Aphids Facing Their Parasitoids: A First Look at How Chemical Signals May Make Higher Densities of the Pea Aphid Acyrthosiphon pisum Less Attractive to the Parasitoid Aphidius ervi

Herbivore-induced plant volatiles constitute the first indicators of insect host presence, and these can affect the foraging behavior of their natural enemies. The density of insect hosts may affect the nature and concentration of these plant-induced volatiles. We tested the impact of infestation density (low, intermediate, and high) of the pea aphid, Acyrthosiphon pisum (Homoptera: Aphididae), feeding on the broad bean Vicia faba, on the attractiveness of the parasitoid Aphidius ervi (Hymenoptera: Braconidae), using a Y-tube olfactometer (infested vs. non-infested plants). The emitted volatile compounds from both infested and non-infested plants were collected and identified. In addition, two series of experiments were carried out to test the impact of the presence of a conspecific female parasitoid within the aphid/plant complex on the attractiveness to other females. Parasitoids were significantly more attracted to the plants with low and intermediate aphid infestation levels. The volatile blend composition of the infested plants changed in relation to aphid density and may explain the low attraction of parasitoids toward high aphid density. The presence of conspecific females on the aphid patch had no apparent impact on the behavioral choices of other parasitoid females. Our study adds a new aspect to understanding plant–aphid–parasitoid interactions, including the possibility that aphids may manipulate chemical cues of host plants affecting the orientation of parasitoids.

Tree Diversity and Damage by Cypress Aphid, Cinara cupressi, on Juniperus procera in Gulele Botanical Garden and Entoto

Juniperus procera is an important tree species in Ethiopia. Increasing demand for fuelwood and construction material has also created markets for J. procera products. The impact of cypress aphid, Cinara cupressi, on J. procera is becoming catastrophic in the country. However, the level of cypress aphid damage on J. procera has never been studied in Entoto Mountain forest and Gullele Botanical Garden (GBG) in Ethiopia. The objective of this study was to assess the cypress aphid densities and extent of damage caused by the cypress aphid on solely and mixed planted J. procera in Entoto Mountain forest and GBG in Addis Ababa, Ethiopia, by evaluating aphid adult and nymph densities and damage to J. procera trees. Eight plots of 50 m ∗ 50 m planted with pure J. procera and mixed J. procerawere selected at each site. Five 5 m ∗ 5 m subplots were formed. To assess aphid density hthree tree twigs per whorl were sampled from each tree in the subplots randomly. The collected samples were examined in the laboratory for the presence of nymph and adult cypress aphid. Tree damage was assessed by visually observing the percentage of infestation in the tree crown and was scored on a 1–5 scale. The results showed significant differences in aphid densities between the sites. The highest aphid count was observed at Entoto Mountain. Besides, the solely planted J. procera trees had higher cypress aphid numbers than the mixed plantation. The mean percentage of aphid-infested trees in Entoto and GBG was 53.6 and 46.4, respectively. Tree mortality was extremely low in both sites and planting systems. The level of aphid damage was also significantly different between the planting systems, where mixed plantations had a lower aphid infestation level. Tree damage was directly correlated with the density of aphids collected. In conclusion, cypress aphid was apparent in both sites, and most of the trees were infested by this pest. However, the extent of damage varied with the planting system. Therefore, it is recommended to consider a mixed plantation of trees as one of cypress aphid management tools in J. procera.

Brevicoryne brassicae (cabbage aphid).

Crops which can suffer severe attack by B. brassicae include cabbage, cauliflower, broccoli, radish, swede and mustard. Kale, oilseed rape and Brussels sprouts are usually only lightly infested, while turnips appear immune. Large colonies feed on the undersides of young leaves, draining plant nutritional resources, and on the flower heads of seed crops, reducing the setting of seed (Blackman and Eastop, 2000). On cabbage in Germany, numbers of aphids on the plants peaked in June-July and again in September-October. The yield was most affected by attack in the second population peak. Control thresholds for fresh consumption were 20% plants attacked with more than 10 apterae/plant, or 10% attacked plants when one or more plants had more than 100 aphids (Hildenhagen and Hommes, 1997).B. brassicae was very numerous on yellow mustard (Sinapis alba) in a study in Poland, in which early infestations (at the bud development stage) prevented stalk development and caused premature plant death; while later infestations (at the peak or end of blooming) caused high yield reductions (Hurej and Preiss, 1997). Among several aphids infesting brassica crops, B. brassicae was generally the most prevalent during the growing season (e.g. Trumble, 1982; Raworth, 1984; Nematollahi et al., 2014a). It occurred primarily on the highest and youngest leaves and stems, with the highest aphid density recorded at the head formation stage on broccoli and the stem elongation stage on oilseed rape (Trumble, 1982; Nematollahi et al., 2014a). B. brassicae significantly preferred the upper parts (upper 10-15 cm of the stem) of oilseed rape plants to the lower parts (the rest of the stem) (Nematollahi et al., 2014a).B. brassicae can sometimes reduce both crop yield and quality of spring and winter oilseed rape in Europe. In field experiments in the UK, yield responses to insecticide treatment tended to be larger in spring-sown than in winter-sown oilseed rape, mainly because it became more heavily infested at an early growth stage. B. brassicae is a sporadic oilseed rape pest, however, that will only rarely reach threshold numbers for control (Ellis et al., 1999). Daebeler and Hinz (1980) presented an analysis of yield loss in winter rape in Germany. They showed that by the time crops become heavily infested, serious injury will already have occurred, so control measures need to be taken early. Experimental studies showed that B. brassicae could reduce fresh and dry weight, leaf area and concentration of amino acid in aphid-infested plants (van Emden, 1990). B. brassicae hampers photosynthesis in a range of oilseed brassicas (Arjad Hussain et al., 2014; Razaq et al., 2014). In regions with warm climates, parthenogenetic reproduction can occur throughout the year. Considerable damage can occur to vegetables, particularly those grown for seed. In the Middle East, alatae migrate to cruciferous vegetable crops in autumn-early winter, migrating to wild Cruciferae in spring where they pass the summer. In Nigeria, cabbages with high uncontrolled infestations usually suffer stunted growth, plant death and low yields (Parh et al., 1987). In Himachal Pradesh, India, the avoidable yield losses caused by an aphid complex (B. brassicae, Lipaphis erysimi and Myzus persicae) to three different cruciferous oilseed crops, Brassica campestris var. toria, B. campestris var. sarson and B. juncea, were 67.61, 62.51 and 50.00%, respectively. Most of the losses occurred when the infestation was prevalent during the flowering stage. These losses were checked by insecticide applications at the initiation of flowering (Sharma and Kashyap, 1998). Late-season insect infestation of Brassica napus, B. rapa, B. juncea and Sinapis alba was studied in Idaho, USA. Aphid colonization (primarily B. brassicae) was observed on all these plant species, but infestation on S. alba and B. rapa occurred too late to have a major effect on seed yield. Seed oil content of rape species was significantly reduced by insect damage (B. brassicae, along with Ceutorhynchus assimilis and Plutella xylostella), although oil quality (indicated by fatty acid profile) was not affected. Uncontrolled insect infestation reduced seed yield of rape species by 37 and 32% in B. napus and B. rapa, respectively (Brown et al., 1999). B. brassicae is a vector of about 20 plant viruses, including Turnip mosaic virus (as cabbage black ringspot, cabbage ring necrosis and radish mosaic) and Cauliflower mosaic virus (Blackman and Eastop, 2000).

Aphid alarm pheromone alters larval behaviour of the predatory gall midge, Aphidoletes aphidimyza and decreases intraguild predation by anthocorid bug, Orius laevigatus

Intraguild predation is the killing and consuming of a heterospecific competitor that uses similar resources as the prey, and also benefit from preying on each other. We investigated the foraging behaviour of the gallmidge, Aphidoletes aphidimyza, a predator of aphids used for biological control that is also the intraguild prey for most other aphid natural enemies. We focus on how aphid alarm pheromone can alter the behaviour of the gallmidge, and predation by the anthocorid bug Orius laevigatus (O. laevigatus). We hypothesised that gallmidges would respond to the presence of (E)-β-farnesene (EBF) by leaving the host plant. Since feeding by Aphidoletes gallmidge larvae does not induce EBF emission by aphids, this emission indicates the presence of an intraguild predator. We found that gallmidge larvae reduced their foraging activities and left the plant earlier when exposed to EBF, particularly when aphids were also present. Contrastingly, gallmidge females did not change the time visiting plants when exposed to EBF, but lay more eggs on plants that had a higher aphid density. Lastly, EBF reduced the number of attacks of the intraguild predator, O. laevigatus, on gallmidge larvae, potentially because more gallmidges stopped aphid feeding and moved off the plant at which point O. laevigatus predated on aphids. Our work highlights the importance of understanding how intraguild predation can influence the behaviour of potential biological control agents and the impact on pest control services when other natural enemies are also present.

Zucchini Plants Alter Gene Expression and Emission of (E)-β-Caryophyllene Following Aphis gossypii Infestation

Zucchini (Cucurbita pepo L.) is widely cultivated in temperate regions. One of the major production challenges is the damage caused by Aphis gossypii (Homoptera: Aphididae), a polyphagous aphid, which can negatively affect its host plant, both directly by feeding and indirectly by vectoring viruses. To gain insights into the transcriptome events that occur during the zucchini–aphid interaction and to understand the early-to-late defense response through gene expression profiles, we performed RNA-sequencing (RNA-Seq) on zucchini leaves challenged by A. gossypii (24, 48, and 96 h post-infestation; hpi). Data analysis indicated a complex and dynamic pattern of gene expression and a transient transcriptional reconfiguration that involved more than 700 differentially expressed genes (DEGs), including a large number of defense-related genes. The down-regulation of key genes of plant immunity, such as leucine-rich repeat (LRR) protein kinases, transcription factors, and genes associated with direct (i.e., protease inhibitors, cysteine peptidases, etc.) and indirect (i.e., terpene synthase) defense responses, suggests the aphid ability to manipulate plant immune responses. We also investigated the emission of volatile organic compounds (VOCs) from infested plants and observed a reduced emission of (E)-β-caryophyllene at 48 hpi, likely the result of aphid effectors, which reflects the down-regulation of two genes involved in the biosynthesis of terpenoids. We showed that (E)-β-caryophyllene emission was modified by the duration of plant infestation and by aphid density and that this molecule highly attracts Aphidius colemani, a parasitic wasp of A. gossypii. With our results we contributed to the identification of genes involved in cucurbit plant interactions with phloem feeders. Our findings may also help pave the way toward developing tolerant zucchini varieties and to identify molecules for sustainable management of harmful insect populations.

Proteome changes in pepper (Capsicum annuum L.) leaves induced by the green peach aphid (Myzus persicae Sulzer)

Background Aphid attack induces defense responses in plants activating several signaling cascades that led to the production of toxic, repellent or antinutritive compounds and the consequent reorganization of the plant primary metabolism. Pepper (Capsicum annuum L.) leaf proteomic response against Myzus persicae (Sulzer) has been investigated and analyzed by LC-MS/MS coupled with bioinformatics tools. Results Infestation with an initially low density (20 aphids/plant) of aphids restricted to a single leaf taking advantage of clip cages resulted in 6 differentially expressed proteins relative to control leaves (3 proteins at 2 days post-infestation and 3 proteins at 4 days post-infestation). Conversely, when plants were infested with a high density of infestation (200 aphids/plant) 140 proteins resulted differentially expressed relative to control leaves (97 proteins at 2 days post-infestation, 112 proteins at 4 days post-infestation and 105 proteins at 7 days post-infestation). The majority of proteins altered by aphid attack were involved in photosynthesis and photorespiration, oxidative stress, translation, protein folding and degradation and amino acid metabolism. Other proteins identified were involved in lipid, carbohydrate and hormone metabolism, transcription, transport, energy production and cell organization. However proteins directly involved in defense were scarce and were mostly downregulated in response to aphids. Conclusions The unexpectedly very low number of regulated proteins found in the experiment with a low aphid density suggests an active mitigation of plant defensive response by aphids or alternatively an aphid strategy to remain undetected by the plant. Under a high density of aphids, pepper leaf proteome however changed significantly revealing nearly all routes of plant primary metabolism being altered. Photosynthesis was so far the process with the highest number of proteins being regulated by the presence of aphids. In general, at short times of infestation (2 days) most of the altered proteins were upregulated. However, at longer times of infestation (7 days) the protein downregulation prevailed. Proteins involved in plant defense and in hormone signaling were scarce and mostly downregulated.

Impact of natural products on Acyrthosiphon pisum density on Pisum sativum L. and forage quality

A field trial was conducted at the Institute of Forage Crops (Pleven, Bulgaria) from 2015 to 2017. It studied the effects of natural products on Acyrthosiphon pisum density, as well as changes in the chemical composition, content of plant fibre components and enzyme degradability in forage pea. Treatments with the natural insecticides Madex and Agricolle, applied alone or in combination with the organic fertilizers Lithovit and Nagro were performed twice - at the beginning of the flowering stage and one week later. The fertilizers used in the trial are environmentally safe and approved for use in organic production. The synthetic products Kristalon, a foliar fertilizer, and Proteus 110 OD, an insecticide, were used for comparison. The application of natural products, either alone and in combination, resulted in a reduction in pea aphid density. Applying Agricolle with Nagro, followed by Lithovit with Agricolle, led to the highest aphid number decrease (70.0 and 51.1%, respectively). An optimal combination of decrease in the content of plant cell wall fibre components, cellulose and lignification degree with a significant increase in forage in vitro enzyme digestibility was established after applying Agricolle with Lithovit and Agricole with Nagro. Digestibility reached 71.8 and 69.8%, respectively, an increase of 8.2-5.2%, while ADF, cellulose and lignification degree decreased from 7.1 and 7.7%, 8.0 and 23.4%, amd 10.5 and 6.8% after applying Agricolle with Lithovit and Agricole with Nagro, respectively. In comparison, the synthetic products Kristalon and Kristalon with Proteus increased forage quality, but to a relatively lesser extent. A stronger linear relationship was found between aphid density and dry matter digestibility, compared to the content of neutral detergent fibres. Pea forage with low content of plant cell wall fibre components, cellulose and lignification degree, high protein content, and digestibility after treatment with the natural product Agricolle, and its combinations with Lithovit and Nagro, make it a very good complement to other forages in dairy cow rations.

Methyl Salicylate and Sesquiterpene Emissions Are Indicative for Aphid Infestation on Scots Pine

Biotic stresses on forest trees are caused by various pest insects and plant pathogens. Attack by these parasites is known to induce the emissions of various biogenic volatile organic compounds (BVOCs), and the profile of these emissions often differs between infested and healthy plants. This difference in emission profile can be used for the non-destructive early-stage diagnosis of the stressor organism. We studied how phloem feeding by a large pine aphid (Cinara pinea Mordvilko) on the branch bark of Scots pine (Pinus sylvestris L.) affects BVOC emissions compared to those of healthy plants in two experiments. We found that in aphid-infested plants, methyl salicylate (MeSA) emissions significantly increased, and the emission rates were dependent on aphid density on the studied branch. Aphid infestation did not significantly affect total monoterpene emission, while the emissions of total sesquiterpenes were substantially higher in aphid-infested saplings than in uninfested plants. Sesquiterpene (E, E)-α-farnesene was emitted at increased rates in both experiments, and the aphid alarm pheromone sesquiterpene (E)-β-farnesene, only in the experiment with higher aphid pressure. We conclude that the rapid increase in MeSA emissions is the most reliable indicator of aphid infestation in pine trees together with (E, E)-α-farnesene.

Intraguild Predation between Chrysoperla carnea (Neuroptera: Chrysopidae) and Hippodamia variegata (Coleoptera: Coccinellidae) at Various Extraguild Prey Densities and Arena Complexities

Intraguild predation (IGP) is a ubiquitous, important and common interaction that occurs in aphidophagous guilds. The effects of extraguild prey (EGP, i.e., aphids) density, predator life stage combinations and duration of the interaction on the level, asymmetry and direction of intraguild predation between lacewing Chrysoperla carnea and ladybird Hippodamia variegata were examined in simple laboratory arena and more complex microcosm environment. Three initial densities of 50, 150 and 400 Aphis fabae third instar nymphs and a control without aphids were provided to six combinations of predator life stages (2nd and 3rd larval instars of lacewing and 3rd and 4th instars and adult females of ladybird). The remaining aphid density and occurrence of IGP were checked after 24, 48 and 72 h. The IGP intensity (IGP level, IL) was similar in the simple arena (reaching 0.6 between larvae in absence of EGP and 0.3 between lacewing larvae and ladybird females) and microcosm environment (0.3 without EGP). In both environments, increasing EGP density lowered IL according to negative exponential relationship. IGP was asymmetric (general average asymmetry was 0.82 in simple arena and 0.93 in microcosm, the difference was not significant) and mostly in favour of larvae of C. carnea, except in the combination of 2nd larvae of C. carnea with the 4th larvae and adults of H. variegata. The direction of IGP, but not other characteristics, partially changed during the duration of the experiment. The incidence of IGP interactions among aphid predators under real conditions and its consequences on aphid biological control are discussed.

Tritrophic interaction between the Mexican sunflower, the aphid Aphis gossypii and natural enemies in a greenhouse experiment

The establishment of invasive plants negatively affects natural environments. Invasive herbivores that attack weeds can be used as a form of biological control, but natural enemies of herbivores must be associated with this interaction to prevent the invasive phytophagous from become a local pest. We performed a greenhouse experiment to evaluate how the cotton aphid, Aphis gossypii, a ok and invasive herbivore, affects the performance of the weed Tithonia diversifolia, the Mexican sunflower. We also examined the relationship between the aphid and local natural enemies. Seedlings of T. diversifolia were divided in two groups: one infested by the aphid and another not infested. After 22 days, we assessed the relationship between aphid abundance and the presence of natural enemies (Coccinelidae and Aphidius platensis) on infested plants, and compared the vegetative performance of the two seedling groups. Both natural enemies were positively related to high aphid density on infested plants. Plants infested by the aphid presented foliar necrosis and senescence, and a reduction of around 50% in leaf number, foliar area, shoot length and shoot, root and total plant weight compared to non-infested plants. These results indicate potential biological control of Mexican sunflower seedlings by the cotton aphid, and control of this aphid by the studied natural enemies.