid Myzus persicae (Hemiptera: Aphididae)

Little is known about the simultaneous effects of drought stress and plant resistance on herbivorous insects. By subjecting the green peach aphid Myzus persicae Sulzer to well‐watered and drought‐stressed plants of both susceptible and resistant peach (Prunus persica), the effects of both stressors on aphid performance and proteomics are tested. Overall, the influence of the water treatment on aphid performance is less pronounced than the effect of host plant genetic resistance. On the susceptible cultivar, aphid survival, host acceptance and ability to colonize the plant do not depend on water treatment. On the resistant cultivar, aphid survival and ability to colonize are higher on drought‐stressed than on well‐watered plants. A study examining the pattern of protein expression aiming to explain the variation in aphid performance finds higher protein expression in aphids on the drought‐stressed susceptible cultivars compared with the well‐watered ones. In the susceptible cultivar, the regulated proteins are related to energy metabolism and exoskeleton functionality, whereas, in the resistant cultivar, the proteins are involved with the cytoskeleton. Comparison of the protein expression ratios for resistant versus susceptible plants reveals that four proteins are down‐regulated in well‐watered plants and 15 proteins are down‐regulated in drought‐stressed plants. Drought stress applied to the susceptible cultivar induces the regulation of proteins in M. persicae that enable physiological adaptation to maintain an almost unaltered aphid performance. By contrast, for aphids on the resistant cultivar subjected to drought stress, the down‐regulation of proteins responds to an induced host susceptibility effect.

Harnessing the aphid life cycle to reduce insecticide reliance in apple and peach orchards. A review

Apple and peach orchards are chemical-intensive systems, and aphids are one of their major pests. Aphids alter fruiting and shoot development, and they can spread viruses. Decades of insecticide use have developed aphid resistance, which calls on research to provide alternatives to chemicals for pest management. Here, we review the literature to identify, for each stage of the aphid life cycle, existing alternatives based on either top-down (i.e. aphid predation or parasitism) or bottom up (i.e. increase of host plant resistance) processes. Firstly, it was found that most studies focus on top-down processes, namely on conservation biological control aiming to preserve existing populations of natural enemies: predators, parasitoids and nematodes. This is achieved by (i) providing shelters (i.e. planting hedges, weed or flower strips) or alternative preys in periods of aphid scarcity or (ii) choosing chemicals with the lowest disruptive effects. Those methods prove more efficient when used early in the season, i.e. before the exponential increase of aphid populations. Fostering the complex of natural enemies is also preferable than just supporting one single enemy. Secondly, other techniques, like (i) releasing biological control agents (entomopathogenic fungi, nematodes) or (ii) using pheromone lures to prevent autumnal sexual reproduction, are currently adapted for their use in orchard conditions. Thirdly, bottom-up regulation has to be devised as a long-term strategy, which could start by choosing a cultivar enabling genetic avoidance or developing genetic resistance. Then, aphid development can be reduced by the control of shoot growth or nitrogen accumulation in response to pruning or moderate water and nutrient inputs. At last, autumnal return of aphids could be disrupted by techniques such as kaolin applications that impair aphid host plant location. It is concluded that these alternative methods have to be adapted to local conditions and combined in long-term strategies in order to decrease the infestation risks throughout the orchard lifespan.