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Biomedical subjects

R Harmsen

Publications and source records attributed to R Harmsen.

15 recordsLinked to original sources
PubMed2000

Some effects of pre-release host-plant on the biological control of Panonychus ulmi by the predatory mite Amblyseius fallacis.

Amblyseius fallacis Garman has been selected for pyrethroid resistance and mass reared for experimental release as a biological control agent for tetranychid mites on a number of crops in Canada. Several releases of this predator onto apple and peach trees have failed to result in the establishment of A. fallacis, or in the biological control of Panonychus ulmi Koch. Here, we test the hypothesis that the change of host-plant at the time of release is a critical factor in the establishment of A. fallacis for biological control of P. ulmi. Functional and numerical response studies were undertaken on two populations of A. fallacis: a wild strain collected from the canopy foliage of an apple orchard near Vineland, Ontario; and a second strain reared on bean plants in a commercial insectary with Tetranychus urticae as prey. Each population consumed significantly more P. ulmi and produced significantly more eggs when on leaf disks from the plant species they were reared on, than on leaf disks from the novel host plant. A further experiment was conducted to determine if establishment and biological control of mass-reared A. fallacis could be affected by rearing a population for a short term on apple leaves prior to release on apple trees. Three release treatments were made into potted apple trees in a glasshouse, using predators commercially mass-reared on bean and T. urticae: A. fallacis released directly; A. fallacis reared in the laboratory for four weeks on bean and T. urticae; A. fallacis reared on apple leaves and T. urticae for four weeks. They were compared with a control treatment lacking predator release. Contrary to results of the functional and numerical response studies, no difference was observed between release treatments. All release treatments adding A. fallacis resulted in a similar, if limited, degree of biological control of P. ulmi. These results indicated that there may be short-term effects of host plant on the establishment of A. fallacis and biological control of P. ulmi, which in our study were observed as an initial reduction of the predatory response. However, in a test, the predators appeared to overcome these short-term effects and successfully established on the new host-plant to control P. ulmi.

Animals↗
PubMed2000

The effect of non-pathogenic phylloplane fungi on life-history traits of urticae (Acari: Tetranychidae).

The direct effect of three common non-pathogenic phylloplane fungi on the life-history traits of Tetranychus urticae Koch was investigated on intact leaves of the garden bean Phaseolus vulgaris L. (var. Tendergreen Improved), under conditions of low and high water-deficit stress. The survival rate of T. urticae was always reduced by an increase in water-deficit stress, whereas the effect of the fungi depended in part on the watering regime. On two of the three fungi tested, Alternaria alternata (Fr.) Keissler and Epicoccum nigrum Link, the mites showed a significant increase in net reproduction compared to those reared under control (i.e. no fungi added) conditions, independent of water-deficit stress, resulting in a higher intrinsic rate of increase. The third fungus tested, Cladosporium cladosporioides (Fres.) de Vries, had a similar response with respect to net reproduction when reared under low water-deficit stress but not when reared under a high water-deficit stress. There was, however, no evidence of an interactive effect with water-deficit stress and presence or absence of fungus affecting the net reproduction of the mites, for any of the three fungal species tested. An interactive effect between fungal species and water-deficit stress was observed for the intrinsic rate of increase of the mites. It is suspected that the fungi are acting as a source of an otherwise limiting resource and the differences observed between the fungal treatments is due to differences in consumption, based on the different sizes of their conidia.

Alternaria↗
PubMed1999

Assessment of Amblyseius fallacis (Acari: Phytoseiidae) for biological control of tetranychid mites in an Ontario peach orchard.

We introduced a mass-reared pyrethroid-resistant strain of the predatory phytoseiid mite Amblyseius fallacis (Garman) into an Ontario peach orchard in an attempt to control populations of the phytophagous mites Panonychus ulmi Koch and Tetranychus urticae Koch (Acari: Tetranychidae). Releases of 1,000 and 2,000 mites per tree were made, at three different times. The release of 2,000 mites per tree in June and in July resulted in significantly higher phytoseiid densities than was observed on control trees. However, densities of P. ulmi or T. urticae were not significantly affected by any release rate or by timing. The release of 1,000 A. fallacis per tree, or of any density in August, did not significantly increase phytoseiid abundance. In the following year, population dynamics of both phytoseiid and phytophagous mites were not significantly affected by the previous year's release. Amblyseius fallacis can be a useful predator in some fruit orchards. However, further research is necessary into the timing and rate of release, modified spray programmes, and with different crops, in order to clarify the role of this species for biological control in Ontario peach orchards.

Aerosols↗
PubMed1975

Photobehavioral differentiation in natural populations of Drosophila pseudoobscura and Drosophila persimilis.

The photoresponses of natural populations of D. pseudoobscura and D. persimilis, occurring sympatrically, are measured in two environmental conditions ("at rest" and "disturbed"). Comparisons of the responses, intraspecifically and interspecifically, lead to the following conclusions. These must be considered within the confines of the operational nature of the measurement of laboratory photoresponses. (1) Within each species population, significant nonenvironmental differentiation has been allowed or produced by selection in the "at rest" photoresponse. No significant nonenvironmental differentiation is found in the photoresponse measured in a disturbed condition. (2) Within each species population, a higher mean "disturbed" photoresponse has been favored. The intensities or patterns of selection acting on these two photoresponses have differed such that more intrapopulation differentiation has been allowed or produced in the "at rest" photoresponse. (3) A higher mean photoresponse has been favored in D. persimilis for both conditions. The intensities or patterns of selection acting between these two species populations on the "at rest" photoresponse have differed such that more intrapopulation differentiation has been allowed or produced in D. persimilis. (4) Comparisons of this study with one on intraspecific and interspecific differentiation in wing length lead to the conclusion that the selective differences inferred above have acted at a level more specifically attuned to photobehavior.

Animals↗