Search

Abstract: The agromyzid leaf-mining fly Phytomyza vitalbae, which was introduced into New Zealand as a biological control agent of the invasive deciduous European vine Clematis vitalba L. (old man’s beard; Ranunculaceae) in 1996, was recorded attacking native non-target Clematis forsteri and Clematis foetida in New Zealand, with C. foetida being most commonly attacked. Both the incidence and levels of P. vitalbae attack were significantly lower on the non-target species, compared to on the target plant and populations of non-target plants that were growing within 4km of the nearest known patch of C. vitalba were most commonly attacked. No-choice starvation tests indicated that survival of P. vitalbae was low and oviposition did not occur on C. foetida unless flies had previously fed on C. vitalba until they began ovipositing, indicating that non-target attack was a “spillover” effect that is unlikely to have a major detrimental impact on the non-target plants. Parasitoid rearing indicated that P. vitalbae shares parasitoids with a closely related native-leaf-miner Phytomyza clematadi. Therefore, P. vitalbae and P. clematadi potentially compete with each other, mediated through shared parasitoids. However, we found no evidence for increasing levels of parasitism of P. clematadi mines, or for a reduction in P. clematadi abundance on C. foetida with increasing proximity to C. vitalba infestations. Both the direct and indirect non-target impacts of P. vitalbae attack, therefore, appear to be minor. Host-range testing performed prior to the release of P. vitalbae in New Zealand predicted the plant species that were most at risk of non-target attack. Our results show that the prevalence of spillover onto non-target species was underestimated and we discuss how host-range testing might be improved in the light of these findings. [Copyright &y& Elsevier]

Losses in grain yield prevented by controlling weeds were measured in 59 fields of (southern hemisphere) spring-sown wheat (Triticum aestivum L.) (cv. Otane) and 45 fields of spring-sown barley (Hordeum vulgare L.) (cv. Corniche) in five consecutive growing seasons from 1988/89 until 1992/93 in the Canterbury region of New Zealand. The losses were measured as the differences in yield between weeded and non-weeded plots located in randomly positioned pairs in the fields, in the first 2 years, the weeding was by push hoe in ‘organically’ grown crops. For the last 3 years, the fields were under prophylactic herbicide regimens with nonweeded plots created by excluding commercial herbicide applications (made mostly in October for wheat and November for barley) with polyethylene sheets placed temporarily over the plots. For each season the distributions of yield losses were modelled using the normal distribution and probabilities of ‘breaking even’ on herbicide use derived by substituting cumulative probability density functions into a simple break-even model for herbicide use. The model assumed that herbicide application in the current crop has no flow-on economic effect for succeeding crops. The mean annual yield losses prevented by herbicide application were positively correlated with September and October rainfall for wheat and barley respectively. As a consequence, the probabilities of breaking even on herbicide use increased with increasing spring rainfall. Using historical rainfall records, probabilities of breaking even were estimated for each of the 48 years from 1947 to 1994. Averaging over these years, the analysis revealed that at current grain prices prophylactic use of the commonly applied herbicides is likely to be uneconomic in 24% (95% confidence limits 6% and 50%) of fields of average-yielding Otane wheat and in 26% (95% confidence limits 1% and 91 %) of fields of average-yielding Corniche barley in Canterbury. It was concluded that there is potential for withholding herbicide treatments without jeopardizing profitability in these crops, particularly in seasons with low spring rainfall. [ABSTRACT FROM AUTHOR]

In the West, weeds have been defined and redefined according to the cultural ideas and outlooks of peoples who have tried to compete with them for open places, over many millennia. In this article, some of the expressions of the changes in human perceptions of a group of plants with which people have had to contend for places, and the deeper cultural significance of the contest itself, are explored. The inter-societal relationships between weeds and humans are also explored in the unique context of New Zealand's discrete landscape and the settler society which transformed it within the comparatively short period of two centuries. Possibilities for ongoing studies of the weeds-human relationship within New Zealand and other regional contexts are focused.