elham ahmed

Effects of volatile odors from leek, Allium porum L., on the behavior of bean fly, Ophiomyia phaseoli (Tryon) (Diptera: Agromyzidae), were tested in laboratory olfactometer bioassays. Aqueous and solvent extracts (dichloromethane and methanol) of leek were repellent to adult flies. Whole leek plants were repellent and prevented attraction to the host plant, beans. Beans that had been exposed to volatiles from living leek plants for 7 d became repellent to the fly. Leek and several other crops were tested in field experiments to identify candidate crops for a mixed cropping system to minimize bean fly attack in beans, Phaseolus vulgaris L. In a wet season field experiment, mixed cropping of bean with leek or three other vegetable crops did not significantly reduce bean fly infestation or infection with Fusarium oxysporum Schltdl. compared with a mono crop, but significantly reduced plant death caused by both agents combined. In two dry season field experiments, mixed cropping of beans with leek significantly reduced adult bean fly settling, emergence, and death of bean plants compared with a mono crop. Bean yield per row was approximately 150% higher for the mixed crop, and economic returns were approximately Sri Lankan Rs. 180,000/ha, higher than for the mono crop. For the mono crop, the farmer had a monetary loss, which would become a small profit only if the costs of family labor are excluded. The study is an example of the first steps toward development of sustainable plant protection in a subsistence system.

The effects of within-species plant genotype mixing on the habitat preference of a polyphagous ladybird were studied. Plant species diversity is often claimed to positively affect habitat preferences of insect predators, but the effects of within-species genotype diversity have not been extensively studied. In a field experiment with different barley (Hordeum vulgare) genotypes in mixed and pure stands, adult seven-spot ladybird Coccinella septempunctata, a polyphagous predator, preferred a specific combination of genotypes over the single genotypes alone before aphids had arrived in the crop, and again when aphids were emigrating. In laboratory experiments on adult ladybird orientation to odour from barley, ladybirds were attracted/arrested by the mixed odour of the same barley genotype mixture that was preferred in the field. Exposure of one barley genotype to volatiles from the other also caused the odour of the exposed plants to become more attractive to ladybirds. The results support the hypothesis that plant volatiles may attract or arrest foraging adult ladybirds, contributing to the selection of favourable habitats, and they show that within-species plant genotype mixing can shape interactions within multitrophic communities.

... As allelopathic plants, the thistles C. arvense and C. vulgare were selected. The aphid model was the bird cherry-oat aphid, Rhopalosiphum padi, the responses of which were compared with two other cereal aphid species. ... G., Beres, I. & Narwal, SS (2001) Allelopathic plants. ...

The response of the bird cherry-oat aphid, Rhopalosiphum padi, to barley plants was investigated following exposure of the plants to root allelochemicals from the aggressive weed couch-grass, Elytrigia (Agropyron) repens. Plants were treated either with root exudates from living couch-grass plants or with previously identified couch-grass root compounds [5-hydroxyindole-3-acetic acid, DL-5-hydroxytryptophan, L-5-hydroxytryptophan hydrate, and 6-hydroxy-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (carboline)] either separately or in mixtures. In choice and no-choice settling tests, aphid acceptance of barley plants was significantly reduced following treatment with root exudates, and the carboline when tested alone or in combination with the other compounds. In contrast, the other compounds without the carboline were less active in reducing aphid acceptance. In a probing bioassay, individual substances were either neutral or stimulatory to aphids, indicating that the reduced settling was probably not due to direct effects on aphids, but rather due to effects on the plant. This was confirmed in olfactometer assays, in which aphids were repelled by odors from barley plants following treatment with a mixture containing all four chemicals.

Aphid ecology and population dynamics are affected by a series of factors including behavioural responses to ecologically relevant chemical cues, capacity for population growth, and interactions with host plants and natural enemies. Using the aphid Rhopalosiphum padi (L.) (Homoptera: Aphididae), we showed that these factors were affected by infection with Rhopalosiphum padi virus (RhPV). Uninfected aphids were attracted to odour of uninfected aphids on the host plant, an aggregation mechanism. However, infected aphids were not attracted, and neither infected nor uninfected aphids were attracted to infected aphids on the plant. Infected aphids did not respond to methyl salicylate, a cue denoting host suitability. Infected aphids were more behaviourally sensitive to aphid alarm pheromone, and left the host plant more readily in response to it. RhPV reduced the lifespan and population growth rate of the aphid. The predacious ladybird, Coccinella septempunctata (L.) (Coleoptera: Coccinellidae), consumed more infected aphids than uninfected aphids in a 24-h period, and the aphid parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) attacked more infected than uninfected aphids. However, the proportion of mummies formed was lower with infected aphids. The results represent further evidence that associated organisms can affect the behaviour and ecology of their aphid hosts.

In order to examine whether the content of cyanogenic glycosides or gramine in barley affects the host plant preference of the aphid Rhopalosiphum padi (L.), two‐choice settling tests were performed with R. padi on different barley varieties with varying amounts of these compounds. The total concentration of cyanogenic glycosides in barley shoots, as determined in 59 varieties, varied between 20 and 1200 nmol (g fw)−1. Of the 59 varieties, six with high [>700 nmol (g fw)−1], six with intermediate [450–650 nmol (g fw)−1] and six with low [<200 nmol (g fw)‐l] content of cyanogenic glycosides were chosen for preference tests with R. padi. The content of gramine was determined in these 18 chosen varieties and was generally either low (close to 0) or high [about 2 μmol (g fw)−1]. There was no correlation between cyanogenic glycoside and gramine content. The aphids did not discriminate between varieties with high, intermediate or low content of cyanogenic glycosides in combinations with high or low gramine content. The results indicate that the concentration of neither of these compounds affected the settling preference of R. padi.

This study investigated the effects of airborne interaction between different barley cultivars on the behaviour of bird cherry-oat aphid Rhopalosiphum padi, the ladybird Coccinella septempunctata and the parasitoid Aphidius colemani. In certain cultivar combinations, exposure of one cultivar to air passed over a different cultivar caused barley to have reduced aphid acceptance and increased attraction of ladybirds and parasitoids. Parasitoids attacked aphids that had developed on plants under exposure more often than those from unexposed plants, leading to a higher parasitisation rate. Ladybirds, but not parasitoids, were more attracted to combined odours from certain barley cultivars than either cultivar alone. The results show that airborne interactions between undamaged plants can affect higher trophic levels, and that odour differences between different genotypes of the same plant species may be sufficient to affect natural enemy behaviour.

Olfactory learning may allow insects to forage optimally by more efficiently finding and using favourable food sources. Although olfactory learning has been shown in bees, insect herbivores and parasitoids, there are fewer examples from polyphagous predators. In this study, olfactory learning by a predatory coccinellid beetle is reported for the first time. In laboratory trials, adults of the aphidophagous ladybird Coccinella septempunctata did not prefer the odour of one aphid-infested barley cultivar over another. However, after feeding on aphids for 24 h on a cultivar, they preferred the odour of that particular cultivar. The mechanism appeared to be associative learning rather than sensitisation. Although inexperienced ladybirds preferred the odour of an aphid-infested barley cultivar over uninfested plants of the same cultivar, after feeding experience on a different cultivar this preference disappeared. This may indicate the acquisition and replacement of olfactory templates. The odour blends of the different aphid-infested barley cultivars varied qualitatively and quantitatively, providing a potential basis for olfactory discrimination by the ladybird. The results show that predatory coccinellids can learn to associate the odour of aphid-infested plants with the presence of prey, and that this olfactory learning ability is sensitive enough to discriminate variability between different genotypes of the same plant.