P. M. Ridland

The effects of local selection versus dispersal on insecticide resistance patterns: longitudinal evidence from diamondback moth ( Plutella xylostella (Lepidoptera: Plutellidae)) in Australia evolving resistance to pyrethroids

When strong directional selection acts on a trait, the spatial distribution of phenotypes may reflect effects of selection, as well as the spread of favoured genotypes by gene flow. Here we investigate the relative impact of these factors by assessing resistance to synthetic pyrethroids in a 12-year study of diamondback moth, Plutella xylostella, from southern Australia. We estimated resistance levels in populations from brassicaceous weeds, canola, forage crops and vegetables. Differences in resistance among local populations sampled repeatedly were stable over several years. Levels were lowest in samples from weeds and highest in vegetables. Resistance in canola samples increased over time as insecticide use increased. There was no evidence that selection in one area influenced resistance in adjacent areas. Microsatellite variation from 13 populations showed a low level of genetic variation among populations, with an AMOVA indicating that population only accounted for 0.25% of the molecular variation. This compared to an estimate of 13.8% of variation accounted for by the resistance trait. Results suggest that local selection rather than gene flow of resistance alleles dictated variation in resistance across populations. Therefore, regional resistance management strategies may not limit resistance evolution.

Pesticide applications on Java potato fields are ineffective in controlling leafminers, and have antagonistic effects on natural enemies of leafminers

Abstract In Indonesia a range of insecticides is routinely applied to control agromyzid leafminers. Insecticide applications can reduce parasitism by indigenous parasitoid wasps and also decrease numbers of the predatory muscid fly, Coenosia humilis, and these effects reduce control of leafminers. In replicated field trials, repeated applications of Profenofos were ineffective in controlling L. huidobrensis numbers on potatoes. Applications of Profenofos and Carbosulfan decreased rates of parasitism byHemiptarsenus varicornis and Opius chromatomyiae, and reduced numbers of C. humilis. These detrimental effects of the pesticides observed may have contributed to the increased damage and decreased yield in the pesticide-treated fields. An alternative control strategy involving the applications of Abamectin led to a reduction in leafminers without harmful effects on parasitoids and predators. Abamectin applications provide one potential component of an effective Liriomyza control strategy for Indonesian potato farmers.

Ecology of diamondback moth in Australian canola: landscape perspectives and the implications for management

The ecology of diamondback moth (DBM), Plutella xylsotella L. (Lepidoptera: Plutellidae), and records of its frequent, but sporadic, population outbreaks in the canola agroecosystems of southern and western Australia are reviewed. The migratory capacity of DBM, possible maintenance of pest populations on brassicaceous weeds and forage crops, resistance to commonly used pyrethroid insecticides, a lack of effective natural enemies (due to disruption by insecticides and difficulties associated with colonising the vast areas of canola crops) and suitable climatic conditions during critical phases of the crop cycle are all likely to contribute to the observed pest outbreaks. A greater understanding of the ecology of DBM in the canola landscape is fundamental to improving its management in the crop but relevant long-term DBM abundance data are currently lacking. Five critical research issues are identified: (i) improved understanding of the factors which determine regional movement patterns of diamondback in canola-growing areas; (ii) the development and implementation of flexible insecticide resistance management strategies; (iii) better understanding of canola crop colonisation by natural enemies of DBM and their population dynamics under current and alternative insecticide application strategies; (iv) greater appreciation of the interactions between DBM and its crop and weedy host plants; and (v) the development of validated simulation models to aid in the forecasting of possible DBM outbreaks. Each issue represents a significant challenge but all must be addressed if the development of a sustainable integrated strategy for the management of DBM in Australian canola is to become a reality.

Molecular and Morphometric Data Indicate a New Species of the Aphid Genus Rhopalosiphum (Hemiptera: Aphididae)

ABSTRACT Here, we provide evidence for a new aphid species from the genus Rhopalosiphum Koch, based upon an Australian survey of variation in the mitochondrial cytochrome oxidase I gene, and subsequently validated by cytochrome b, nuclear microsatellites, nuclear sequence characterized amplified region locus, and karyotypic analyses. Despite the new species being morphologically very similar to Rhopalosiphum padi (L.), there was significant genetic differentiation, with the new species being closer to the Rhopalosiphum insertum (Walker) group. Karyotypic analyses indicated a putative hybrid origin for the new species, but mitochondrial and nuclear DNA sequence data showed that the available Australian and overseas populations of Rhopalosiphum spp. did not serve as parental lineages. Diagnostic methods were developed that enabled the identification of the new species based on morphometric data and a polymerase chain reaction-restriction fragment length polymorphism based molecular technique.

Patterns of Genetic Variation and Host Adaptation in an Invasive Population of Rhopalosiphum padi (Hemiptera: Aphididae)

ABSTRACT Adaptive changes to local conditions are often thought to be required for successful invasions. However, there are also successful invasive asexual species that may have limited potential for evolutionary change. The bird cherry-oat aphid, Bhopalosiphum padi (L.) (Hemiptera: Aphididae), is thought to have been introduced in Australia 100 yr ago and is now an established pest of cereal crops. Here, we use mitochondrial DNA, cytochrome c oxidase subunit I, and nuclear microsatellites to characterize the genetic diversity in invasive R. padi. Asexual reproduction was detected and clonal diversity was low; with two multilocus genotypes, Rp1 and Rp2, dominating all populations. No geographic or host association was detected between the dominant genotypes on cereals but Rp2 was not found on sweet corn, Zea mays L., crops. The fitness of the predominant multilocus genotypes, Rp1 and Rp2, and an uncommon genotype, Rp26, were investigated on different host types (wheat | Triticum aestivum L. |, barley |Hordeum vulgare L. |, triticale | × Triticosecale Wittm. ex A. Camus. |, and sweet corn | Zea mays L. var. rugosa|) at two temperatures. Rp1 and Rp2 performed similarly on cereals and Rp26 performed poorly on all hosts, patterns that might explain field distributions and abundance of these clones. The results suggest that a high level of clonal diversity is not required for successful invasion and that host associations may develop despite low diversity.

Australia leads the way in the fight against the Diamondback Moth

Richard Vickers from CSIRO Entomology, and Nancy Endersby and Peter Ridland from Institute for Horticultural Development, Australia (all from Australia) describe the latest international efforts in the fight against a major horticultural insect pest.

Evaluation of Fixed Sample-Size Plans for Plutella xylostella (Lepidoptera: Plutellidae) on Broccoli Crops in Australia

Abstract Fixed sample-size plans for monitoring Plutella xylostella (L.) (Lepidoptera: Plutellidae) on broccoli and other Brassica vegetable crops are popular in Australia for their simplicity and ease of application. But the sample sizes used are often small, ≈10–25 plants per crop, and it may be that they fail to provide sufficient information upon which to base pest control decisions. We tested the performance of seven fixed sample-size plans (10, 15, 20, 30, 35, 40, and 45 plants) by resampling a large data set on P. xylostella in commercial broccoli crops. For each sample size, enumerative and presence-absence plans were assessed. The precision of the plans was assessed in terms of the ratio of the standard error to the mean; and at least 45 and 35 samples were necessary for the enumerative and presence-absence plans, respectively, to attain the generally accepted benchmark of ≤0.3. Sample sizes of 10–20 were highly imprecise. We also assessed the consequences of classifications based on action thresholds (ATs) of 0.2 and 0.8 larvae per plant for the enumerative case, and 0.15 and 0.45 proportion of plants of infested for the presence-absence case. Operating characteristic curves and investigations of the frequency of correct decisions suggest improvements in the performance of plans with increased sample size. In both the enumerative and presence-absence cases, the proportion of incorrect decisions was much higher for the lower of the two ATs assessed, and type II errors (i.e., failure to suggest pest control upon the AT is exceeded) generally accounted for the majority of this error. Type II errors are the most significant from a producer’s standpoint. Further consideration is necessary to determine what is an acceptable type II error rate.

Attending To Risk In Sequential Sampling Plans

Researchers typically tackle questions by constructing powerful, highlyreplicated sampling protocols or experimental designs. Such approaches often demand large samples sizes and are usually only conducted on a once-off basis. In contrast, many industries need to continually monitor phenomena such as equipment reliability, water quality, or the abundance of a pest. In such instances, costs and time inherent in sampling preclude the use of highlyintensive methods. Ideally, one wants to collect the absolute minimum number of samples needed to make an appropriate decision. Sequential sampling, wherein the sample size is a function of the results of the sampling process itself, offers a practicable solution. But smaller sample sizes equate to less knowledge about the population, and thus an increased risk of making an incorrect management decision. There are various statistical techniques to account for and measure risk in sequential sampling plans. We illustrate these methods and assess them using examples relating to the management of arthropod pests in commercial crops, but they can be applied to any situation where sequential sampling is used.