Barbara Ekbom

Myrosinase: gene family evolution and herbivore defense in Brassicaceae

Glucosinolates are a category of secondary products present primarily in species of the order Capparales. When tissue is damaged, for example by herbivory, glucosinolates are degraded in a reaction catalyzed by thioglucosidases, denoted myrosinases, also present in these species. Thereby, toxic compounds such as nitriles, isothiocyanates, epithionitriles and thiocyanates are released. The glucosinolate-myrosinase system is generally believed to be part of the plants defense against insects, and possibly also against pathogens. In this review, the evolution of the system and its impact on the interaction between plants and insects are discussed. Further, data suggesting additional functions in the defense against pathogens and in sulfur metabolism are reviewed.

Landscape heterogeneity and farming practice influence biological control

Summary We measured the impact of natural enemies on the population development on the bird cherry-oat aphid ( Rhopalosiphum padi ) on conventional and organic farms. By pairing farms with similar landscape features but different farming systems, we were able to separate the effects of farming practice and landscape features on the influence of natural enemies on R. padi abundance. Natural enemies had a greater impact on R. padi establishment on organic farms than on conventional farms. Irrespective of farming system, landscapes with abundant field margins and perennial crops were associated with low R. padi establishment. After establishment, there was no difference in ground-living enemy impact on R. padi population growth rate between farming systems, but impact was greater in landscapes where arable land was contiguous. Wir untersuchten die Wirkung naturlicher Feinde auf die Populationsentwicklung der Haferblattlaus (Rhopalosiphum padi) in konventionell und organisch bewirtschafteten landwirtschaftlichen Betrieben. Indem wir Betriebe paarten, die ahnliche Landschaftseigenschaften aber unterschiedliche Betriebsformen aufwiesen, konnten wir die Wirkung der Bewirtschaftung und der Landschaftseigenschaften auf den Einflus der naturlichen Feinde auf die Abundanz von R. padi trennen. Die naturlichen Feinde hatten in organisch bewirtschafteten Betrieben eine grosere Wirkung auf die Ansiedlung von R. padi als in konventionell bewirtschafteten Betrieben. Unabhangig von der Bewirtschaftungsform waren Landschaften mit zahlreichen Feldrainen und perennierenden Feldkulturen mit einer geringen Ansiedlung von R. padi verbunden. Die Wirkung bodenlebender Feinde auf die Wachstumsrate der Populationen von R. padi nach der Ansiedlung unterschied sich nicht zwischen den Bewirtschaftungsformen. Die Wirkung war jedoch in Landschaften groser, die zusammenhangende Ackerflachen aufwiesen.

Oviposition mistakes in herbivorous insects: confusion or a step towards a new host plant?

Insects sometimes lay eggs on host plants unsuitable for their offspring. One explanation for this behavior has been that the association of insect and plant is new, and therefore, the insect has not had time to develop a capacity for discrimination. We explore an alternative explanation using a simulation model of gall-forming cecidomyiid oviposition behavior. It is demonstrated that insects which can discriminate between plant types have little advantage over «confused» (non-discriminating) counterparts when the proportion of poor host plants is large and the time available for oviposition is short. When non-discriminating females also oviposit on their eclosion plant the advantage of discrimination is negligible. We suggest that for insects where a host shift is costly, e.g. gall-forming cecidomyiids with an intimate relationship with the plant, there are only small penalties to retaining a non-discriminating oviposition strategy. Insect traits such as short adult life span and poor capacity for directed flight further act to maintain the insect in a state of perpetual «confusion»

Yield increase attributable to aphid predation by ground-living polyphagous natural enemies in spring barley in Sweden

It has been claimed that natural enemies of pests perform important ecosystem services in agricultural landscapes. These services have rarely been evaluated in yield or monetary terms at farm level. We have examined the degree to which ground-living natural enemies of the bird cherry-oat aphid (Rhopalosiphum padi (L.)) reduced aphid abundance and influenced barley yields on ten commercial farms in central Sweden. On average, ground-living natural enemies of pests increased barley yields by 303 kg/ha. This corresponded to a potential 52% reduction in yield loss from R. padi compared with a scenario where no natural enemies were present. Measured as the percentage increase of actual yield, ground-living natural enemies of aphids increased the yield by 23%. The increase in yields in absolute terms was larger on conventional farms than organic farms, but the percentage increase of yields was larger on organic farms. The study only considered 1 year, with high aphid abundance. However, we also discuss the effects of predators as well as insecticides in the long-term. Our study shows that yield increases attributable to predators can be compared with yield increases from insecticide use for the evaluation of different management strategies.

Movements of carabid beetles (Coleoptera: Carabidae) inhabiting cereal fields: a field tracing study

SummaryThe individual movements of four carabid species were investigated by tracing beetles in the field using a portable radar system (harmonic radar). The field-inhabiting a Pterostichus melanarius Illiger, P. niger Schaller and Harpalus rufipes De Geer were traced in cereal fields. The movements of P. melanarius Mnd P. niger were also tracked in a wood and then compared with the movements of the forest-species Carabus nemoralis Müller. P. melanarius and P. niger displayed directed movements in cereal fields, although P. niger moved faster than P. melanarius. The movement pattern of H. rufipes in the field, and C. nemoralis and P. malanarius in the wood can be described as a c correlated random walk. In cereal fields, P. melanarius burrowed actively into the soil, and P. niger searched out crevices. In contrast, none of these two carabids burrowed in the forest soil, whereas C. nemoralis did. No statistically significant difference was observed between the movements of hungry and satiated beetles, although starved beetles tended to disperse shorter distances per move and turned back more often (especially P. niger). The significance of the various movement patterns exhibited by the species studied for their reproduction and survival on arable land is discussed.

Variation in trichome density and resistance against a specialist insect herbivore in natural populations of Arabidopsis thaliana

Abstract.  1. The annual herb Arabidopsis thaliana is a prime model organism of plant molecular genetics, and is currently used to explore the molecular basis of resistance to herbivores. However, both the magnitude and the causes of variation in resistance among natural populations of A. thaliana are poorly known. The hypotheses (a) that resistance to a specialist herbivore, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is positively related to the density of leaf trichomes, and (b) that oviposition preference by female moths is positively correlated to larval performance on different populations and maternal lines of the host plant were examined.

Diet, fecundity and egg size in some polyphagous predatory carabid beetles

Egg size was measured at different rates of egg laying in three polyphagous carabid species, known to be useful predators of cereal aphids; the small Bembidion lampros Herbst and the medium‐sized Pterostichus cupreus L. and P. melanarius Illiger. Variations in fecundity, as well as the ability of the medium‐sized species to also build up fat reserves, were obtained when beetles were subjected to different dietary regimes consisting of aphids, or foods with a lower or higher protein content. Egg size was found to be dependent on the rate of egg laying within a species. A diet of cereal aphids appeared to be adequate for egg production in these polyphagous carabids, but female P. cupreus were unable to build up fat reserves when they ingested aphids contaminated with the aphicide pirimicarb. Beetles were able to devote resources to more and larger eggs (B. lampros), or to larger eggs and/or fat reserves (P. melanarius/P. cupreus) when given access to a carbohydrate‐rich food with low protein content. The highest rate of egg laying was obtained when female P. cupreus and P. melanarius were given a more varied diet at frequent intervals; including regular shifts between unsprayed aphids, carbohydrate‐rich food and protein‐rich maggots. Within the varied diet treatment a negative relationship was obtained between egg size and egg number among similar‐sized individuals of P. cupreus and P. melanarius; females producing the largest number also laid the smallest eggs. Egg size affected larval survival, since first instars hatching from large eggs were found to survive longer than those hatching from small eggs. The influence of differences in food intake on reproduction, maintenance metabolism, and survival of field‐inhabiting carabids is discussed.

Flow and stability of natural pest control services depend on complexity and crop rotation at the landscape scale

Increasing landscape complexity can enhance biodiversity and ecosystem services in agroecosystems. However, policies based on conversion of arable land into semi-natural habitats to increase landscape complexity and ecosystem services can be difficult to implement. Although it appears to be a promising management option, nothing is known about the effect of increasing landscape diversity through crop rotations on the delivery of ecosystem services. In this study, we examined how landscape complexity and crop rotation intensity in the landscape at different spatial scales affect the flow and the stability of natural pest control services in barley fields using manipulative cage experiments. Exclusion experiments revealed that natural enemies can have a strong impact on aphid population growth and that the delivery of pest control services is strongly dependent on the landscape context. We found that the overall level of pest control increased with landscape complexity and that this effect was independent of crop rotation intensity. In addition, the within-field stability in pest control services increased with crop rotation intensity in the landscape, although stability in parasitism rates decreased. Multiple spatial scales analyses showed that the mean level of natural pest control was best predicted by landscape complexity at the 0 center dot 5-km and the 1-km spatial scales. The stability in overall pest control decreased with proportion of ley at the 2 center dot 5-km and the 3-km spatial scales. Synthesis and applications. Our study disentangled, for the first time, the relative effects of landscape complexity and crop rotation intensity on the delivery of an ecosystem service. We show that combined management of semi-natural habitat and crop rotation can stabilize and enhance natural pest control in agricultural landscapes. Our findings have important implications in terms of management options to maintain and enhance ecosystem services in agroecosystems. They suggest that conservation of heterogeneous landscapes, characterized by a higher proportion of semi-natural habitats such as pastures and relatively small fields, is essential for maintaining and enhancing effective biological control in agroecosystems.

LANDSCAPE COMPLEXITY AND FARMING PRACTICE INFLUENCE THE CONDITION OF POLYPHAGOUS CARABID BEETLES

Previous work has suggested that the condition of polyphagous predatory carabid beetles can be influenced by landscape structure and farming practice. However, earlier studies aimed at testing the effect of farming practice on insect condition and reproduction have not been designed to test this accurately because farms representing different cultivation systems have not had similar landscape complexity. In this study we measured the condition of the polyphagous predatory carabid beetles Harpalus rufipes, Pterostichus cupreus, P. melanarius, and P. niger from five pairs of conventional and organic farms in central east Sweden. The farms within each pair were near to each other and had similar size, land use, and landscape structure. The condition of the beetles was measured as the residuals from the regression between ln(elytra length) and ln(body mass). We show that the residuals for P. melanarius were positively correlated with their fat reserves. For P. melanarius, residuals were higher on farms where the fields had high perimeter-to-area ratio. The pooled residuals of H. rufipes, P. cupreus, and P. niger were positively correlated with the perimeter-to-area ratio and crop diversity. Pterostichus me- lanarius from organic farms had better condition than conspecifics at conventional farms, and for H. rufipes, P. cupreus, and P. niger combined there was a tendency toward better condition on organic farms than at conventional farms. Harpalus rufipes, P. cupreus, P. melanarius, and P. niger, which are all polyphagous predatory carabid beetles, are natural enemies of pest aphids in the agroecosystem. We conclude that, by manipulating landscape structure and composition, improvement in the condition of polyphagous predatory carabid beetles could be achieved, thereby potentially increasing populations of these beneficial insects.

Interchanges of Insects Between Agricultural and Surrounding Landscapes

Perhaps the single most important event in the history of humankind has been the domestication of the land. The rise and fall of civilizations has hinged on control and success of agricultural production. Feeding the populace has meant adding new land to the area devoted to crop production and technological advances have steered modes of production. Large units of production, fields and farms, worked by large machinery tailored to use of agrochemicals have dominated the development of agriculture in the last 50 years. No human activity has so changed the look of the landscape as has agriculture.