We developed and validated a management plan for Tetranychus urticae in strawberries, based on the natural control exerted by Neoseiulus californicus and on acaricide applications made only when necessary. The plan has two components: a sampling protocol and a decision chart. Systematic presence-absence sampling of active T. urticae and N. californicus was used to predict prey and predator densities relying only on the proportion of T. urticaeinfested leaflets, once the occurrence of the predator was detected in at least one of them. The decision chart, which was constructed taking into account the pest and predator densities and the pest's rate of increase, determines the range in the proportion of T. urticae-infested leaflets that will require different actions: to use selective acaricides and re-check at 7 days, to take no action but re-check at 7 days, or to take no action but re-check at 14 days. The management plan was potentially effective and feasible, showing that natural populations of N. californicus can consistently exert strong top-down suppression of T. urticae. Thus, N. californicus is a promising candidate for conservation biological control.
Wild vegetation surrounding crops may provide temporary habitat and potential food sources for phytoseiids in different seasons. Monthly vegetation samples of wild plants adjacent to strawberry plants and wild plants in a vegetation strip close to the crop were taken. The frequency of Neoseiulus californicus, Tetranychus urticae and other mites and insects was recorded. In addition, in a laboratory assay, the survival, developmental time and fecundity of females fed on pollen of strawberry and pollen of wild plants where N. californicus was recorded during their flowering, were estimated. Pollen from Urtica urens, Lamium amplexicaule, Convolvulus arvensis, Sonchus oleraceous, Galega officinalis, and Fragaria x ananassa (strawberry) allowed development of N. californicus to adult, but not reproduction. Survival was 70-80 % when fed on pollen from S. oleraceus, G. officinalis and C. arvensis, 80-90 % when fed on pollen from U. urens and F. x ananassa, and more than 90 % when fed on T. urticae and on pollen from L. amplexicaule. In autumn and winter, U. urens, L. amplexicaule and S. oleraceous could promote the persistence of N. californicus when prey density in strawberry is low, offering T. urticae, thrips and pollen. In summer, pollen of C. arvensis and G. officinalis would contribute to the persistence of N. californicus when the strawberry crop is ending and offers scarce food resources. Although the pollen of these plants would not enable the predator population to increase, the presence of these plants in the vicinity of strawberry could contribute to the persistence of N. californicus population and help to limit T. urticae growth when this pest begins to colonize the crop.
The performance of Tetranychus urticae and its predator Neoseiulus californicus on ten strawberry cultivars was determined in the laboratory. Development time and survival of T. urticae from egg to adult were recorded on Albión, Aromas, Camarosa, Diamante, Festival, Kp, Sabrosa, Selva, Sweet Charlie, and Whitney. Fecundity of newly molted and mated females was recorded during the first 10 days of oviposition. Predation rate and fecundity of N. californicus were tested on Albión, Aromas, Festival, Kp, Sabrosa, and Whitney. Predator females reared on each cultivar were placed individually in experimental units, and the number of eggs per day was counted during 3 days. Cultivars with high hairiness (Albión, Aromas, and Festival) and cultivars with low hairiness (Sabrosa, Whitney and Kp) were identified, to assess the effect on the performance of both species. Development time, survival from egg to adult, and fecundity of T. urticae differed among cultivars. Festival was classified as moderately resistant, Aromas and Kp were moderately susceptible, and the others were intermediate. The number of prey consumed per day per female of N. californicus differed between cultivars and time. Fecundity of N. californicus did not differ among cultivars; however, it did over time. The development time and fecundity of T. urticae did not differ among high and low hairiness cultivars. The glandular hairiness affected neither consumption nor fecundity of N. californicus. According to detrimental and propitious effect on T. urticae and N. californicus performance, respectively, we concluded that Festival and Albión could be used along with this predator in T. urticae management programs.
Among the herbivorous arthropods that feed on strawberry, the most important are the two-spotted spider mite (TSSM), Tetranychus urticae Koch, and several species of aphids. Mites and aphids belong to different guilds that coexist in the field and feed on the undersides of strawberry leaflets. However, the occurrence of large numbers of individuals of both species on the same leaflet is rarely recorded. We hypothesize that negative interactions between TSSM and aphids explain the intraplant distribution of these herbivores. We first examined the spatial coincidence of both herbivores in the field. Under experimental conditions, we then analyzed: (i) the rate of increase of TSSM and the aphid Chaetosiphon fragaefolii (Cockerell), growing individually and together; (ii) the effect of honeydew on TSSM preference; and (iii) the effect of previous strawberry leaflet damage by TSSM on C. fragaefolii preference. The proportion of TSSM that coincided with at least one aphid decreased as the percentage of leaflets with TSSM increased. The spatial coincidence index between aphids and TSSM increased together with the percentage of TSSM-infested leaflets. TSSM showed both a lower rate of increase when they shared the same leaflet with C. fragaefolii and lower fecundity on strawberry discs with honeydew. The rate of increase of C. fragaefolii did not change on co-occupied leaves, but the aphid species moved to the other side of leaflets shared with TSSM. Negative interactions resulting in a tendency for species to avoid each other, such as demonstrated herein, can affect distribution and performance of herbivorous arthropods.
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