Aphid Parasitism Research Articles

Landscape composition and configuration influence cereal aphid–parasitoid–hyperparasitoid interactions and biological control differentially across years

The loss of landscape heterogeneity through agricultural intensification is known to affect aphid–parasitoid–hyperparasitoid interactions, with consequences for biological control. Various aspects of landscape heterogeneity (e.g. landscape composition and configuration) are expected to affect these interactions differentially, but there were few attempts to empirically compare the influence of separate landscape features on pest-parasitoid dynamics. To address these questions, we conducted three simultaneous studies in wheat fields in northern Serbia, to compare the effects of contrasting landscape contexts: (1) simple vs. more complex landscapes; (2) large- vs. small-field landscapes; (3) large-field areas with contrasting character of their marginal vegetation. We (1) found that aphid densities, parasitism rates and species richness of parasitoids and hyperparasitoids were higher in landscapes with more extensive and diversified non-crop habitats, positively affecting the biological control. We (2) did not find significant differences in aphid abundance and parasitism between large- and small-field landscapes, but we detected some contradictory patterns in aphid growth and parasitism increase; we relate both findings to certain region-specific landscape features of wider relevance. The character of marginal vegetation (3) had mixed effects on aphid–parasitoid interactions and dynamics, with respect to source of colonization. Parasitism rates above 22–24% were associated with population decline in the aphids, consistently across analyzed landscape contrasts. Other relationships were subject to significant interannual variability (over 2–4 years period), suggesting that effectiveness of landscape management for conservation biological control would also fluctuate year by year. Our findings show that a well-founded landscape-scale management for biological pest control in agriculture must be adjusted for differential aspects of landscape heterogeneity effects on pest–parasitoid interactions.

Effects of surrounding landscape on parasitism of alfalfa aphids in an IPM crop system in northern Catalonia

The parasitism (Hymenoptera, Braconi- dae, Aphidiinae) of alfalfa aphids (Hemiptera, Aph- ididae) was determined during three crop growing seasons. The highest rates of parasitism of Acyrtho- siphon pisum Harris and of Therioaphis trifolii Monell occurred during the first-second and the fourth alfalfa intercuts, respectively. The landscape surrounding the fields affected only parasitism rates of A. pisum. During the second intercut the forest area had a negative influence on parasitism rates, whereas the bordering surfaces of alfalfa and winter cereals had a positive influence. Positive correlations were found between the rates of parasitism of A. pisum and the rate of aphid population increase in the first intercut. By contrast, negative correlations were found in the second intercut, when the aphid population peaked, suggesting that parasitoids contribute to the regulation of the aphid. This study stresses that regulation of alfalfa aphids cannot be explained solely on the basis of one functional group of natural enemies.

Drought and Root Herbivory Interact to Alter the Response of Above-Ground Parasitoids to Aphid Infested Plants and Associated Plant Volatile Signals

Multitrophic interactions are likely to be altered by climate change but there is little empirical evidence relating the responses of herbivores and parasitoids to abiotic factors. Here we investigated the effects of drought on an above/below-ground system comprising a generalist and a specialist aphid species (foliar herbivores), their parasitoids, and a dipteran species (root herbivore).We tested the hypotheses that: (1) high levels of drought stress and below-ground herbivory interact to reduce the performance of parasitoids developing in aphids; (2) drought stress and root herbivory change the profile of volatile organic chemicals (VOCs) emitted by the host plant; (3) parasitoids avoid ovipositing in aphids feeding on plants under drought stress and root herbivory. We examined the effect of drought, with and without root herbivory, on the olfactory response of parasitoids (preference), plant volatile emissions, parasitism success (performance), and the effect of drought on root herbivory. Under drought, percentage parasitism of aphids was reduced by about 40–55% compared with well watered plants. There was a significant interaction between drought and root herbivory on the efficacy of the two parasitoid species, drought stress partially reversing the negative effect of root herbivory on percent parasitism. In the absence of drought, root herbivory significantly reduced the performance (e.g. fecundity) of both parasitoid species developing in foliar herbivores. Plant emissions of VOCs were reduced by drought and root herbivores, and in olfactometer experiments parasitoids preferred the odour from well-watered plants compared with other treatments. The present work demonstrates that drought stress can change the outcome of interactions between herbivores feeding above- and below-ground and their parasitoids, mediated by changes in the chemical signals from plants to parasitoids. This provides a new insight into how the structure of terrestrial communities may be affected by drought.

‘Attract and reward’: Combining a herbivore-induced plant volatile with floral resource supplementation – Multi-trophic level effects

Two field experiments were conducted to assess whether a concept termed ‘attract and reward’ (A&R) could enhance conservation biological control (CBC). In A&R, a synthetically-produced herbivore-induced plant volatile (HIPV) (‘attract’) is combined with a floral resource (‘reward’). It is anticipated that the two will work synergistically, attracting natural enemies into the crop (‘attract’) and maintaining them within it (‘reward’).The study was conducted in Canterbury, New Zealand and the system consisted of brassica crop, commonly occurring brassica herbivores, their natural enemies and higher order natural enemies. The HIPV deployed was methyl salicylate (MeSA) and the floral resource was buckwheat Fagopyrum esculentum.The first experiment assessed the abundance of arthropods from three trophic levels and the second evaluated herbivore abundance, parasitism and hyper-parasitism rates. No synergistic effect of ‘attract’ and ‘reward’ was observed in either experiment. Populations of three parasitoids, one hoverfly and one lacewing from the third trophic level and a fourth trophic level lacewing parasitoid increased significantly in treatments with buckwheat. One hoverfly species was significantly more abundant in treatments with MeSA, but less abundant in treatments with buckwheat. The effect of MeSA on Diadegma semiclausum abundance depended on sex, with fewer males and more females being caught. Treatments with MeSA had significantly higher aphid parasitism rate.Combining MeSA and buckwheat could be beneficial because the two techniques increase the abundance of different natural enemies. Thus, these results indicate that A&R has potential as a CBC technique, as long as any unwanted side effects can be managed.

Infestation and natural parasitism of aphids in single and mixed pastures of black oats and ryegrass

Some species of aphids are major pests on cereal crops and grass pastures. Usually these pests are not adequately controlled in pasture lands that become sources of aphid infestations to cereal crops. The dynamics of aphids and the incidence of natural enemies are less known in pasture systems than in cereal fields. The objective of this work was to assess the aphid infestation and natural aphid parasitism in different pasture composition. Three hypotheses were tested: 1- the aphid species composition in pastures may vary according to the cereal species in the field; 2- the mixture of two plant species can modify the amount and diversity of aphids; 3- the plant species composition of pasture fields influences the parasitism of aphids. Empirical data were obtained from three Poaceae fields: black oats (Avena strigosa L.), ryegrass (Lolium multiflorum L.), and a mixed field of black oats and ryegrass. The most abundant aphid species was Rhopalosiphum padi followed by Sitobion avenae. Plant species composition increases the amount and the parasitism rates of aphids. The mixture of heavily infested black oats with a poorly infested ryegrass resulted in reduction of aphid infestation in comparison with heavily-infested single plant species field. This is possible because the conditions are favorable for the development of parasitoid populations. Aphidius colemani was the main parasitoid found in these areas.

Preliminary study on parasitism of aphids (Hemiptera: Aphididae) in relation to characteristics of alfalfa fields (Medicago sativa L.) in the Argentine Pampas

Alfalfa is a perennial crop and is one of the most relevant forage resources for cattle in the Argentine Pampas, with aphids (Hemiptera Aphididae), one of the main pest insects in alfalfa, being frequently attacked by a rich fauna of aphidiinae parasitoids (Hymenoptera: Braconidae). The aim of this study was to identify the possible field characteristics that influence the parasitism rates of aphids in alfalfa, in order to recommend simple methods of environmental manipulation to enhance the action of parasitoids. The abundance of aphids and their parasitism were estimated fortnightly, over a period of seven months in fourteen alfalfa fields located near the city of Rafaela, Santa Fe, through the collection of stem cuttings. The influence of field size, age of crop, and percentage of borders with spontaneous vegetation, on aphid abundance and their parasitism rates, were assessed through general lineal models with repeated measures. Greater aphid populations were observed in fields with a low percentage of natural borders, whereas the impact by parasitoids was higher in older crops and in those fields with a high percentage of natural borders. The relative importance of the characteristics of fields on parasitism of aphids is discussed, bearing in mind that conservation is the strategy of biological control recommended for developed countries given its low cost and potential sustainability.

Biological control of aphids in the presence of thrips and their enemies

Generalist predators are often used in biological control programs, although they can be detrimental for pest control through interference with other natural enemies. Here, we assess the effects of generalist natural enemies on the control of two major pest species in sweet pepper: the green peach aphid Myzus persicae (Sulzer) and the western flower thrips Frankliniella occidentalis (Pergande). In greenhouses, two commonly used specialist natural enemies of aphids, the parasitoid Aphidius colemani Viereck and the predatory midge Aphidoletes aphidimyza (Rondani), were released together with either Neoseiulus cucumeris Oudemans, a predator of thrips and a hyperpredator of A. aphidimyza, or Orius majusculus (Reuter), a predator of thrips and aphids and intraguild predator of both specialist natural enemies. The combined use of O. majusculus, predatory midges and parasitoids clearly enhanced the suppression of aphids and consequently decreased the number of honeydew-contaminated fruits. Although intraguild predation by O. majusculus on predatory midges and parasitoids will have affected control of aphids negatively, this was apparently offset by the consumption of aphids by O. majusculus. In contrast, the hyperpredator N. cucumeris does not prey upon aphids, but seemed to release aphids from control by consuming eggs of the midge. Both N. cucumeris and O. majusculus did not affect rates of aphid parasitism by A. colemani. Thrips were also controlled effectively by O. majusculus. A laboratory experiment showed that adult predatory bugs feed on thrips as well as aphids and have no clear preference. Thus, the presence of thrips probably promoted the establishment of the predatory bugs and thereby the control of aphids. Our study shows that intraguild predation, which is potentially negative for biological control, may be more than compensated by positive effects of generalist predators, such as the control of multiple pests, and the establishment of natural enemies prior to pest invasions. Future work on biological control should focus on the impact of species interactions in communities of herbivorous arthropods and their enemies.

Interaction between the aphid parasitoid Lysiphlebus testaceipes (Hymenoptera: Aphidiidae) and its hyperparasitoid Syrphophagus africanus (Hymenoptera: Encyrtidae)

The interactions between the cowpea aphid Aphis craccivora Koch, its parasitoid Lysiphlebus testaceipes Cresson and hyperparasitoid Syrphophagus africanus Gahan were assessed in the laboratory. Host age preference for hyperparasitism was evaluated using parasitized aphids of different ages (i.e. 1, 2, 3, 4, 5 and 6 days old and mixed ages). We studied the influence of the hyperparasitoid on the parasitism efficiency of L. testaceipes, the effect of host age on some biological parameters of S. africanus, and its hyperparasitism based on aggregated or isolated mummies. The presence of S. africanus significantly affected aphid parasitism by L. testaceipes (21.2 ± 3.6 vs 36.6 ± 4.5%). Except for 1-day-old hosts, S. africanus successfully parasitized hosts of all tested ages, with preference for newly formed mummies (39.16 ± 3.31% parasitism in 5-day-old hosts). The development time of immature stages of the S. africanus progeny was significantly affected by host age. The sex ratio of the hyperparasitoid progeny was largely female biased and did not vary with host age. Syrphophagus africanus females from live parasitized aphids produced more offspring (36.55 ± 6.28 vs 25.00 ± 7.16) and lived longer (21.09 ± 1.57 vs 10.88 ± 2.31 days) than those from mummy hosts. Hyperparasitism rates were higher on aggregated mummies than on dispersed ones (36.00 ± 2.86 vs 20.66 ± 4.00%).

Development and Implementation of an Environmentally Conscious System for Producing Cruciferous Vegetables by Small Farms in a Hilly and Mountainous Area of Western Japan

We report here on the development of a system for producing cruciferous vegetables without using chemical pesticides. This system enables small-scale farmers in hilly and mountainous areas in the western region of Japan to grow products of equivalent volume and quality to those produced by standard practices. The first steps in developing the system involved modifying and creating methods to protect cruciferous vegetables against insect pests. Crops were protected from flying pests, such as the white cabbage butterfly, cabbage armyworm, diamondback moth, cabbage sawfly, leafminer fly, cabbage bug, striped flea beetle, and brassica leaf beetle using a 0.6 mm mesh screen. Bacillus thuringiensis preparations, the use of which is permitted by Japanese Agricultural Standards for organic plants and organic processed foods of plant origin, were used to prevent damage caused by butterfly larvae, (white cabbage butterfly, cabbage armyworm, and diamondback moth) that hatch from eggs deposited on the outer surface of the screen through which they can invade. Invasion by crawling pests, such as the striped flea and brassica leaf beetles, was suppressed by covering the ground around the greenhouse with a mulching sheet to control weeds that form the habitat for these pests. Aphids passing through the 0.6 mm mesh screen were eliminated by a banker plant system employing the aphid parasite Diaeretiella rapae (McIntosh), a natural predator. Solarizing the soil before cultivation destroyed the pests' larvae, pupae, or eggs residing underground, such as those of the striped flea beetle and cutworm, while pest damage by vegetable weevils was prevented by inserting our newly invented traps just underground. The second step in developing the system involved dealing with higher greenhouse temperatures during summer that would result from the 0.6 mm mesh screen cover. To keep the wet bulb globe temperature (WBGT) around the worker within acceptable limits, a battery-powered ventilator consisting of a cart, fan, solar battery, and ordinary battery was used as well as installing simple roof windows, all of which effectively lowered the WBGTs. By combining these methods, we were able to demonstrate that the system was highly effective for producing quality cruciferous vegetables in greenhouses and in open fields.

Effects of organic and conventional fertilizer treatments on host selection by the aphid parasitoid Diaeretiella rapae

AbstractThe type and quantity of fertilizer supplied to a crop will differ between organic and conventional farming practices. Altering the type of fertilizer a plant is provided with can influence a plant’s foliar nitrogen levels, as well as the composition and concentration of defence compounds, such as glucosinolates. Many natural enemies of insect herbivores can respond to headspace volatiles emitted by the herbivores’ host plant in response to herbivory. We propose that manipulating fertilizer type may also influence the headspace volatile profiles of plants, and as a result, the tritrophic interactions that occur between plants, their insect pests and those pests’ natural enemies. Here, we investigate a tritrophic system consisting of cabbage plants, Brassica oleracea, a parasitoid, Diaeretiella rapae, and one of its hosts, the specialist cabbage aphid Brevicoryne brassicae. Brassica oleracea plants were provided with either no additional fertilization or one of three types of fertilizer: Nitram (ammonium nitrate), John Innes base or organic chicken manure. We investigated whether these changes would alter the rate of parasitism of aphids on those plants and whether any differences in parasitism could be explained by differences in attractivity of the plants to D. rapae or attack rate of aphids by D. rapae. In free‐choice experiments, there were significant differences in the percentage of B. brassicae parasitized by D. rapae between B. oleracea plants grown in different fertilizer treatments. In a series of dual‐choice Y‐tube olfactometry experiments, D. rapae females discriminated between B. brassicae‐infested and undamaged plants, but parasitoids did not discriminate between similarly infested plants grown in different fertilizer treatments. Correspondingly, in attack rate experiments, there were no differences in the rate that D. rapae attacked B. brassicae on B. oleracea plants grown in different fertilizer treatments. These findings are of direct relevance to sustainable and conventional farming practices.

Biological parameters and thermal requirements of the parasitoid Praon volucre (Hymenoptera: Braconidae) with Macrosiphum euphorbiae (Hemiptera: Aphididae) as host

The effect of temperature on the biology of Praon volucre (Haliday, 1833) (Hymenoptera: Braconidae) in Macrosiphum euphorbiae (Thomas, 1878) (Hemiptera: Aphididae) hosts was studied and the thermal requirements of the parasitoid were determined. Experiments were carried out at 16, 19, 22, 25, and 28 ± 1°C with 70 ± 10% relative humidity and 12 h photophase. The developmental time of P. volucre males and females decreased between 16 and 23°C. The highest percentages of aphid parasitism and adult parasitoid emergence were observed at 16, 19, and 22°C. The sex ratio (expressed as % females) decreased between 16 and 23°C. Male and female longevity was high between 19 and 22°C and decreased strongly at 25°C. The lower temperature threshold for P. volucre was 5.17°C and the thermal constant was 243 degrees-days.

Effect of Bt maize on the plant-aphid–parasitoid tritrophic relationships

The effect of Bt maize on aphid parasitism and the aphid–parasitoid complex was measured in field conditions on three transgenic varieties, two derived from Event MON810 and one from Bt176, and their near-isogenics in a two-year study. No differences in aphid abundance were found between Bt maize varieties and their near-isogenics. Differences within Bt and within near-isogenic varieties were found, but only in one year. Differences in aphid abundance were probably better accounted for the variety background and year conditions than by the transgenesis or Event. Lysiphlebus testaceipes (Cresson), Lipolexis gracilis Forster (Hymenoptera, Braconidae, Aphidiinae) and Aphelinus sp. (Hymenoptera, Aphelinidae) were the prevalent parasitoids. Bt maize did not alter the aphid–parasitoid associations and had no effect on the aphid parasitism and hyperparasitism rates. The results suggest that Bt maize has no negative impact on second, third and fourth levels of the trophic relationships studied.

Evolutionary ecology of the interactions between aphids and their parasitoids

Many organisms, including entomopathogenous fungi, predators or parasites, use aphids as ressources. Parasites of aphids are mostly endoparasitoid insects, i.e. insects which lay eggs inside the body of an other insect which will die as a result of their development. In this article, we review the consequences of the numerous pecularities of aphid biology and ecology for their endoparasitoids, notably the Aphidiinae (Hymenoptera: Braconidae). We first examine the various mechanisms used by aphids for defence against these enemies. We then explore the strategies used by aphidiine parasitoids to exploit their aphid hosts. Finally, we consider the responses of both aphids and parasitoids to ecological constraints induced by seasonal cycles and to environmental variations linked to host plants and climate. The fundamental and applied interest of studying these organisms is discussed.

Effects of Aposymbiotic and Symbiotic Aphids on Parasitoid Progeny Development and Adult Oviposition Behavior Within Aphid Instars

This study aims at exploring the potential relationship between aphidiine parasitoid development and the primary endosymbiont in aphids by focusing on specific aphid instars and the relative effects on parasitoid oviposition behavior and progeny development. Lysiphlebus ambiguus (Aphidiidae, Hymenoptera) is a solitary parasitoid of several species of aphids, including Aphis fabae. In this study, A. fabae was treated with antibiotic rifampicin to obtain aposymbiotic hosts and exposed to parasitism. L. ambiguus launched significantly more attacks on symbiotic L(2) (the second instar), aposymbiotic L(3) (the third instar) and L(4) (the forth instar) hosts than on the corresponding hosts at the same age. L. ambiguus also parasitized more L(1) aphids compared with adults irrespective of whether the aphid was asymbiotic or not. Pupa mortality rate of parasitoid progeny was significantly lower from aposymbiotic hosts than from the corresponding symbiotics at all stages. Female-biased parasitoid progeny was produced from aposymbiotic aphids without respect to host ages, but female progeny increased linearly with host ages at parasitism from symbiotic aphids. Body size of parasitoid progeny increased linearly with host instars at parasitism in symbiotic aphids but did not significantly change across host instars in aposymbiotic aphids. The offspring parasitoids turned out to be generally large in body size from attacking aposymbiotic aphids compared with the symbiotics. Development time of egg-to-adult of parasitoid progeny decreased with host instars in both symbiotic and aposymbiotic aphids but was generally much longer in aposymbiotic aphids than in symbiotic aphids. Our study suggests that age or body size of host aphids may not be the only cue exercised by L. ambiguus to evaluate host quality and that offspring parasitoids may be able to compensate for the nutrition stress associated with disruption of primary endosymbiotc bacteria in aposymbiotic aphids.

Spatial distribution of flower vs. honeydew resources in cereal fields may affect aphid parasitism

The potential impact of nectar availability for biological pest control is of increasing relevance due to the disappearance of non-crop plants in intensified agricultural landscapes. However, little is known about how floral resources and their spatial distribution in and around crop fields influence cereal aphid parasitoid (Hymenoptera, Aphidiidae) survival and population dynamics. Cereal aphid parasitoids may be less dependent on flowers as they can use honeydew from their aphid hosts as an alternative food resource. However, honeydew appears to be less suitable as a food source for parasitoids when compared to nectar. Here the effects of flower distribution (nectar), aphid abundance (honeydew quantity) and honeydew quality on parasitoid life history parameters were explored using a spatially explicit, individual-based simulation model. In addition, effects of the availability of floral resources in field margins on parasitism levels in winter wheat were investigated in a field study. In the simulation study three flower distributions were compared: (i) flowers restricted to the field margin (most common in agricultural landscapes) or (ii) flowers in strips within the crop, and (iii) random flower distribution in the field. Simulations suggest that nectar feeding increases parasitoid longevity and parasitism at low aphid densities, but only in fields with randomly distributed flowers. In contrast, the spatial distribution of flowers did not affect the parasitoid longevity and parasitism rate at high aphid densities, where parasitoid feeding was restricted to the abundant honeydew resources. In simulations, parasitoid survival and the number of aphids parasitized were also affected by the nutritional quality of the honeydew, with low quality honeydew having a less positive effect on parasitoid longevity and parasitism rate. The field study did not indicate evidence of a significant relationship between flower cover in field margins and parasitism rates in wheat near the margins. This finding is compatible with results from simulations and suggests that honeydew feeding is more prevalent in cereal aphid parasitoids than nectar feeding. In conclusion, nectar may be an important factor for the potential biocontrol success of cereal aphid parasitoids, but most likely only at low aphid densities and especially when flowers are broadly distributed in the field.

Specific bottom-up effects of arbuscular mycorrhizal fungi across a plant-herbivore-parasitoid system.

The majority of plants are involved in symbioses with arbuscular mycorrhizal fungi (AMF), and these associations are known to have a strong influence on the performance of both plants and insect herbivores. Little is known about the impact of AMF on complex trophic chains, although such effects are conceivable. In a greenhouse study we examined the effects of two AMF species, Glomus intraradices and G. mosseae on trophic interactions between the grass Phleum pratense, the aphid Rhopalosiphum padi, and the parasitic wasp Aphidius rhopalosiphi. Inoculation with AMF in our study system generally enhanced plant biomass (+5.2%) and decreased aphid population growth (−47%), but there were no fungal species-specific effects. When plants were infested with G. intraradices, the rate of parasitism in aphids increased by 140% relative to the G. mosseae and control treatment. When plants were associated with AMF, the developmental time of the parasitoids decreased by 4.3% and weight at eclosion increased by 23.8%. There were no clear effects of AMF on the concentration of nitrogen and phosphorus in plant foliage. Our study demonstrates that the effects of AMF go beyond a simple amelioration of the plants’ nutritional status and involve rather more complex species-specific cascading effects of AMF in the food chain that have a strong impact not only on the performance of plants but also on higher trophic levels, such as herbivores and parasitoids.

Volatile emission in wheat and parasitism by Aphidius avenae after exogenous application of salivary enzymes of Sitobion avenae

AbstractEnzymes in the saliva of the English green aphid, Sitobion avenae (F.) (Homoptera: Aphididae), may be responsible for inducing indirect defence in wheat. In this study, we examined the effects of exogenous application of three salivary enzymes of S. avenae, namely, polyphenol oxidase, pectinase, and cellulase, on the release of volatiles from wheat and the attractiveness of these volatiles to the parasitoid Aphidius avenae Haliday (Hymenoptera: Aphidiidae). In greenhouse experiments, parasitism of S. avenae on plants surrounded by pectinase‐treated plants and aphid‐damaged plants was significantly higher than parasitism of aphids on plants surrounded by cellulase‐ and polyphenol oxidase‐treated plants. In Y‐tube olfactometer tests, female A. avenae significantly preferred the volatiles collected from pectinase‐treated plants and aphid‐damaged plants. Gas chromatography‐mass spectrometry analysis showed that two kinds of semiochemicals, sulcatol and sulcatone, were identified only in pectinase‐treated plants and aphid‐damaged plants. In field experiments, parasitism of S. avenae by A. avenae was significantly higher on plants sprayed with a mixture of sulcatol and sulcatone than parasitism on plants sprayed with volatiles collected from cellulase‐ and polyphenol oxidase‐treated plants. These results show that pectinase in S. avenae watery saliva can trigger wheat plant indirect defence, and play a crucial role in interaction between wheat, wheat aphid, and A. avenae.

Host location and parasitism of Brevicoryne brassicae in diversified broccoli cropping systems

AbstractHost location is a crucial step in the life cycle of the cabbage aphid, Brevicoryne brassicae (L.) (Homoptera: Aphididae). Diversified cropping systems have the potential to inhibit or interfere with host location processes to reduce the severity of aphid outbreaks. We altered the vegetation mix of a broccoli [Brassica oleracea var. italica (Plenck) (Brassicaceae)] cropping system by substituting broccoli with strips of potatoes [Solanum tuberosum (L.) (Solanaceae)], planting broccoli into a cereal rye [Secale cereale (L.) (Poaceae)] cover crop, or both. The probability of aphid infestation was significantly reduced by the presence of the cover crop, whereas strips of potatoes slightly increased initial numbers. The effectiveness of the cover crop treatments was primarily due to fewer alate aphids initially colonizing broccoli plants. Aphid parasitism by Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae) did not explain the observed differences in the number of aphid colonies present in each treatment.

Laboratory studies on extracts of Melia fruit and Neem fruit on B. brassicae, B. tabaci and their predators and parasites

The biological effect of plant extracts on the two predators C. carnea and C. undecimpunctata was studied in the laboratory. Treated B. brassicae was offered to the newly hatched larvae of the two predators. The rate of prey consumption and duration of the larval instars of the two predators were slightly affected as a result of the treatments of the plant extracts. Adults of the aphid parasite D. Rapae and the whitefly parasite E. Mundus were exposed to the plant extracts via feeding on treated honey droplets or parasitized treated hosts. Mortality percentages of D. Rapae fed on honey contaminated with plant extracts were 50 and 80% for extracts of Melia fruits in ethanol and in petroleum ether, respectively. Mortalities in parasites developed in hosts treated with plant extracts of Neem fruits in ethanol and Melia flowers in petroleum ether in were 33.3 and 81.5%, respectively. Similar results were obtained when E. Mundus received the same treatments of the plant extracts.

Floral diversity, parasitoids and hyperparasitoids – A laboratory approach

Adding floral resources to agro-ecosystems to improve biological control can enhance the survival, egg load, and parasitism rate of insect parasitoids. However, this may not always be the case because the herbivore may benefit from the added resource as much as, or more than the third-trophic level. In addition, the natural enemies of those in the third-trophic level may also derive improved fitness from the added resources. Both these processes will dampen trophic cascades, leading to less-effective biological control. In this study, the effect of adding different flowering plants on the longevity, egg load, aphid parasitism rates and hyperparasitism of Aphidius ervi Haliday (Hymenoptera: Braconidae) by its hyperparasitoid Dendrocerus aphidum Rondani (Hymenoptera: Megaspilidae) were investigated, using the pea aphid Acyrthosiphon pisum Harris (Homoptera: Aphididae) as the herbivore. Parasitoids exposed to buckwheat survived, on average, between four to five times as long as those in the control (water) and those in phacelia, alyssum and coriander treatments survived three to four times as long. Hyperparasitoids exposed to buckwheat survived five to six times as long as those in the control and three to five times longer with the other plants compared with the control. Almost all flower species significantly increased parasitoid and hyperparasitoid egg loads and the number of parasitised aphids and parasitised mummies compared with control. Understanding the factors influencing the dynamics of multitrophic interactions involving flowering plants, herbivores, parasitoids and hyperparasitoids is a fertile area for future research. One of the most challenging areas in contemporary ecology concerns the relative importance of different types of biodiversity mediating trophic interactions and thereby influencing the structure of communities and food webs. This paper begins to explore this using an experimental, laboratory-based approach.