Elicitors of induced disease resistance in postharvest horticultural crops: a brief review

Increasing loss of conventional fungicides due to pathogen resistance and general unacceptability in terms of public and environmental risk have favoured the introduction of integrated pest management (IPM) programmes. Induction of natural disease resistance (NDR) in harvested horticultural crops using physical, biological and/or chemical elicitors has received increasing attention over recent years, it being considered a preferred strategy for disease management. This article reviews the enhancement of constitutive and inducible antifungal compounds and suppression of postharvest diseases through using elicitors. The effect of timing of pre- and/or postharvest elicitor treatment and environment on the degree of elicitation and the potential for inducing local acquired resistance, systemic acquired resistance and/or induced systemic resistance to reduce postharvest disease is discussed. The review highlights that more applied and basic research is required to understand the role that induced NDR can play in achieving practical suppression of postharvest diseases as part of an IPM approach. (C) 2003 Elsevier B.V. All rights reserved.

Is the potential of Coccidoxenoides perminutus, a mealybug parasitoid, limited by climatic or nutritional factors?

The encyrtid Coccidoxenoides perminutus is a widely distributed parasitoid of citrus mealybug (Planococcus citri). Worldwide, it has been implicated in successful biocontrol in only a few widely separated localities. C perminutus contributes little to control P. citri in field situations in south-east Queensland, Australia, but invades insectary cultures and reduces mealybug populations considerably under these controlled conditions. This discrepancy between poor field performance and good performance under controlled conditions was investigated to establish whether climatic factors inhibit the field performance of this species in the biological control of P. citri. Subsequent laboratory examination of the influence of varied humidities and temperatures on the activity levels and survival of C perminutus revealed a low tolerance for high saturation deficits (i.e., low % RH at high T degreesC) with reduced reproductive output. The influence of different food sources on adult survival and reproduction was also quantified, to establish if the adverse effects of climate could be overcome by supplementing adult diet. Neither honeydew from their mealybug hosts nor nectar from Alphitonia flowers significantly enhanced parasitoid survival. A subsequent test of five nectar species revealed a significant difference in their influence on C. perminutus survival and reproduction, with only Alpinia zerumbet proving to be as suitable as honey. The floral species that proved suitable in the laboratory need to be checked for their attractiveness to C perminutus in the field and for their ability to enhance the survival and reproductive output of parasitoids. This information suggests that the prevailing dry conditions in south-east Queensland citrus-growing areas apparently impede successful biological control of P. citri by C perminutus, but possibilities are available for habitat manipulation (by providing suitable nectar sources for adult parasitoids) to conserve and enhance C perminutus activity in the field. (C) 2004 Elsevier Inc. All rights reserved.

Egg parasitoids of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), from south-east Queensland

Field surveys of egg parasitoids of the diamondback moth, Plutella xylostella, were conducted at Redlands and Gatton, south-east Queensland. Eggs of P. xylostella were present all year round in both localities, and parasitized eggs were consistently found between late spring and early winter. Percent parasitism in the range 30-75% was recorded on many occasions, although rates less than 10% were more common. The major parasitoids included Trichogrammatoidea bactrae Nagaraja and Trichogramma pretiosum Riley. Laboratory evaluation showed that the T. pretiosum from Gatton has a high capacity to parasitize P. xylostella eggs under suitable conditions. This study represents the first record of egg parasitoids of P. xylostella from Australia.

Infection of the immature stages of Diadegma semiclausum, an endolarval parasitoid of the diamondback moth, by Beauveria bassiana

Interactions between the immature stages of Diadegma semiclausum, an endolarval parasitoid of Plutella xylostella, and the fungal entomopathogen Beauveria bassiana were investigated in the laboratory. Detrimental effects of B. bassiana on D. semiclausum cocoon production and adult parasitoid emergence increased with increasing pathogen concentration and some parasitoid larvae became infected by B. bassiana within hosts. The negative impact of B. bassiana on D. semiclausum cocoon production decreased as temporal separation between parasitism and pathogen exposure increased. Adult parasitoid emergence was significantly compromised by the highest rates of B. bassiana tested even when exposure of host larvae to the pathogen was delayed until one day before predicted parasitoid cocoon formation. Parasitoid pupae were infected by the pathogen in all B. bassiana treatments which did not preclude their development. (C) 2004 Elsevier Inc. All rights reserved.

Wolbachia-induced cytoplasmic incompatibility as a means for insect pest population control

Biological control is the purposeful introduction of parasites, predators, and pathogens to reduce or suppress pest populations. Wolbachia are inherited bacteria of arthropods that have recently attracted attention for their potential as new biocontrol agents. Wolbachia manipulate host reproduction by using several strategies, one of which is cytoplasmic incompatibility (CI) [Stouthamer, R., Breeuwer, J. A. J. & Hurst, G. D. D. (1999) Annu. Rev. Microbiol. 53,71-102]. We established Wolbachia-infected lines of the medfly Ceratitis capitata using the infected cherry fruit fly Rhagoletis cerasi as donor. Wolbachia induced complete CI in the novel host. Laboratory cage populations were completely suppressed by single releases of infected males, suggesting that Wolbachia-induced CI could be used as a novel environmentally friendly tool for the control of medfly populations. The results also encourage the introduction of Wolbachia into pest and vector species of economic and hygenic relevance to suppress or modify natural populations.

Evaluation of the impact of natural enemies on Plutella xylostella L. (Lepidoptera: Yponomeutidae) populations on commercial Brassica farms

1 Accurate assessment of the impact of natural enemies on pest populations is fundamental to the design of robust integrated pest management programmes. In most situations, diseases, predators and parasitoids act contemporaneously on insect pest populations and the impact of individual natural enemies, or specific groups of natural enemies, is difficult to interpret. These problems are exacerbated in agro-ecosystems that are frequently disrupted by the application of insecticides. 2 A combination of life-table and natural enemy exclusion techniques was utilized to develop a method for the assessment of the impact of endemic natural enemies on Plutella xylostella populations on commercial Brassica farms. 3 At two of the experimental sites, natural enemies had no impact on P. xylostella survival, at two other sites, natural enemy impact was low but, at a fifth site, natural enemies drastically reduced the P. xylostella population. 4 The calculation of marginal death rates and associated k-values allowed the comparison of mortality factors between experimental sites, and indicated that larval disappearance was consistently the most important mortality factor, followed by egg disappearance, larval parasitism and pupal parasitism. The appropriateness of the methods and assumptions made to calculate the marginal death rates are discussed. 5 The technique represents a robust and easily repeatable method for the analysis of the activity of natural enemies of P. xylostella, which could be adapted for the study of other phytophagous pests.

Resistance of three different populations of mulberry silkworm (Bombyx mori L.) to Bacillus thuringiensis

The relative resistance levels of three different populations of mulberry silkworm, Bombyx mori L. to Bacillus thuringiensis (Bt) has been studied. All three populations (two Australian and one Indonesian) were observed for similar characteristics including 3rd instar larval mortality at 24, 48, 72 and 96 h after treatment (HAT), LD50 ratio and probit mortality. Among the Australian two populations, the QuBill (yellow coloured, oval shaped cocoon) population showed higher larval mortality to Bt toxicity compared to the QuBite (white coloured, oval shaped cocoon) population. When all the populations were compared, the Insab (Indonesian population with white Coloured, peanut shaped cocoon) showed lower larval mortality and highest LD50 ratio up to 48 HAT. The Insab population also showed a 24 It longer incubation/latent period prior to the start Of Mortality.

Nucleocapsid gene variability reveals two subgroups of Lettuce necrotic yellows virus

The complete nucleocapsid (N) genes of eight Australian isolates of Lettuce necrotic yellows virus (LNYV) were amplified by reverse transcription PCR, cloned and sequenced. Phylogenetic analyses of these sequences revealed two distinct subgroups of LNYV isolates. Nucleotide sequences within each subgroup were more than 96% identical but heterogeneity between groups was about 20% at the nucleotide sequence level. However, less than 4% heterogeneity was noted at the amino acid level, indicating mostly third nucleotide position changes and a strong conservation for N protein function. There was no obvious geographical or temporal separation of the subgroups in Australia.