Diaeretiella Research Articles

Host fitness of different aphid species for Diaeretiella rapae (M’Intosh): a life table approach

Diaeretiella rapae [M’Intosh (Hymenoptera: Braconidae)] is a koinobiont, cosmopolitan, solitary and polyphagous endo-parasitoid of various species of aphids especially attacking on cruciferous plants. Present study intended to assess the quality of three aphid species i.e. Myzus persicae (Sulzer), Lipaphis erysimi (Kaltenbach), and Brevicoryne brassicae (Linn.) (Hemiptera: Aphididae) as host influencing the survival rate, demography, population growth, longevity and sex ratio of D. rapae. Diaeretiella rapae exhibited considerably highest life expectancy on M. persicae followed by L. erysimi and B. brassicae. However, mortality survival ratio and indispensable mortality was highest at pupal stage when D. rapae was reared on M. persicae. Moreover, natality rate (mx), potential fecundity (pf), net reproductive rate (R0) and intrinsic rate of increase (rm) was significantly (P < 0.05) highest on M. persicae followed by L. erysimi and B. brassicae. Similarly, adult longevity was prolonged on M. persicae with female biased sex ratio. Based on the present study it can be concluded that M. persicae proved to be the best host for D. rapae, however, L. erysimi and B. brassicae proved to be the moderate or less suitable host for the multiplication of this parasitoid species. Moreover, it may be difficult to find whether the use of a particular food for nutrition by a host of parasitoid will have a positive or negative impact on biological performance of parasitoid, therefore, there is an urge to determine these facts experimentally using biochemical analysis.

Performance of the parasitoid species Aphelinus asychis Walker (Hymenoptera: Aphelinidae), Aphidius ervi (Haliday)\xa0(Hymenoptera: Braconidae) and Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae), using Myzus persicae (Sulzer) (Hemiptera: Aphididae) as host

Three parasitoid species viz. Aphelinus asychis Walker (Hymenoptera: Aphelinidae), Aphidius ervi (Haliday) (Hymenoptera: Braconidae) and Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae) parasitizing the aphid species Myzus persicae (Sulzer) (Hemiptera: Aphididae) in mid-hills of north India were studied. At different locations and times of the year, the parasitization by A. asychis, A. ervi, and D. rapae ranged from 7.53 to 37.58, 4.26 to 80.45, and 74.25 to 80.48%, respectively. All the 3 parasitoids successfully completed their development on different nymphal instars of the aphid host and the total developmental duration of A. asychis, A. ervi, and D. rapae ranged 10.4–14.6, 24.2–29.6, and 10.2–15.2 days, respectively. It was significantly longer on the 1st nymphal instar of the host. The longevity of the female parasitoids was significantly longer than their counterparts. Differences in host age significantly influenced the longevity of female parasitoids and it was more on 1 to 2-day-old nymphs than that on 4 to 5-day-old nymphs. Fecundity and ovipositional periods of the parasitoids on younger (1–2 days old) host age group were considerably prolonged than on the older ages of the aphid. Average total fecundity of A. asychis and D. rapae was significantly higher when parasitizing 1–2-day-old nymphs. In A. asychis, host feeding behavior was also observed by a total host feeding of 89.2 aphids (1–2 days old) and 43.4 aphids (4–5 days old) during its life span. It is concluded that A. asychis, A. ervi, and D. rapae can be mass reared using M. persicae as host and can be utilized successfully in augmentative biological control program.

Effect of the Utilization of the Bioinsecticide Spinosad on the Whitefly, Bemisia tabaci (Genn.) and the Cabbage Aphid, Brevicoryne brassicae (L.), Attacking the Red Cabbage Plants and Also Estimating its Effect on the Mummies of the Aphid Parasitoid Diaeretiella rapae (McIntoch), in the Greenhouses Located in Giza Governorate.

Effect of the Utilization of the Bioinsecticide Spinosad on the Whitefly, Bemisia tabaci (Genn.) and the Cabbage Aphid, Brevicoryne brassicae (L.), Attacking the Red Cabbage Plants and Also Estimating its Effect on the Mummies of the Aphid Parasitoid Diaeretiella rapae (McIntoch), in the Greenhouses Located in Giza Governorate.

Responsibility of Certain Biotic Factors for Management Populations of Brevicoryne brassicae L in Cabbage fields

This work was carried out at the experimental farm of the Faculty of Agriculture, Assiut University during the 2017-2018 and 2018-2019 cabbage growing seasons. Evaluation of the seasonal abundance of the cabbage aphid Brevicoryne brassicae L in cabbage fields and responsibility of certain biotic factorsfor managing populations of this insect pest was the cornerstone of this investigation. Cabbage aphid, B. brassicae was found to be active on cabbage plantations and recorded in low numbers (99.71 individuals / 7 Cm2 / leaf / infested plant) at the beginnings of the season (November) with gradual increase until harvesting. Three amongst the six evaluated cabbage cultivars and/or hybrids were appeared as susceptible (S) and harbored so high numbers of the pest. However, the remaining cultivars showed some sort of resistance to this insect pest. The local cultivar (Ganzory) harbored the least aphid numbers and appeared as moderately resistant (MR) cultivar. This finding could be attributed to the antixenosis and/or antibiosis phenomena presented by this local cultivar. Proteins and amino acid contents showed a highly significant positive correlation (r) with B. brassicae populations, however, chlorophyll showed highly significant negative (r). The obtained data reflect the role of the cabbage aphid parasitoid Diaeretiella rapae (McIntosh) to manage B. brassicae populations. Mummified aphids (parasitism rate) before harvesting were found to be equal 6.16-fold of that recorded before ripening stages. High percentages of D. rapae were succeeded to emerge from the mummified B. brassicae with diverse extrusion rates. Multiple increases were recorded on the percentages of infested plants when cabbage undergo toward ripening at the 9th stage till harvesting. No or less existence of unmarketable plants (through the periods of the low infestation)before cup formation and/or plant ripening period was recorded. So, it can be concluded that cabbage cultivar, plant age, and stage in addition to plant metabolites combined with the naturally occurring biological control agent (Aphid parasitoid) can be affected and manage the population trend of cabbage aphid B. brassicae infesting cabbage in the field.

Parasitoid communities and interactions with Diuraphis noxia in Australian cereal production systems

New information is presented on previously introduced aphid parasitoids using the newly arrived Russian wheat aphid (RWA), Diuraphis noxia Kurdjumov 1913 (Hemiptera: Aphididae) in South Australia including species diversity and relative abundance, population dynamics and regional differences in communities. In total, eight species of primary braconids (Hymenoptera) were identified from D. noxia: Aphidius colemani Viereck, 1912 A. platensis Brethes 1913, A. ervi Haliday 1834 (Hymenoptera: Braconidae), A. rhopalosiphi De Stefani Perez, 1902, A. salicis Haliday, 1834, A. smithi Sharma and Subba Rao, 1959, Diaeretiella rapae (McIntosh, 1855) and Lysiphlebus testaceipes (Cresson, 1880). The predominant parasitoids of D. noxia were D. rapae, A. colemani and A. platensis which was consistent across regions. This study indicates the pre-emptive importation of biological control agents (generalist parasitoid wasps) contributed to limiting an invasive pest. Data presented provides a baseline for further research on parasitoid species to reduce D. noxia, a new pest species in Australian cereal ecosystems.

Host-range study about four aphid parasitoid species among 16 aphid species for constructing banker–plant systems

To provide fundamental information for the biological control of aphids in vegetable greenhouses, we compared the host ranges of four aphid parasitoid species, Aphidius colemani Viereck, Aphidius gifuensis Ashmead, Diaeretiella rapae (M’Intosh), and Ephedrus nacheri Quilis (Hymenoptera: Braconidae: Aphidiinae). The acceptability as host of 11 vegetable-pest aphids, Acyrthosiphon pisum (Harris), Aphis craccivora Koch, Aphis gossypii Glover, Aulacorthum solani (Kaltenbach), Brevicoryne brassicae (Linnaeus), Chaetosiphon fragaefolii (Cockerell), Lipaphis erysimi (Kaltenbach), Macrosiphoniella sanborni (Gillette), Macrosiphum euphorbiae (Thomas), Myzus persicae (Sulzer), and Uroleucon formosanum (Takahashi), in addition to five aphid species, Melanaphis sacchari (Zehntner), Rhopalosiphum maidis (Fitch), Rhopalosiphum padi (Linnaeus), Schizaphis graminum (Rondani), and Sitobion akebiae (Shinji) (Hemiptera: Aphididae) that serve as alternative hosts in banker–plant systems for the four aphid parasitoid species, were investigated. A newly emerged pair of parasitoid adults were provided to 100 aphids of each species on caged host plants in a 25 °C chamber for 24 h. The numbers of mummified aphids and emerged adults were counted in 10 trials for each aphid species. Aphidius colemani, A. gifuensis, D. rapae and E. nacheri parasitized four, two, three, and eight pest species, respectively, and four, three, three, and five alternative host species, respectively. Ephedrus nacheri had the broadest host range among the four species, and all the four species parasitized M. persicae, R. maidis, and S. graminum. This information will be useful for selecting candidate of biological control agents for aphids and for constructing banker–plant systems.

Potato aphids and their natural enemies in tizi-ouzou northern of algeria: Biodiversity and importance

Survey of potato aphids and their natural enemies in Tizi Ouzou Northern of Algeria during a year 2013, a total of 308 individual natural enemies comprised of 07 predatory species and 3 parasitoid species were collected and identified. The results showed the mean abundance of predators, especially ladybird's species and Adonia variegata (Goeze, 1777) is represented by 53.8%. Only some bugs were found and the aphid parasitoids were represented by three species Aphidius ervi(Haliday, 1833), Diaeretiella rapae (M'Intosh, 1855) and Praon volucre (Haliday, 1833).

Phytophagous insects and natural enemies in kale under organic and chemical fertilizers

ABSTRACT: Organic agriculture is distinguished by practices that benefit the environment and support sustainable agriculture. In the present study, leafy kale, Brassica oleracea L. var. acephala DC., was submitted to chemical and organic (castor bean cake and bovine manure) fertilization treatments aiming to verify the influence of these fertilizers on the occurrence of insect pests and natural enemies. The insects were sampled by visual examination of plants and pitfall traps. Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) (Hemiptera: Aphididae) were the phytophagous insects that occurred more significantly in kale, under chemical rather than organic fertilization, whereas Harmonia axyridis (Pallas), Hippodamia convergens (Guérin-Méneville) (Coleoptera: Coccinellidae), Diaeretiella rapae (M’Intosh) (Hymenoptera: Braconidae) and spiders (Arachnida: Araneae) were abundant in kale under organic fertilizer. The study showed that castor bean cake and bovine manure as leafy kale fertilizers can reduce the use of insecticides and provide quality food.

Biotic Factors Responsible for Management the Population Trend of Cabbage Aphid Brevicoryne brassicae L, Inhabiting Canola Plants

The present study was conducted to determine the species composition and seasonal abundance of the prevalent arthropod pests and natural enemies inhabiting canola plantations during 2017 and 2018, growing seasons in Assiut, Upper Egypt. Plant relative susceptibility to cabbage aphid Brevicoryne brassicae L.and effect of the naturally occurring parasitoid species to manage the population trend of this insect pest was evaluated. Data revealed the presence of 14 species belonging to 14 genera, 10 families and 5 orders. Predatory species (Campylomma impicta Wagner and Coccinella undecimpunctata L) were constituted the highest proportion of entomophagous species, while parasitoids were presented by one species only [Diaeretiella rapae (McIntosh)]. The predatory species showed less than 6% dominance and 40% abundance. However, the aphid parasitoid was presented by more than 90% dominance and 60% abundance. Thrips tabaci Lindeman was the highest prevalent phytophagous species, while cabbage aphid B. brassicae ranked the second by less dominance and abundance percentages. Cabbage aphid B. brassicae was found to be active on canola plantations all over both growing seasons. A gradual increase in B. brassicae populations was recorded until the appearance of its peak in March, 5. A gradual decrease in the pest populations was recorded until the end of the season. Bactol cultivar harbored the highest aphid mean numbers (132.47individuals/plant) throughout both seasons of study and classified as susceptible (S) cultivar. Serw 4 and Serw 6 cultivars harbored fewer numbers (98.05 and 85.08 individuals/plant) and classified as low resistant (LR) and moderately resistant (MR) cultivars, respectively. The mean percentage of B. brassicae parasitized by D. rapae during both growing seasons was estimated. Although, the pest populations decreased at mid-March, the parasitism percentages increased until the end of both seasons. The seasonal mean of parasitism percentage recorded 34.76%. Simultaneous decrease of the pest numbers with the increase of its parasitoid confirms the responsibility of D. rapae as a good biological control agent against B. brassicae populations

The preference-performance relationship as a means of classifying parasitoids according to their specialization degree.

Host range in parasitoids could be described by the preference–performance hypothesis (PPH) where preference is defined as host acceptance and performance is defined as the sum of all species on which parasitoid offspring can complete their life cycle. The PPH predicts that highly suitable hosts will be preferred by ovipositing females. However, generalist parasitoids may not conform to this hypothesis if they attack a large range of hosts of varying suitability. Under laboratory conditions, we tested the PPH relationship of three aphid parasitoids currently considered as generalist species (Aphelinus abdominalis, Aphidius ervi, Diaeretiella rapae). As expected, the three parasitoids species showed low selectivity, i.e., females stung all aphid species encountered (at least in some extent). However, depending on the parasitoid species, only 42%–58% of aphid species enabled producing parasitoid offspring. We did not find a correlation between the extent of preference and the performance of three generalist aphid parasitoids. For A. ervi, host phylogeny is also important as females showed higher attack and developmental rates on hosts closely related to the most suitable one. In addition, traits such as (a) the presence of protective secondary endosymbionts, for example, Hamiltonella defensa detected in Aphis fabae and Metopolophium dirhodum and (b) the sequestration of plant toxins as defense mechanism against parasitism, for example, in Aphis nerii and Brevicoryne brassicae, were likely at play to some extent in narrowing parasitoid host range. The lack of PPH relationship involved a low selectivity leading to a high adaptability, as well as selection pressure; the combination of which enabled the production of offspring in a new host species or a new environment. Testing for PPH relationships in parasitoids may provide useful cues to classify parasitoids in terms of specialization degree.

Spatial distribution of cabbage aphid, Brevicoryne brassicae (L.) and its parasitoid,Diaeretiella rapae (Mc Intosh) under sub-temperate conditions of Himachal Pradesh, India

Cabbage aphid, Brevicoryne brassicae (L) (Aphididae: Hemiptera) is one of the most serious pest of cauliflower throughout the world. Knowledge of spatial distribution is important for developing an effective sampling plan and ultimately for IPM strategies for a given pest. In the present study spatial distribution of B. brassicae and its parasitoid, Diaeretiella rapae (Mc lntosh) (Braconidae: Hymenoptera) was studied on cauliflower ( Brassica oleracea var botrytis ) under sub-temperate conditions of India during 2017. Both B. brassicae and D. rapae assumed activity in the fourth week of January and remained active till May end with peak activity during fourth week of March, 2017. Dispersion indices like variance to mean ratio (Iƒ 2 /X), David-Moore index (IDM = Iƒ 2 /X-1), mean crowding (X*), Lloyd’s mean crowding index (X*/X) and ‘k’ of negative binomial indicated that both the aphid and the parasitoid followed negative binomial distribution throughout the cropping season, Taylor’s power equation was Iƒ 2 = 1.7013X0.5314 for B. brassicae and Iƒ 2 = 2.2057X1.4467 for D. rapae , while Iwao’s patchiness regression equation was X*= 32.0099+1.7947X and X*= -2.0678+2.2746X for B. brassicae and D. rapae , respectively. Optimum number of samples required varied with the mean density and the desired precision level for both the aphid and the parasitoid.

'Bottom-up' effects in a tritrophic plant-aphid-parasitoid system: Why being the perfect host can have its disadvantages.

In this study the host plant genotype effect on cabbage aphid, Brevicoryne brassicae (L.)(Hemiptera: Aphididae) preference and performance, the effect of aphid genotype on parasitoids performance, as well as the indirect effects of plant genotypes on aphid parasitoid performance, were tested using different population samples of the aphid and its primary endoparasitoid wasp, Diaeretiella rapae (M'Intosh) (Hymenoptera: Braconidae). Experiments were run as fully-factorial randomized block design in a greenhouse. Accordingly, host plant cultivar had significant effects on the total number of aphids and aphid-load whilst the fitness of the aphid genotypes were also influenced by plant cultivar. The effect of parasitism on cabbage aphids was significantly different between plant cultivars. Overall, the results revealed that cabbage aphid is under different selective pressures arising from both higher (parasitoid) and lower (host plant cultivar) trophic levels. The host plant cultivar had a significant effect on both aphid fitness and parasitism rate on particular aphid genotypes. This indicates that host-plant-adapted aphid species can create much context-dependency in the nature and strength of 'fitness benefits parasitism', which may in turn alter the costs and benefits of host specialization.

Early Season Parasitism of Cereal Aphids1 in Wheat Fields and Field Borders

Winter wheat, Triticum aestivum L., is the most widely grown crop in Oklahoma and typically is planted in early autumn and harvested in June. Wheat often is infested by insect pests, the most important of which are cereal aphids. The objective of the research was to determine the relationship between parasitism rates of cereal aphids on plants in the wheat field border versus in the wheat field. Aphid abundance in borders and fields was assessed by D-vac suction sampling. Three parasitoid species were reared from cereal aphids on sentinel plants in field borders and wheat fields during the 2-year study. Lysiphlebus testaceipes (Cresson) was the most abundant species, followed by Aphelinus nigritus Howard and Diaeretiella rapae (M'Intosh). There was no significant correlation for any species between the number of individuals emerging from aphids on sentinel plants in field borders before wheat was planted to the number emerging from sentinel plants in borders after emergence of wheat plants (r ranged from -0.12 to 0.22). Nor was there correlation for any species for the number of individuals emerging from sentinel plants in field borders after emergence of wheat to the number emerging at the same time from sentinel plants in adjacent wheat fields (r ranged from -0.08 to - 0.07). In a habitat there were significant correlations among species for the number of individuals emerging from sentinel plants (r ranged from -0.08 to 0.94). While the same parasitoid species forage for aphids in field borders as for cereal aphids in wheat fields, parasitism levels in borders did not reflect amounts in adjacent fields. This suggested that the amount of activity of cereal aphid parasitoids at field borders had minimal relationship to the amount of activity and hence to parasitism of cereal aphids in adjacent wheat fields.

Possible coexistence of Harmonia axyridis Pallas (Coleoptera: Coccinellidae) and Diaeretiella rapae M'Intosh (Hymenoptera: Braconidae) in the biological control of Lipaphis erysimi (Homoptera: Aphididae)

Abstract To study the interactions between the aphidophagous predator Harmonia axyridis Pallas (Coleoptera: Coccinellidae) and the specialist aphid parasitoid Diaeretiella rapae M'Intosh (Hymenoptera: Braconidae) in the biological control of mustard aphid, Lipaphis erysimi (Homoptera: Aphididae), the prey discrimination by H. axyridis among unparasitized, non-mummified parasitized and mummified aphids was examined under laboratory conditions. Prey/host selections were also tested by offering L. erysimi at various developmental stages to assess the possibility of coexistence between the two species, so the prey preference of H. axyridis when D. rapae parasitize aphids, and the host preference of D. rapae when H. axyridis interfered with the parasitization were detected. We found that H. axyridis could discriminate against mummies rather than non-mummified parasitized aphids. The ladybug showed a significantly positive preference for adult prey when D. rapae turned aphids into mummies, while D. rapae tended to parasitize younger nymphal aphids when H. axyridis was introduced. The present study suggests the prey discrimination against mummies by H. axyridis, and indicates that H. axyridis and D. rapae can avoid resource competition by attacking different and non-overlapping developmental stages of aphid. Thus, H. axyridis and D. rapae can potentially coexist and establish a stable ecosystem in the biological control of L. erysimi.

Different pepper cultivars affect performance of second (Myzus persicae) and third (Diaeretiella rapae) trophic levels

Abstract While chemical application can control aphid outbreaks, the overuse of insecticides can make aphids more resistant to these insecticides. These effects make strategies such as host plant resistance combined with biological control agents as an important part of alternative control methods. In this study, the performance of the green peach aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) and its parasitoid wasp, Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae) were determined on different pepper cultivars (California, Hamedan, Minab, Sabz-Farangi, Sabz-Ghalami, Semnan, Shahroud, and Tehran) in a growth chamber at 25 ± 1 °C, 60 ± 5% RH and a 16:8 h L:D photoperiod. The intrinsic rate of increase (r) of the aphid on the above-mentioned cultivars was 0.3203, 0.3891, 0.3594, 0.3152, 0.3512, 0.3032, 0.3352 and 0.2787 day−1, respectively. Comparison of the intrinsic rate of increase (r) on different pepper cultivars revealed that Hamedan was the most susceptible and Tehran was the most resistant cultivar. The parasitoids reared on the aphids fed on the susceptible cultivar (Hamedan) had a higher intrinsic rate of increase (0.2641 day−1) than those (0.2259 day−1) reared on the resistant cultivar (Tehran). These results demonstrated that the quality of host plant affected the performance of M. persicae and its parasitoid D. rapae.

The effect of rearing history and aphid density on volatile‐mediated foraging behaviour of Diaeretiella rapae

1. Parasitoid females foraging for hosts rely on cues derived from the insect host, the host plant and/or their interaction, and all of these can be learned during the immature and adult stages.2. The present study investigated the importance of rearing history on foraging behaviour of Diaeretiella rapae, an endoparasitoid often associated with aphids feeding on brassicaceous plant species.3. Parasitoids were reared on one of the four possible combinations, comprising two brassicaceous host plant species, Brassica nigra or Raphanus sativus, and two aphid species Brevicoryne brassicae or Myzus persicae. These parasitoids were tested in a Y‐tube olfactometer and given the choice between volatiles emitted by an aphid‐infested plant (25 or 100 aphids per plant) and an uninfested control plant. The parasitoid's responses were compared when offered: (i) the same plant–aphid combination as the one on which it had been reared; (ii) the same host plant infested with the alternative aphid species; or (iii) an alternative plant with the alternative aphid species.4. Aphid density did affect the behavioural responses to the various odour sources, but rearing history did not. Diaeretiella rapae only preferred aphid‐induced to non‐induced plant volatiles at low aphid infestation level, whereas they did not discriminate between volatiles at high aphid infestation level.5. It is concluded that aphid‐induced volatiles of brassicaceous plants play an important role during host habitat location, but seem less important for parasitoids to locate the aphid host itself. The data are discussed in the light of manipulation of host plant defences by aphids.

Effects of paracress (Acmella oleracea) extracts on the aphids Myzus persicae and Lipaphis erysimi and two natural enemies

This study was conducted to evaluate the toxicity of paracress (Acmella oleracea) extracts to the aphids Myzus persicae (Sulz.) and Lipaphis erysimi (Kalt.) (Hemiptera: Aphididae), and the extracts selectivity to the aphid parasitoid Diaeretiella rapae McIntosh (Hymenoptera: Braconidae) and to the predator Orius insidiosus (Say) (Hemiptera: Anthocoridae). We evaluated water and ethanol extracts from A. oleracea. The ethanol extract caused 90% mortality of both aphid species within 70 h and reduced their fecundity, whereas the water extract was inactive. In addition, this extract caused no mortality of the natural enemies. In an attempt to identify bioactive compounds in the ethanol extract, three alkamides, spilanthol, undeca-2E-en-8,10-diynoic acid isobutylamide (compound 2) and (2E)-N-(2-methylbutyl)-2-undecene-8,10-diynamide (compound 3), were isolated and tested on L. erysimi. From the isolated alkamides, spilanthol and compound 2 showed high insecticidal activity against L. erysimi, indicating that they contributed to the observed extract activity. The results of this study demonstrate the potential of the ethanol extract of A. oleracea as a botanical insecticide in the management of aphids in brassicas.

Behavioral and Ovipositional Response of Diaeretiella rapae (Hymenoptera: Braconidae) to Rhopalosiphum padi and Brevicoryne brassicae in Winter Wheat and Winter Canola.

Winter canola Brassica napus L. (Brassicales: Brassicaceae) was introduced to U.S. Southern Great Plains (Kansas, Oklahoma, Texas) growers to manage some difficult-to-control grassy weeds in winter wheat Triticum aestivum L. (Poales: Poaceae). Two braconid parasitoids, Diaeretiella rapae (M'Intosh) and Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae) are active in this cropping landscape. Both wasps move between crops but D. rapae has a limited ability to develop in the main wheat aphid hosts, so L. testaceipes could influence D. rapae's ability to maintain itself when canola is absent in the landscape. We compared behavioral responses of naturally emerged D. rapae and wasps that were excised before emergence to odor volatiles of host plant, aphid host and aphid-infested plants using two plant/aphid combinations (wheat/Rhopalosiphum padi (L.) and canola/Brevocoryne brassicae L. (Hemiptera: Aphididae). We also compared parasitism rates of D. rapae that were naturally emerged and excised from R. padi or B. brassicae on subsequent parasitism rates of R. padi or B. brassicae hosts. Naturally emerged wasps responded more strongly to host plant and host plant + aphid odors compared to excised wasps regardless of the host origin. Neither wasp group responded to odors from aphids alone. Both wasp groups were most attracted to odors from aphid-infested host plants, regardless of the combination. D. rapae parasitism rates on canola-reared aphids were higher than on wheat-reared aphids. D. rapae parasitism rates were lower when switched from its original host to the alternate host. Results suggest that D. rapae faces challenges to maintain significant populations in the wheat/canola landscape of the Southern Great Plains, especially in years when canola is not locally present.

Bacteria and Competing Herbivores Weaken Top-Down and Bottom-Up Aphid Suppression.

Herbivore suppression is mediated by both plant defenses and predators. In turn, plant defenses are impacted by soil fertility and interactions with soil bacteria. Measuring the relative importance of nutritional and microbial drivers of herbivore resistance has proven problematic, in part because it is difficult to manipulate soil-bacterial community composition. Here, we exploit variation in soil fertility and microbial biodiversity across 20 farms to untangle suppression of aphids (Brevicoryne brassicae) through bottom–up and top–down channels. We planted Brassica oleracea plants in soil from each farm, manipulated single and dual infestations of aphids alone or with caterpillars (Pieris rapae), and exposed aphids to parasitoid wasps (Diaeretiella rapae) in the open field. We then used multi-model inference to identify the strongest soil-based predictors of herbivore growth and parasitism. We found that densities of Bacillus spp., a genus known to include plant-growth-promoting rhizobacteria, negatively correlated with aphid suppression by specialist parasitoids. Aphid parasitism also was disrupted on plants that had caterpillar damage, compared to plants attacked only by aphids. Relative abundance of Pseudomonas spp. bacteria correlated with higher aphid growth, although this appeared to be a direct effect, as aphid parasitism was not associated with this group of bacteria. Non-pathogenic soil bacteria are often shown to deliver benefits to plants, improving plant nutrition and the deployment of anti-herbivore defenses. However, our results suggest that these plant growth-promoting bacteria may also indirectly weaken top–down aphid suppression by parasitoids and directly improve aphid performance. Against a background of varying soil fertility, microbial biodiversity, competing herbivores, and natural enemies, we found that effects of non-pathogenic soil microbes on aphid growth outweighed those of nutritional factors. Therefore, predictions about the strength of plant defenses along resource gradients must be expanded to include microbial associates.

Dual-guild herbivory disrupts predator-prey interactions in the field.

Plant defenses often mediate whether competing chewing and sucking herbivores indirectly benefit or harm one another. Dual-guild herbivory also can muddle plant signals used by specialist natural enemies to locate prey, further complicating the net impact of herbivore-herbivore interactions in naturally diverse settings. While dual-guild herbivore communities are common in nature, consequences for top-down processes are unclear, as chemically mediated tri-trophic interactions are rarely evaluated in field environments. Combining observational and experimental approaches in the open field, we test a prediction that chewing herbivores interfere with top-down suppression of phloem feeders on Brassica oleracea across broad landscapes. In a two-year survey of 52 working farm sites, we found that parasitoid and aphid densities on broccoli plants positively correlated at farms where aphids and caterpillars rarely co-occurred, but this relationship disappeared at farms where caterpillars commonly co-occurred. In a follow-up experiment, we compared single and dual-guild herbivore communities at four local farm sites and found that caterpillars (P.rapae) caused a 30% reduction in aphid parasitism (primarily by Diaeretiella rapae), and increased aphid colony (Brevicoryne brassicae) growth at some sites. Notably, in the absence of predators, caterpillars indirectly suppressed, rather than enhanced, aphid growth. Amid considerable ecological noise, our study reveals a pattern of apparent commensalism: herbivore-herbivore facilitation via relaxed top-down suppression. This work suggests that enemy-mediated apparent commensalism may override constraints to growth induced by competing herbivores in field environments, and emphasizes the value of placing chemically mediated interactions within their broader environmental and community contexts.