Graduate Thesis Or Dissertation
 

Interspecific predation and cannibalism of immatures by adult female Metaseiulus occidentalis, Typhlodromus pyri (Acari: Phytoseiidae) and Zetzellia mali Schueten (Acari: Stigmaeidae)

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  • Interspecific predation and cannibalism and associated rates of oviposition were assessed for adult female Metaseiulus occidentalis (Nesbitt) and Typhlodromus pyri Scheuten when provided non-limiting amounts of either eggs, larvae, protonymphs, or deutonymphs. Tests with all four stages of phytoseiids and larvae of Tetranychus urticae Koch were carried out at 25°C. At 15 and 12.5°C only larvae of phytoseiids and larvae of T. urticae were prey items. Predation by T. pyri was higher than M. occidentalis at 12.5 and 15°C, but more similar at 25°C. M. occidentalis did not feed appreciably on phytoseiid larvae at 15° and 12.5°C. Neither phytoseiid oviposited at 12.5°C when fed phytoseiid larvae, but T. pyri did at 15°C. We concluded that T. pyri was a more active predator at low temperatures and early-season predation on M. occidentalis immatures by T. pyri could contribute to displacement of M. occidentalis from apple orchards in western Oregon. The effects of prey species and prey density on the rates of inter- and intraspecific predation and oviposition of the two phytoseiid mite predators Metaseiulus occidentalis (Nesbitt) and Typhlodromus pyri Schueten were investigated through a series of laboratory experiments. Adult female predators were given mixed populations of phytoseiid larvae and larvae of a more preferred prey, the spider mite, Tetranychus urticae Koch, at different rates and proportions. T. pyri, more of a generalist predator, consistently showed higher rates of predation and cannibalism on phytoseiid immatures at most prey populations and proportions. Manly Preference Indices indicated that T. pyri switched to feed on phytoseiid immatures at higher population levels and proportions of T. urticae than did M. occidentalis. This ability to readily utilize phytoseiid immatures as prey indicated that maintaining both predators in a biological control program at low prey densities may require the use of active selective techniques that favour M. occidentalis. The differential impact of Zetzellia mali on the phytoseiids Metaseiulus occidentalis and Typhlodromus pyri was studied in laboratory experiments and by analysis of population data from experimental orchard plots containing either of the phytoseiid species, similar numbers of prey mites, and high or low populations of Z. mali. Five hypotheses were evaluated to explain why Z. mali had more impact on M. occidentalis in the field than on T. pyri.. Given equal opportunity, Z. mali adult females did not consume greater numbers of M. occidentalis eggs than T. pyri eggs nor did adult females of either phytoseiid species inflict greater mortality on Z. mali eggs or larvae through attack or consumption. There was no difference in the within tree association of Z. mali adult females with the eggs of either phytoseiid species nor were there differences in the way prey mites (all stages) were spatially partitioned between adult female Z. mali as compared with adults and deutonymphs of either of the two phytoseiids. The foraging area of adult female Z. mali and the oviposition locations of the two phytoseiids from both field and laboratory assessments were compared using spatial statistical procedures. M. occidentalis laid significantly more eggs in the primary foraging area of adult female Z. mali than did T. pyri. This spatial difference was the only factor tested which might explain the observed greater impact of Z. mali on M. occidentalis. Impact of these interspecific interactions and competition on the persistence of predatory mite guilds for the biological control of plant-feeding mites are discussed.
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