Cascading effects of N input on tritrophic (plant–aphid–parasitoid) interactions

Because N is frequently the most limiting mineral macronutrient for plants in terrestrial ecosystems, modulating N input may have ecological consequences through trophic levels. Thus, in agro‐ecosystems, the success of natural enemies may depend not only from their herbivorous hosts but also from the host plant whose qualities may be modulated by N input. We manipulated foliar N concentrations by providing to Camelina sativa plants three different nitrogen rates (control, optimal, and excessive). We examined how the altered host‐plant nutritional quality influenced the performances of two aphid species, the generalist green peach aphid, Myzus persicae, and the specialist cabbage aphid, Brevicoryne brassicae, and their common parasitoid Diaeretiella rapae. Both N inputs led to increased N concentrations in the plants but induced contrasted concentrations within aphid bodies depending on the species. Compared to the control, plant biomass increased when receiving the optimal N treatment but decreased under the excessive treatment. Performances of M. persicae improved under the optimal treatment compared to the control and excessive treatments whereas B. brassicae parameters declined following the excessive N treatment. In no‐choice trials, emergence rates of D. rapae developing in M. persicae were higher on both optimum and excessive N treatments, whereas they remained stable whatever the treatment when developing in B. brassicae. Size of emerging D. rapae females was positively affected by the treatment only when it developed in M. persicae on the excessive N treatment. This work showed that contrary to an optimal N treatment, when N was delivered in excess, plant suitability was reduced and consequently affected negatively aphid parameters. Surprisingly, these negative effects resulted in no or positive consequences on parasitoid parameters, suggesting a buffered effect at the third trophic level. Host N content, host suitability, and dietary specialization appear to be major factors explaining the functioning of our studied system.     

Prey‐mediated effects of glucosinolates on aphid predators

1. Plant resistance against herbivores can act directly (e.g. by producing toxins) and indirectly (e.g. by attracting natural enemies of herbivores). If plant secondary metabolites that cause direct resistance against herbivores, such as glucosinolates, negatively influence natural enemies, this may result in a conflict between direct and indirect plant resistance. 2. Our objectives were (i) to test herbivore‐mediated effects of glucosinolates on the performance of two generalist predators, the marmalade hoverfly (Episyrphus balteatus) and the common green lacewing (Chrysoperla carnea) and (ii) to test whether intraspecific plant variation affects predator performance. 3. Predators were fed either Brevicoryne brassicae, a glucosinolate‐sequestering specialist aphid that contains aphid‐specific myrosinases, or Myzus persicae, a non‐sequestering generalist aphid that excretes glucosinolates in the honeydew, reared on four different white cabbage cultivars. Predator performance and glucosinolate concentrations and profiles in B. brassicae and host‐plant phloem were measured, a novel approach as previous studies often measured glucosinolate concentrations only in total leaf material. 4. Interestingly, the specialist aphid B. brassicae selectively sequestered glucosinolates from its host plant. The performance of predators fed this aphid species was lower than when fed M. persicae. When fed B. brassicae reared on different cultivars, differences in predator performance matched differences in glucosinolate profiles among the aphids. 5. We show that not only the prey species, but also the plant cultivar can have an effect on the performance of predators. Our results suggest that in the tritrophic system tested, there might be a conflict between direct and indirect plant resistance.     

Intercropping oilseed rape as a potential relay crop for enhancing the biological control of green peach aphids and aphid‐transmitted virus diseases

Tobacco viruses transmitted by green peach aphids, Myzus persicae (Sulzer) (Hemiptera: Aphididae), cause severe disease in flue‐cured tobacco, Nicotiana tabacum L. (Solanaceae), in China and throughout the world. Field experiments were conducted in 2016 and 2017 in Longyan City, Fujian Province, China, to determine whether M. persicae and aphid‐transmitted virus diseases are affected by intercropping of oilseed rape, Brassica napus L. (Brassicaceae), in tobacco fields. The results showed that, compared with those in monocultured fields, the densities of M. persicae and winged aphids in intercropped fields significantly decreased in both 2016 and 2017. In particular, the appearance of winged aphids was delayed by ca. 7 days. Moreover, the densities of Aphidius gifuensis Ashmead (Hymenoptera: Aphidiidae), a parasitoid of the aphid, significantly increased in 2016 and 2017. Accordingly, the incidence rates of aphid‐transmitted virus diseases (those caused by the cucumber mosaic virus, potato virus Y, and tobacco etch virus) significantly decreased in the intercropped fields in 2016 and 2017. Tobacco yields and monetary value significantly increased in 2016 (by 10–25 and 14–29%, respectively) and 2017 (by 17–22 and 22–34%, respectively). Consequently, our results suggest that intercropping oilseed rape in tobacco fields is a good approach to regulating and controlling aphids and tobacco mosaic viruses, for example potyvirus, and this intercropping can help control aphid‐transmitted virus diseases in tobacco.     

Feeding behavior of aphids on narrow‐leafed lupin (Lupinus angustifolius) genotypes varying in the content of quinolizidine alkaloids

Since the beginning of breeding narrow‐leafed lupins [Lupinus angustifolius L. (Fabaceae)] with a low alkaloid content, susceptibility to several aphid species has increased. Therefore, the probing and feeding behavior of Aphis fabae Scopoli, Aphis craccivora Koch, Acyrthosiphon pisum (Harris), Myzus persicae (Sulzer), and the well‐adapted Macrosiphum albifrons Essig (all Hemiptera: Aphididae) was studied over 12 h on narrow‐leafed lupin genotypes containing varying amounts and compositions of alkaloids. We used the electrical penetration graph (EPG) technique to obtain information on the influence of alkaloid content and composition on the susceptibility to various aphid species. Results indicated that the total time of probing of A. fabae, A. craccivora, A. pisum, and M. persicae increased with a reduced alkaloid content, whereas the alkaloid content had no influence on M. albifrons. Almost all of the individuals (>93%) conducted sieve element phases on the highly susceptible genotype Bo083521AR (low alkaloid content). A reduced occurrence of phloem phases was observed during the 12‐h recording on the alkaloid‐rich cultivar Azuro, especially for A. pisum (37.5%) and A. fabae (55.0%). Furthermore, aphids feeding on genotypes with low alkaloid content had in most cases significantly longer sieve element phases than when feeding on resistant genotypes (Kalya: low alkaloid content, yet resistant; Azuro: high alkaloid content, resistant), whereas M. albifrons showed the longest phloem phase on the alkaloid‐rich cultivar Azuro. As most significant differences were found in phloem‐related parameters, it is likely that the most important plant factors influencing aphid probing and feeding behavior are localized in the sieve elements. The aphids’ feeding behavior on the cultivar Kalya, with a low alkaloid content but reduced susceptibility, indicates that not only the total alkaloid content influences the feeding behavior but additional plant factors have an impact.     

Plant‐mediated interactions between two herbivores differentially affect a subsequently arriving third herbivore in populations of wild cabbage

• Plants are part of biodiverse communities and frequently suffer from attack by multiple herbivorous insects. Plant responses to these herbivores are specific for insect feeding guilds: aphids and caterpillars induce different plant phenotypes. Moreover, plants respond differentially to single or dual herbivory, which may cascade into a chain of interactions in terms of resistance to other community members. Whether differential responses to single or dual herbivory have consequences for plant resistance to yet a third herbivore is unknown.
• We assessed the effects of single or dual herbivory by Brevicoryne brassicae aphids and/or Plutella xylostella caterpillars on resistance of plants from three natural populations of wild cabbage to feeding by caterpillars of Mamestra brassicae. We measured plant gene expression and phytohormone concentrations to illustrate mechanisms involved in induced responses.
• Performance of both B. brassicae and P. xylostella was reduced when feeding simultaneously with the other herbivore, compared to feeding alone. Gene expression and phytohormone concentrations in plants exposed to dual herbivory were different from those found in plants exposed to herbivory by either insect alone. Plants previously induced by both P. xylostella and B. brassicae negatively affected growth of the subsequently arriving M. brassicae. Furthermore, induced responses varied between wild cabbage populations.
• Feeding by multiple herbivores differentially activates plant defences, which has plant‐mediated negative consequences for a subsequently arriving herbivore. Plant population‐specific responses suggest that plant populations adapt to the specific communities of insect herbivores. Our study contributes to the understanding of plant defence plasticity in response to multiple insect attacks.

     

The NIa‐Pro protein of Turnip mosaic virus improves growth and reproduction of the aphid vector,Myzus persicae (green peach aphid)

Many plant viruses depend on aphids and other phloem‐feeding insects for transmission within and among host plants. Thus, viruses may promote their own transmission by manipulating plant physiology to attract aphids and increase aphid reproduction. Consistent with this hypothesis, Myzus persicae (green peach aphids) prefer to settle on Nicotiana benthamiana infected with Turnip mosaic virus (TuMV) and fecundity on virus‐infected N. benthamiana and Arabidopsis thaliana (Arabidopsis) is higher than on uninfected controls. TuMV infection suppresses callose deposition, an important plant defense, and increases the amount of free amino acids, the major source of nitrogen for aphids. To investigate the underlying molecular mechanisms of this phenomenon, 10 TuMV genes were over‐expressed in plants to determine their effects on aphid reproduction. Production of a single TuMV protein, nuclear inclusion a‐protease domain (NIa‐Pro), increased M. persicae reproduction on both N. benthamiana and Arabidopsis. Similar to the effects that are observed during TuMV infection, NIa‐Pro expression alone increased aphid arrestment, suppressed callose deposition and increased the abundance of free amino acids. Together, these results suggest a function for the TuMV NIa‐Pro protein in manipulating the physiology of host plants. By attracting aphid vectors and promoting their reproduction, TuMV may influence plant–aphid interactions to promote its own transmission.     

Infection of Arabidopsis by cucumber mosaic virus triggers jasmonate-dependent resistance to aphids that relies partly on the pattern-triggered immunity factor BAK1

Many aphid-vectored viruses are transmitted nonpersistently via transient attachment of virus particles to aphid mouthparts and are most effectively acquired or transmitted during brief stylet punctures of epidermal cells. In Arabidopsis thaliana, the aphid-transmitted virus cucumber mosaic virus (CMV) induces feeding deterrence against the polyphagous aphid Myzus persicae. This form of resistance inhibits prolonged phloem feeding but promotes virus acquisition by aphids because it encourages probing of plant epidermal cells. When aphids are confined on CMV-infected plants, feeding deterrence reduces their growth and reproduction. We found that CMV-induced inhibition of growth as well as CMV-induced inhibition of reproduction of M. persicae are dependent upon jasmonate-mediated signalling. BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) is a co-receptor enabling detection of microbe-associated molecular patterns and induction of pattern-triggered immunity (PTI). In plants carrying the mutant bak1-5 allele, CMV induced inhibition of M. persicae reproduction but not inhibition of aphid growth. We conclude that in wildtype plants CMV induces two mechanisms that diminish performance of M. persicae: a jasmonate-dependent and PTI-dependent mechanism that inhibits aphid growth, and a jasmonate-dependent, PTI-independent mechanism that inhibits reproduction. The growth of two crucifer specialist aphids, Lipaphis erysimi and Brevicoryne brassicae, was not affected when confined on CMV-infected A. thaliana. However, B. brassicae reproduction was inhibited on CMV-infected plants. This suggests that in A. thaliana CMV-induced resistance to aphids, which is thought to incentivize virus vectoring, has greater effects on polyphagous than on crucifer specialist aphids.     

Estimating costs of aphid resistance to parasitoids conferred by a protective strain of the bacterial endosymbiont Regiella insecticola

Heritable bacterial endosymbionts are common in aphids (Hemiptera: Aphididae), and they can influence ecologically important traits of their hosts. It is generally assumed that their persistence in a population is dependent on a balance between the costs and benefits they confer. A good example is Hamiltonella defensa Moran et al., a facultative symbiont that provides a benefit by strongly increasing aphid resistance to parasitoid wasps, but becomes costly to the host in the absence of parasitoids. Regiella insecticola Moran et al. is another common symbiont of aphids and generally does not influence resistance to parasitoids. In the green peach aphid, Myzus persicae (Sulzer), however, one strain (R5.15) was discovered that behaves like H. defensa in that it provides strong protection against parasitoid wasps. Here we compare R5.15‐infected and uninfected lines of three M. persicae clones to test whether this protective symbiont is costly as well, i.e., whether it has any negative effects on aphid life‐history traits. Furthermore, we transferred R5.15 to two other aphid species, the pea aphid, Acyrthosiphon pisum (Harris), and the black bean aphid, Aphis fabae Scopoli, where this strain is also protective against parasitoids and where we could compare its effects with those of additional, non‐protective strains of R. insecticola. Negative effects of R5.15 on host survival and lifetime reproduction were limited and frequently non‐significant, and these effects were comparable or in one case weaker than those of R. insecticola strains that are not protective against parasitoid wasps. Unless the benefit of protection is counteracted by detrimental effects on traits that were not considered in this study, R. insecticola strain R5.15 should have a high potential to spread in aphid populations.     

Feeding behavior of Myzus persicae on asparagus species susceptible and resistant to Asparagus virus 1

Asparagus virus 1 (AV‐1) infects Asparagus officinalis L. (Asparagaceae) in the field worldwide. However, various wild relatives of A. officinalis are resistant to AV‐1. Here we study the behavior of the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), on 19 AV‐1‐resistant wild relatives of A. officinalis. We focus on behavior that is associated with regular cell penetration, relevant for inoculation of AV‐1, and sieve element penetration to check for vector resistance and its potential influence on AV‐1 transmission. Parameters, relevant for the transmission of non‐persistent viruses and host plant acceptance, were obtained by the electrical penetration graph technique. Furthermore, phylloclade architecture of A. officinalis and its wild relatives was examined to study its influence on aphid behavior. Behavior of M. persicae displays many cell penetrations and long ingestion periods on A. officinalis, compared to the generally shorter cell penetrations (reduced potential for virus transmission) and reduced or no ingestion (phloem‐located aphid resistance) on wild relatives. Because effects on aphid behavior are not consistent throughout the group of the tested wild relatives of A. officinalis, with some wild relatives being susceptible to M. persicae, a common genetic background for AV‐1 and aphid resistance appears to be unlikely. However, the reduced potential of virus transmission as well as aphid resistance shown by some wild relatives may be useful for future breeding programs.     

Do native and invasive herbivores have an effect on Brassica rapa pollination?

• Mutualistic (e.g. pollination) and antagonistic (e.g. herbivory) plant–insect interactions shape levels of plant fitness and can have interactive effects.
• By using experimental plots of Brassica rapa plants infested with generalist (Mamestra brassicae) and specialised (Pieris brassicae) native herbivores and with a generalist invasive (Spodoptera littoralis) herbivore, we estimated both pollen movement among treatments and the visiting behaviour of honeybees versus other wild pollinators.
• Overall, we found that herbivory has weak effects on plant pollen export, either in terms of inter‐treatment movements or of dispersion distance. Plants infested with the native specialised herbivore tend to export less pollen to other plants with the same treatment. Other wild pollinators preferentially visit non‐infested plants that differ from those of honeybees, which showed no preferences. Honeybees and other wild pollinators also showed different behaviours on plants infested with different herbivores, with the former tending to avoid revisiting the same treatment and the latter showing no avoidance behaviour. When taking into account the whole pollinator community, i.e. the interactive effects of honeybees and other wild pollinators, we found an increased avoidance of plants infested by the native specialised herbivore and a decreased avoidance of plants infested by the invasive herbivore.
• Taken together, our results suggest that herbivory may have an effect on B. rapa pollination, but this effect depends on the relative abundance of honeybees and other wild pollinators.

     

Bottom‐up effects mediated by an organic soil amendment on the cabbage aphid pests Myzus persicae and Brevicoryne brassicae

Earthworm‐produced compost or vermicompost has been shown to increase resistance of plants to a variety of insect pests, but it is still unclear whether this resistance is dose dependent and whether the mechanisms responsible are the same for insect species with differing feeding habits and preferences. Therefore, we tested the effects of plants grown in various vermicompost concentrations (0, 20, 40, and 60%) on the preference and performance of generalist, Myzus persicae L., and specialist, Brevicoryne brassicae L. (both Hemiptera: Aphididae), aphid pests. Preference was evaluated with leaf disk (apterous) and whole plant (alate) choice assays. After 24 h of feeding, there was no significant negative effect on the feeding preference noted for apterae of either species of any of the treatments tested. To the contrary, apterae B. brassicae showed a significant preference for vermicompost treatments over control leaf disks. Alate M. persicae preferred alighting on control plants over vermicompost‐grown plants, but B. brassicae showed no preference toward any of the treatments tested. Both aphid species deposited significantly more nymphs on control plants than on those grown in 20% vermicompost. Furthermore, plants grown in soil amended with 20% vermicompost significantly suppressed mass accumulation, as well as numbers of adults and nymphs of both aphid species compared to controls. These data clearly show that vermicompost soil amendments can significantly influence pest aphid preference and performance on plants and that these effects are not dose dependent, but rather species and morph dependent.     

Quantifying aphid behavioral responses to environmental change

Aphids are the most common vector of plant viruses, and their feeding behavior is an important determinant of virus transmission. Positive effects of global change on aphid performance have been documented, but effects on aphid behavior are not known. We assessed the plant‐mediated behavioral responses of a generalist aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), to increased CO₂ and nitrogen when feeding on each of three host species: Amaranthus viridis L. (Amaranthaceae), Polygonum persicaria L. (= Persicaria maculosa Gray) (Polygonaceae), and Solanum dulcamara L. (Solanaceae). Via a family of constrained Markov models, we tested the degree to which aphid movements demonstrate preference among host species or plants grown under varying environmental conditions. Entropy rates of the estimated Markov chains were used to further quantify aphid behavior. Our statistical methods provide a general tool for assessing choice and quantitatively comparing animal behavior under different conditions. Aphids displayed strong preferences for the same host species under all growth conditions, indicating that CO₂‐ and N‐induced changes in plant chemistry have minimal effects on host preference. However, entropy rates increased in the presence of non‐preferred hosts, even when preferred hosts were available. We conclude that the presence of a non‐preferred host species affected aphid‐feeding behavior more than changes in plant leaf chemistry when plants were grown under elevated CO₂ and increased N availability.     

Resistance to Asparagus virus 1 in the Wild Relative Asparagus amarus

Five asparagus cultivars, three breeding lines and the wild relative Asparagus amarus were tested for natural infection by Asparagus virus 1 (AV‐1) in experimental fields at two locations over 3 and 4 years, respectively. In the first year after re‐planting the annual crowns in the field, more than 90% of tested plants of cultivars were infected by AV‐1. In the third and fourth year, 100% of tested plants of cultivars were AV‐1 infected. In comparison, all plants of the wild relative A. amarus were completely free of AV‐1, suggesting a high level of resistance. Additionally, 1‐year‐old glasshouse‐cultivated plants of A. officinalis and A. amarus were placed in an AV‐1 provocation cabin under field conditions. Seven months later, 100% of the A. officinalis plants showed a high virus concentration in ELISA, whereas no AV‐1 was detectable in the A. amarus plants. This result was confirmed by highly sensitive AV‐1‐specific RT‐PCR. To exclude vector resistance, the feeding behaviour of green peach aphid Myzus persicae was tested over 12 h using the electrical penetration graph method. Both asparagus genotypes were accepted by the aphids as potential hosts, but the feeding time was significantly longer on A. amarus. A genetic distance analysis of the various cultivars of Asparagus officinalis and selected wild relatives of the JKI collection was carried out, resulting in a clear discrimination of cultivars and wild relatives, especially A. amarus. The potential breeding value of the putative resistance carrier is discussed.     

Effect of cultivars with varying levels of resistance to aphids on development time,sex ratio,size and longevity of the parasitoid <Emphasis Type="Italic">Aphidius colemani</Emphasis>

The influence of cultivars of common cabbage, Brassica oleracea var. capitata with varying levels of resistance to Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) on key biological characteristics of Aphidius colemani (Viereck) was investigated under laboratory conditions. The total development time for female parasitoids reared on M. persicae did not differ significantly between Minicole (green-leaved, partially resistant with antibiosis factors for B. brassicae) and Derby Day (green-leaved, susceptible to both aphid species); but development was significantly faster (ca 10%) on Ruby Ball (red-leaved, partially resistant with antixenosis factors for B. brassicae). Total development time for females reared on B. brassicae was slightly shorter on Ruby Ball than on Minicole. Males reared on M. persicae developed into adults significantly faster (ca 10%) on Ruby Ball than on Minicole. However, when B. brassicae was the host, no significant variations in development time were observed. Sex ratios, size and longevity of both male and female parasitoids on either host were not significantly influenced by cultivar. The results are discussed in relation to the compatible utilisation of host-plant resistance and biological control in the integrated management of aphids.     

Influence of water stress on the glucosinolate profile of Brassica oleracea var. italica and the performance of Brevicoryne brassicae and Myzus persicae

Drought stress alters the chemical composition of plants, which can influence their tolerance to insect herbivory. To evaluate plant chemical responses to drought stress, broccoli, Brassica oleracea L. var. italica Plenck (Brassicaceae), was grown under well‐watered, drought, and water‐logged conditions. The glucosinolate (GS) levels and the performance of two aphid species, the specialist Brevicoryne brassicae (L.) and the generalist Myzus persicae (Sulzer) (both Hemiptera: Aphididae), in relation to water stress conditions were studied. High Performance Liquid Chromatography analysis showed that water stress changed the levels of GS in broccoli plants. Plants grown for 2 weeks under drought stress were significantly smaller and showed decreased levels of total GS when compared with GS contents of well‐watered plants, whereas water‐logged conditions led to a slight increase in the GS contents. A substantial decrease in indolyl GS was detected in water‐deficient plants, whereas aliphatic GS decreased slightly. Analysis of sugar levels in phloem sap of broccoli plants revealed that plants under water‐logged conditions contained the highest amounts of sugars followed by drought‐stressed and well‐watered plants. The two aphid species responded differently to water stress‐induced changes in their host plants. Significantly larger populations of M. persicae were recorded on plants with a limited water supply than on plants grown under well‐watered or water‐logged conditions. Brevicoryne brassicae was less affected by water stress, and similar population sizes were found on plants that were subject to different treatments. Analysis of covariance showed a significant effect of the plants’ water condition but no significant effect of GS content on the performance of M. persicae. However, the specialist B. brassicae remained unaffected by changes induced under water stress conditions.     

Hydrosols evaluation in pest control: insecticidal and settling inhibition potential against Myzus persicae (Sulzer)

The hydrosols are by‐products derived during the extraction of essential oils. Although essential oils have been widely evaluated for their insecticidal activities, the possible use of hydrosols in pest control has been almost unknown. The effects of the hydrosols of Origanum majorana (marjoram), Mentha pulegium (pennyroyal), and Melissa officinalis (lemon balm) on the survival and settling behaviour of the aphid pest Myzus persicae were investigated. The hydrosols were isolated using Clevenger hydrodistillation (i) with conventional heating (HD) and (ii) assisted by microwaves (MWHD). GC‐MS analysis showed that the volatiles occurring in the hydrosols were similar between the two techniques. Hydrosols were assayed for possible settling inhibitory effects on M. persicae in Petri dishes (15 cm diameter). In each dish, a sprayed together with an unsprayed eggplant leaf piece was placed. An adult aphid <24 h old was released on the treated leaf and its path length was recorded during the initial 10 min. Then, its position (on the treated or untreated leaf) was recorded 10, 20, 30, 60 min and 24 h after spraying. M. officinalis HD hydrosol resulted in fourfold increase of the path length compared with the control (deionized water). The last observation (24 h) revealed that M. officinalis and M. pulegium HD hydrosols had the strongest inhibitory effect. Additionally, O. majorana hydrosols caused 10–15% aphid mortality after 24 h. In this study, the wider use of the MWHD technique is further supported as in addition to its well‐known advantages (i.e. shorter distillation time, less energy consumption), the effects of the hydrosols produced are comparable to those obtained by HD. Most importantly, the results clearly showed that the potential of hydrosols in pest control ought not to be ignored and should attract the interest of future studies.     

Maternal host plant effects on aphid performance: contrasts between a generalist and a specialist species on Brussels sprout cultivars

• 1 The performance of the second generation (G₂) of alates and apterae of a generalist, Myzus persicae, and a specialist, Brevicoryne brassicae, aphid species reared on Chinese cabbage or cabbage was evaluated on five cultivars of Brussels sprout.
• 2 Aphid performance was influenced both by the type of host on which the parent aphid had been reared and by the host on which it was feeding when reproducing.
• 3 The fecundity of the G₂ of alates of both aphid species reared on Chinese cabbage differed significantly between all the cultivars of Brussels sprout and, on average, was 25% higher than those reared on cabbage. These differences were also apparent for the intrinsic rate of increase of B. brassicae but not for M. persicae.
• 4 There was a trend for the G₂ of alates from Chinese cabbage to have greater fecundity compared with aphids from cabbage. These differences were significant for the fecundity of the G₂ of alates of both aphid species on Brussels sprout cultivars Fillbasket (30% higher), Red Delicious (35% higher) and Winter Harvest (25% higher) than those reared on cabbage.
• 5 The intrinsic rate of increase for the G₂ of alates of B. brassicae from Chinese cabbage was significantly different on all Brussels sprout cultivars tested. The intrinsic rate of increase differed significantly between aphids reared on either Chinese cabbage or cabbage on cultivars Oliver and Darkmar‐21 (M. persicae) and Red Delicious and Winter Harvest (B. brassicae). The cv. Oliver appeared to be the most consistently good host; Red Delicious was the poorest host overall.

     

Genetic characterization,pathogenicity and benomyl susceptibility of Lecanicillium fungal isolates from Argentina

The aim of this study was to investigate biological and molecular characteristics of Lecanicillium strains isolated from Hemipteran hosts in Argentina. Morphology‐based taxonomic characterization together with molecular taxonomy based on rRNA operon internal transcribed spacer (ITS), mitochondrial nad1 gene, and nuclear ef1a gene sequences resulted in the assignment of nine out of ten isolates to the Lecanicillium lecanii sensu lato complex. However, whereas several isolates were thus unequivocally characterized as Lecanicillium muscarium or Lecanicillium longisporum, species assignment was not possible for three isolates that might represent a new species within the L. lecanii s.l. complex. We found two group‐I introns on 18S and 28S rRNA gene on only one isolate. Pathogenicity tests were conducted against the peach aphid using conidial suspensions (1 × 10⁷ conidia/ml), and the Kaplan–Meier analysis was performed to evaluate the survival of Myzus persicae. Lecanicillium longisporum CEP 155 and L. muscarium CEP 182 were significantly more pathogenic to M. persicae than all the Lecanicillium isolates causing aphid mortalities >85%. Determination of susceptibility to the benzimidazole fungicide benomyl revealed important differences between Lecanicillium strains. The inhibitory effect of benomyl appeared less pronounced for the L. muscarium fungal isolates than for those belonging to a different taxon. Based in our results, the best candidate strain as microbial biological control agent against M. persicae is L. muscarium CEP 182. However, further research under field conditions in greenhouses should be done in order to confirm the compatibility of entomopathogenic fungi and fungicides within an IPM strategy.