Plaagonderdrukking in mengteelten

In monoculturen in de landbouw worden planten vaak beschadigd door plaaginsecten, terwijl in natuurlijke ecosystemen en mengteelten interacties tussen soorten voork'omen dat planten kaalgevreten worden. Bestaande hypothesen geven onvoldoende verklaring van de plaagonderdrukkende eigenschappen van mengteelten. De hier beschreven mechanistische benadering is belangrijk voor het ontwerpen van plaagonderdrukkende mengteelte

De speurneus van een sluipwesp

Sluipwespen zijn de bekendste biologische bestrijders. Met gewone wespen hebben deze diertjes niets te maken. Het zijn kleine, tengere insecten die heel goed zijn in het opsporen van hun slachtoffers. Die slachtoffers - of beter gezegd, gastheren waar zij hun eitjes in kunnen leggen - zijn meestal de goed gecamoufleerde eitjes of rupsen van vlinders. Eigenlijk is het een wonder dat de sluipwespen die slachtoffers zo goed weten te vinden op de oneindige akkers. Bij het zoeken naar eitjes en rupsen maken sluipwespen vooral gebruik van hun reukvermogen. De planten waarvan ze eten produceren een kenmerkende geur die alleen wordt afgegeven als er insecten van de plant eten. De geuren die een plant produceert bij insectenvraat zijn vaak specifiek voor de soort of zelfs de leeftijd van de rups die op de plant zit. Een sluipwesp kan dus van een afstand ruiken welke gastheer er op een plant te vinden i

Hoe kunnen we plaagonderdrukkende mengteelten ontwerpen?

Plaagpopulaties zijn vaak kleiner in mengteelten dan in monocultures. Met mengteelten kan daarom worden geprobeerd plaagproblemen in agro-ecosystemen te verkleinen. Een grotere vegetatiediversiteit kan plaagorganismen onderdrukken doordat waardplanten minder goed zichtbaar of van geringere kwaliteit zijn en doordat natuurlijke vijanden meer voedsel vinden. Helaas is de respons op mengteelten van zowel plaaginsecten als van hun natuurlijke vijanden. Vaak moeilijk te voorspellen en er is nog onvoldoende kennis om in concrete gevallen de plaagpopulatieverschillen tussen mengteelten die plagen onderdrukken vereist daarom meer studie. In dit artikel laten we zien dat recent verworven kennis van het gedrag en de ecologie van plant-plaag-natuurlijke vijand interacties een nieuw beeld oplevert van het verschil tussen mono- en mengculture

Disentangling above-and belowground neighbor effects on the growth, chemistry, and arthropod community on a focal plant

Abstract. Neighboring plants can influence arthropods on a focal plant, and this can result in associational resistance or associational susceptibility. These effects can be mediated by above-and belowground interactions between the neighbor and focal plant, but determining the relative contribution of the above-and belowground effects remains an open challenge. We performed a common garden experiment with a design that enabled us to disentangle the above-and belowground effects of five different plant species on the growth and chemistry of the focal plant ragwort (Jacobaea vulgaris), and the arthropod community associated with this plant. Aboveground effects of different neighboring plant species were more important for the growth and quality of J. vulgaris and for the arthropod abundance on this plant than belowground effects of neighbors. This remained true when only indirect neighbor effects (via affecting the biomass or quality of the focal plant) were considered. The aboveground neighbor effects on arthropod abundance on the focal plant were strongly negative. However, the magnitude of the effect depended on the identity of the neighboring species, and herbivore abundance on the focal plant was higher when surrounded by conspecific than when surrounded by heterospecific plants. We also observed interactions between above-and belowground neighbor effects, indicating that these effects may be nonadditive. We conclude that above-and belowground associational effects are not equally strong, and that neighbor effects on plant-arthropod interactions occur predominantly aboveground

Effects of pollen species composition on the foraging behaviour and offspring performance of the mason bee Osmia bicornis (L.)

The effects of floral species composition on offspring performance of solitary bees are rarely studied under conditions where foraging behaviour of mothers is allowed to play a role. In a semi-field experiment, we restricted foraging choices of the polylectic mason bee Osmia bicornis L. to flower species belonging to plant families presumably used to different extent: Borago officinalis L. (Boraginaceae), Centaurea cyanus L. (Asteraceae) and Brassica napus L. (Brassicaceae). We quantified the foraging behaviour and brood cell production by mother bees, and compared the quality of offspring in pure and mixed flower species stands. Offspring survival in pure stands was expected to reflect the mothers’ foraging preferences in the mixed stand. Pure stands of B. napus supported highest offspring survival, body mass and fraction of females produced. Offspring survival on C. cyanus and B. officinalis was very low. Larval mortality occurred earlier in brood cells provided with B. officinalis pollen than in brood cells provided with C. cyanus pollen suggesting different effects of pollen quality on early larval and later development. The time spent on different foraging activities correlated with lifetime reproductive output. However, in mixed stands, the proportion of time the bees were foraging on the different flower species did not differ significantly. Foraging behaviour may therefore not generally be a good proxy for the quality of floral resources for offspring production. Our results suggest that resources collected from one plant species may influence the usefulness of resources from another plant species. Bees may therefore overcome potentially deleterious effects of the suboptimal resources by mixing low- and high-quality resources. This may help generalist bees, such as O. bicornis, to cope with an unpredictable environment

Effects of pollen species composition on the foraging behaviour and offspring performance of the mason bee Osmia bicornis (L.)

The effects of floral species composition on offspring performance of solitary bees are rarely studied under conditions where foraging behaviour of mothers is allowed to play a role. In a semi-field experiment, we restricted foraging choices of the polylectic mason bee Osmia bicornis L. to flower species belonging to plant families presumably used to different extent: Borago officinalis L. (Boraginaceae), Centaurea cyanus L. (Asteraceae) and Brassica napus L. (Brassicaceae). We quantified the foraging behaviour and brood cell production by mother bees, and compared the quality of offspring in pure and mixed flower species stands. Offspring survival in pure stands was expected to reflect the mothers’ foraging preferences in the mixed stand. Pure stands of B. napus supported highest offspring survival, body mass and fraction of females produced. Offspring survival on C. cyanus and B. officinalis was very low. Larval mortality occurred earlier in brood cells provided with B. officinalis pollen than in brood cells provided with C. cyanus pollen suggesting different effects of pollen quality on early larval and later development. The time spent on different foraging activities correlated with lifetime reproductive output. However, in mixed stands, the proportion of time the bees were foraging on the different flower species did not differ significantly. Foraging behaviour may therefore not generally be a good proxy for the quality of floral resources for offspring production. Our results suggest that resources collected from one plant species may influence the usefulness of resources from another plant species. Bees may therefore overcome potentially deleterious effects of the suboptimal resources by mixing low- and high-quality resources. This may help generalist bees, such as O. bicornis, to cope with an unpredictable environment

Testing the paradox of enrichment along a land use gradient in a multitrophic aboveground and belowground community

In the light of ongoing land use changes, it is important to understand how multitrophic communities perform at different land use intensities. The paradox of enrichment predicts that fertilization leads to destabilization and extinction of predator-prey systems. We tested this prediction for a land use intensity gradient from natural to highly fertilized agricultural ecosystems. We included multiple aboveground and belowground trophic levels and land use-dependent searching efficiencies of insects. To overcome logistic constraints of field experiments, we used a successfully validated simulation model to investigate plant responses to removal of herbivores and their enemies. Consistent with our predictions, instability measured by herbivore-induced plant mortality increased with increasing land use intensity. Simultaneously, the balance between herbivores and natural enemies turned increasingly towards herbivore dominance and natural enemy failure. Under natural conditions, there were more frequently significant effects of belowground herbivores and their natural enemies on plant performance, whereas there were more aboveground effects in agroecosystems. This result was partly due to the “boom-bust” behavior of the shoot herbivore population. Plant responses to herbivore or natural enemy removal were much more abrupt than the imposed smooth land use intensity gradient. This may be due to the presence of multiple trophic levels aboveground and belowground. Our model suggests that destabilization and extinction are more likely to occur in agroecosystems than in natural communities, but the shape of the relationship is nonlinear under the influence of multiple trophic interactions.

Herbivore-Mediated Effects of Glucosinolates on Different Natural Enemies of a Specialist Aphid

The cabbage aphid Brevicoryne brassicae is a specialist herbivore that sequesters glucosinolates from its host plant as a defense against its predators. It is unknown to what extent parasitoids are affected by this sequestration. We investigated herbivore-mediated effects of glucosinolates on the parasitoid wasp Diaeretiella rapae and the predator Episyrphus balteatus. We reared B. brassicae on three ecotypes of Arabidopsis thaliana that differ in glucosinolate content and on one genetically transformed line with modified concentrations of aliphatic glucosinolates. We tested aphid performance and the performance and behavior of both natural enemies. We correlated this with phloem and aphid glucosinolate concentrations and emission of volatiles. Brevicoryne brassicae performance correlated positively with concentrations of both aliphatic and indole glucosinolates in the phloem. Aphids selectively sequestered glucosinolates. Glucosinolate concentration in B. brassicae correlated negatively with performance of the predator, but positively with performance of the parasitoid, possibly because the aphids with the highest glucosinolate concentrations had a higher body weight. Both natural enemies showed a positive performance-preference correlation. The predator preferred the ecotype with the lowest emission of volatile glucosinolate breakdown products in each test combination, whereas the parasitoid wasp preferred the A. thaliana ecotype with the highest emission of these volatiles. The study shows that there are differential herbivore-mediated effects of glucosinolates on a predator and a parasitoid of a specialist aphid that selectively sequesters glucosinolates from its host plant

The influence of metabolically engineered glucosinolates profiles in Arabidopsis thaliana on Plutella xylostella preference and performance

The oviposition preference and larval performance of the diamondback moth (DBM), Plutella xylostella, was studied using Arabidopsis thaliana plants with modified glucosinolate (GS) profiles containing novel GSs as a result of the introduction of individual CYP79 genes. The insect parameters were determined in a series of bioassays. The GS content of the plants as well as the number of trichomes were measured. Multivariate analysis was used to determine the possible relationships among insect and plant variables. The novel GSs in the tested lines did not appear to have any unequivocal effect on the DBM. Instead, the plant characteristics that affected larval performance and larval preference did not influence oviposition preference. Trichomes did not affect oviposition, but influenced larval parameters negatively. Although the tested A. thaliana lines had earlier been shown to influence disease resistance, in this study no clear results were found for P. xylostella