Velemir Ninkovic

Barley exposed to aerial allelopathy from thistles (Cirsium spp.) becomes less acceptable to aphids

Abstract.  1. Recent studies have shown that plant–plant interaction via chemicals (allelopathy) can affect insects. Here the effects on aphid acceptance of barley after exposure to volatiles and root exudates from two common weeds, the thistles Cirsium arvense and Cirsium vulgare, were investigated.

Mixing barley cultivars affects aphid host plant acceptance in field experiments

Four barley varieties with no significant difference in aphid acceptance were sown in pure stands and in pairwise combinations with varieties side by side in separate rows. Settling tests were done in situ in the field plots with apterae of Rhopalosiphum padi (L.) (Homoptera:Aphididae) and showed that aphid acceptance was changed in some combinations of cultivars. In a laboratory test, in which plants of one cultivar were exposed to air from the other cultivars, aphid acceptance was significantly reduced in three of the four cultivars when treated with air from certain other cultivars. Two of these three cultivars showed the same reduction under field conditions. This supports the hypothesis that plant/plant communication may release responses in neighbouring plants that change aphid host plant acceptance. The results also show that this mechanism is not restricted to optimal growing conditions in the laboratory, although it may be modified under field conditions depending on plant genotype.

Effects of two types of semiochemical on population development of the bird cherry oat aphid Rhopalosiphum padi in a barley crop

1 Field experiments were performed in barley using volatile semiochemicals affecting population density (density‐related substances – DRS) and spring migration (methyl salicylate) in bird cherry oat aphid Rhopalosiphum padi (L.). Natural infestations of aphids were used, and semiochemical dosages were chosen to be biologically relevant based on previous studies. A simple formulation method for active substances using wax pellets was developed.

Change in acceptability of barley plants to aphids after exposure to allelochemicals from couch-grass (Elytrigia repens)

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.

Chemical interaction between undamaged plants--effects on herbivores and natural enemies.

Most research on plant-plant chemical interactions has focussed on events following herbivore or pathogen attack. However, undamaged plants also interact chemically as a natural facet of their behaviour, and this may have consequences for insects that use the plants as hosts. In this review, the links between allelopathy and insect behaviour are outlined. Findings on how chemical interactions between different plant species and genotypes affect aphid herbivores and their natural enemies are reviewed, and the role of plant diversity and chemical interaction for trophic interactions in crops is discussed.

Airborne interactions between undamaged plants of different cultivars affect insect herbivores and natural enemies

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.

Aphid acceptance of Hordeum genotypes is affected by plant volatile exposure and is correlated with aphid growth

Nineteen genotypes of Hordeum vulgare (L.) were screened and characterized as partially resistant or susceptible regarding growth of the bird cherry—oat aphid (Rhopalosiphum padi L.). In a separate test, these same genotypes were treated with volatiles from undamaged plants of barley cultivar Alva. As a result of this treatment, aphid host acceptance (AHA) was significantly affected in seven genotypes, and the magnitude of the effect was positively correlated with aphid growth (AG) in the independent resistance screening test. Changes in AHA induced by volatiles from the same genotype as the volatile receiver were also positively correlated with AG. All the 19 genotypes were also tested as inducers with cultivar Kara as the receiver of volatiles. Five genotypes induced significant reductions in AHA of Kara. The results show that aphids are able to detect changes in responding plants induced by volatiles from another plant. Plant volatile interactions may thus be a component of induced resistance to aphids. These interactions could influence the results of experiments used to select for insect-resistant plants in plant breeding programmes, where normally plant genotypes are mixed at testing.

Effect of within-species plant genotype mixing on habitat preference of a polyphagous insect predator

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.

Weed–barley interactions affect plant acceptance by aphids in laboratory and field experiments

Increased botanical diversity can lead to suppression of insect pests. One route by which botanical diversity is increased in crops is through the occurrence of weeds, which increasingly interact with crop plants as organic production expands. However, the mechanisms by which this might affect insect herbivores are poorly understood. This study examined whether volatile chemical interactions between weeds and barley, Hordeum vulgare L. (Poaceae), can affect plant acceptance by the bird cherry oat aphid, Rhopalosiphum padi L. (Hemiptera: Aphididae). In laboratory experiments, exposure of barley to volatiles from Chenopodium album L. (Amaranthaceae) and Solanum nigrum L. (Solanaceae) resulted in significantly reduced aphid acceptance compared with unexposed plants. In a series of field experiments in which the occurrence of weeds was manipulated in plots of barley, significantly lower aphid acceptance was recorded on barley plants grown in plots with C. album compared with barley plants in weedless plots. The results indicate that interaction between weeds and barley can affect aphid–plant interactions in the field as well as in the laboratory and provide further evidence that the effects of chemical interactions between visibly undamaged plants can extend to higher trophic levels.

Volatile exchange between undamaged plants - a new mechanism affecting insect orientation in intercropping

Changes in plant volatile emission can be induced by exposure to volatiles from neighbouring insect-attacked plants. However, plants are also exposed to volatiles from unattacked neighbours, and the consequences of this have not been explored. We investigated whether volatile exchange between undamaged plants affects volatile emission and plant-insect interaction. Consistently greater quantities of two terpenoids were found in the headspace of potato previously exposed to volatiles from undamaged onion plants identified by mass spectrometry. Using live plants and synthetic blends mimicking exposed and unexposed potato, we tested the olfactory response of winged aphids, Myzus persicae. The altered potato volatile profile deterred aphids in laboratory experiments. Further, we show that growing potato together with onion in the field reduces the abundance of winged, host-seeking aphids. Our study broadens the ecological significance of the phenomenon; volatiles carry not only information on whether or not neighbouring plants are under attack, but also information on the emitter plants themselves. In this way responding plants could obtain information on whether the neighbouring plant is a competitive threat and can accordingly adjust their growth towards it. We interpret this as a response in the process of adaptation towards neighbouring plants. Furthermore, these physiological changes in the responding plants have significant ecological impact, as behaviour of aphids was affected. Since herbivore host plants are potentially under constant exposure to these volatiles, our study has major implications for the understanding of how mechanisms within plant communities affect insects. This knowledge could be used to improve plant protection and increase scientific understanding of communication between plants and its impact on other organisms.