Biocontrol of Pests of Apples and Pears in Northern and Central Europe: 2. Parasitoids

The parasitoids of arthropod pests of apple and pear in northern and central Europe and their use as biological control agents are reviewed. The review demonstrates that apple and pear pests are host to a large and varied parasitoid fauna. All important pests are known to be host of parasitoids, but many parasitoids play only a minor part in regulating populations of their host. However, many parasitoid species are important natural enemies and some effectively regulate pest populations in unsprayed and/or commercial (insecticide sprayed) apple or pear orchards either individually or as part of parasitoid guilds. Exploitation/fostering of existing populations of parasitoids has been demonstrated to be an effective or partially effective approach for natural control of several important pest species. Important examples include natural regulation of the apple sawfly by Lathrolestes ensator and Aptesis nigrocincta, of the summer fruit tortrix moth by Colpoclypeus florus and Teleutaea striata, of leaf midges by Platygaster demades, of woolly aphid by Aphelinus mali and of leaf mining moths by guilds of parasitoid species. Introduction of parasitoids is an alternative approach to the exploitation of parasitoids already present in the orchard. This approach has been little explored and its success rate has been low, mainly confined to the control of non-indigenous pests by introducing parasitoids from their native region. Mass production methods for parasitoids are difficult and costly and are likely to be economic only where long-term populations can be established. Even where low cost mass culture techniques are developed, the degree of control may not be high enough to prevent economic pest damage as demonstrated by negative results with mass release of Trichogramma egg parasites for control of tortricids in orchards. Suitability of the orchard habitat is recognized as crucial to the success of individual parasitoids. Key requirements are adequate populations of the pest(s) and/or alternative hosts, suitable shelter, overwintering sites or food sources and avoidance of harmful effects of pesticides. Many species are highly sensitive to broad-spectrum insecticides, especially in the adult life-stage. Avoiding the harmful affects of insecticides is crucial to successful exploitation. The use of insecticides needs to be avoided, either altogether or at crucial times in the parasitoids' life cycle, or less harmful alternatives need to be used. Numerous parasitoids could potentially be exploited as biological control agents but hitherto have received little attention because little is known about them and/or because they are sensitive to broad-spectrum pesticides and are thus virtually absent from commercial orchards. The aim of future studies should be to develop effective strategies for establishing equilibria between important pests and their parasitoids, with pest damage rarely exceeding the economic threshold.     

Moth outbreaks in relation to oak masting and population levels of small mammals: an alternative explanation to the mammal-predation hypothesis

The relationship between population outbreaks of the gypsy moth (Lymantria dispar) and oak masting in North America has been interpreted as an effect of reduced predation on moth pupae from small mammals after years of acorn failure. However, moth defoliation could be a consequence of high acorn production rather than of acorn failure, as all moth outbreaks in two time series presented by Liebhold et al. [Popul Ecol (2000) 42:257–266] were reported shortly after mast years. A similar pattern has been found for the green oak leaf roller moth (Tortrix viridana) in southern Norway. Because predation from small mammals should be less important for the latter species, I argue that the acorn-moth relationships are most likely caused by mast-induced changes in the chemical composition of oak leaves. Given the high number of eggs laid by each moth female, there is a huge potential for population growth in or shortly after a mast year if larval survival is no longer limited by low food quality.     

Autosomal and sex-linked microsatellite loci in the green oak leaf roller Tortrix viridana L. (Lepidoptera, Tortricidae)

Eight microsatellite markers were developed for the lepidopteran species Tortrix viridana using an enrichment protocol. The loci were highly variable with number of alleles ranging from four to 38. Six of the eight loci were in Hardy-Weinberg equilibrium. The other two were linked to the Z-chromosome. Values of observed heterozygosity ranged for the autosomal loci from 0.510 to 0.957. All loci will be useful to study dispersal and the autosomal loci, as well for phylogeographical studies.     

The potential for interspecific competition between two abundant defoliators on oak: leaf damage and habitat quality

Abstract. 1. The pedunculate oak, Quercus robur L., suffers high annual levels of spring defoliation in Wytham Woods, Oxon. The two major defoliators, Tortrix viridana L. and Operophtera brumata L., sometimes reach high enough densities to completely defoliate trees.
2. Experiments with larvae of both species enclosed on oak trees in Wytham Woods suggest considerable potential for intra- and interspecific competition between T. viridana and O. brumata.
3. T. viridana and O. brumata consume equivalent amounts of leaf tissue per unit caterpillar density. Despite this, O. brumata is the superior competitor, measured by mortality during the larval feeding period.
4. Evidence is presented that T. viridana larvae depend upon the protected microclimate of leaf rolls to maintain water balance, and that leaf damage may disrupt the construction of, and microclimate within, leaf rolls. O. brumata may therefore be competitively superior because it indirectly interferes with the water balance of T. viridana.
5. The importance of interspecific competition in the dynamics of T. viridana and O. brumata populations is discussed. Stochastic mortality factors acting on first instar larvae of both species probably make intense competition a rare event.     

Differential susceptibility to variable plant phenology and its role in competition between two insect herbivores on oak

Abstract. 1. The population dynamics of Tortix viridana L. (Lepidoptera: Tortricidae) and Operophtera brumata L. (Lepidoptera: Geometridae) on the pedunculate oak Quercus robur L. are driven by the degree to which larval hatch coincides with oak budburst in spring.
2. Experiments with first instar larvae demonstrate that T. viridana has a much narrower dietary breadth, is less inclined to disperse from closed oak buds, and can resist starvation for longer periods than the polyphagous O.brumata.
3. As a result of its specialist foraging strategy, T. viridana may be less susceptible to mortality associated with colonization of oak in spring than O.brumata. This may explain why T. viridana populations are, on average, higher.
4. These results are discussed in the light of previous work which described the competitive advantage of O. brumata in field cage experiments. Stochastic mortality factors, which drive population change in both species independently of competition, favour T.viridana larvae, and over-ride the competitive superiority of O.brumata.