Conditioning Treatments Affect Insect and Mite Populations on Bedding Plants in the Greenhouse

During greenhouse production in Spring 1995, conditioning treatments were applied to columbine (Aquilegia ×hybrida Sims ‘McKana Giants’), New Guinea impatiens (Impatiens hawkeri Bull. ‘Antares’), marigold (Tagetes erecta L. ‘Little Devil Mix’) and ageratum (Ageratum houstonianum Mill. ‘Blue Puffs’) plants. Treatments included: mechanical conditioning (brushing 40 strokes twice daily); moisture stress conditioning (MSC) (wilting for ≈2 hours per day); undisturbed ebb-and-flow irrigation; overhead irrigation; high (500 mg·L N) or low (50 mg·L N) 3×/week N fertilizer regimes; daminozide (5000 mg·L); or paclobutrazol (30, 45, or 180 mg·L). One week after initiation of treatments, individual plants in separate greenhouses were inoculated with two adult green peach aphids (Myzus persicae Sulzer) or five two-spotted spider mites (Tetranychus urticae Koch). A natural infestation of western flower thrips (Frankliniella occidentalis Pergande) in the mite-inoculated greenhouse provided an additional insect treatment. Brushing was the only treatment that consistently reduced thrips and mite populations. Aphid populations were lower on low-N than on high-N plants, but thrips and mite populations were not consistently affected by plant fertilization. Moisture stress conditioning tended to increase aphid populations on New Guinea impatiens and marigold, but had little effect on spider mite or thrips populations. Ebb-and-flow irrigation reduced the mite population on ageratum relative to that on overhead irrigated (control) plants. Plant growth regulators did not consistently affect pest populations. Chemical names used: butane-dioic acid mono(2,2-dimethylhydrazide) (daminozide); β-[(4chlorophenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-1-ethanol (paclobutrazol). Received for publication 31 July 1997. Accepted for publication 11 Sept. 1998. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Associate Professor, Horticulture. Professor, Entomology. Aphids and other sucking insects generally respond more positively to host stress than do other foliar feeding insects, presumably because stress increases the soluble nitrogen available to these insects (Larsson, 1989; Larsson and Bjorkman, 1993). Leafminer (Lyriomyza trifolii Burgess) populations increased linearly with increasing leaf nitrogen content of chrysanthemum (Harbaugh et al., 1983) and tomato (Lycopersicon esculentum Mill.) (Minkenberg and Ottenheim, 1990). Overhead irrigation resulted in higher green peach aphid infestations on chrysanthemum than did capillary watering (Wyatt, 1969). In greenhouse experiments, drought stress increased two-spotted spider mite populations on chrysanthemum (Price et. al., 1982), but reduced aphid (Aphis varians Patch) populations on fireweed (Epilibium angustifolium L.) (Morris, 1992). Moderate to severe drought generally reduces insect pest population development, but intermittent drought stress can increase aphid fecundity (Tingey and Singh, 1980). Brushing vegetable transplants reduced thrips feeding damage and aphid populations (Latimer and Oetting, 1994). In preliminary experiments (unpublished) on the effect of conditioning treatments on insect pest infestations of bedding plants, we found that treatment with paclobutrazol (45 mg·L) or brushing (40 strokes twice daily) reduced the number of two-spotted spider mites per marigold plant after 4 weeks of treatment. In the same test, spider mite populations were reduced when ageratum plants were grown under ebb-and-flow irrigation as opposed to overhead irrigation or were conditioned by brushing. The objective of the tests reported here was to evaluate the effects of several greenhouse management practices on insect populations on herbaceous perennial and annual bedding plants. Materials and Methods Plugs for ‘Little Devil Mix’ marigold, ‘Blue Puffs’ ageratum, ‘McKana Giants’ columbine, and ‘Antares’ New Guinea impatiens were obtained from a commercial grower in Mar. 1995, and planted in 10 × 10-cm plastic pots (soil volume 580 cm) filled with MetroMix 300 (Grace Horticultural Products, Cambridge, Mass.). Marigold, ageratum, and columbine plugs were planted on 3 Mar. and the New Guinea impatiens on 14 Mar. Conditioning treatments were initiated on each crop 7 d after planting, when the plants had resumed active growth. The control plants were well-watered with overhead irrigation, and fertilized with N at 250 mg·L three times per week using a 20N–4.4P–16.6K watersoluble fertilizer (Peters peatlite special 20– 10–20; The Scotts Co., Marysville, Ohio). Two fertilization rates were selected to represent extreme ends of the normal range. The water soluble fertilizer was applied three times per week with N at 50 mg·L (“low N”) or at 500 mg·L (“high N”). All plants not assigned to specific N treatments were fertilized with N at 250 mg·L three times per week. deficits, also reduces growth and improves plant resistance to additional stresses (Eakes et al., 1991). The effect of growth regulation or conditioning treatments on population dynamics of greenhouse pests has received little attention. However, management factors can increase or reduce plant resistance to insect pests and affect pest populations (Tingey and Singh, 1980). For example, sterol-inhibiting fungicides (triazole chemicals similar to paclobutrazol or uniconazole) can alter the population structure of both pest and beneficial arthropods. They reportedly increase populations of European red mites (Panonychus ulmi Koch) and green apple aphids (Aphis pomi Degeer) in apple (Malus ×domestica Borkh.) orchards (Biggs and Hagley, 1988). Aphid population growth was reduced on black currants (Ribes nigrum L.) by treatment with chlormequat chloride (2-chloro-N,N,Ntrimethylethanaminium chloride), but was unaffected by daminozide (Smith, 1969). Daminozide reduced green peach aphid and two-spotted spider mite populations on chrysanthemum (Dendranthema ×grandiflora Kitam.), presumably indirectly by reducing the survival rate of the immatures, as the chemical showed no direct toxicity to the adults (Worthing, 1969). Horticultural practices used in greenhouse production are designed to control plant growth, form, and condition to produce a healthy, attractive, and marketable plant. Growth regulation and conditioning of greenhouse-grown bedding plants can be accomplished chemically with plant growth regulators (PGRs), or nonchemically by mechanical conditioning (brushing), and irrigation or nutrient management. Herbaceous ornamentals are commonly treated with PGRs, such as daminozide, paclobutrazol, or uniconazole [(E)-1-(4chlorophenyl)-4,4-dimethyl-2-(1,2,4triazol1-yl)-penten-3-ol]. Mechanical conditioning via brushing or shaking of plant shoots affects the growth and condition of a variety of vegetable transplants (Latimer, 1991) and ornamental bedding plants (Autio et al., 1994). Moisture stress conditioning (MSC), the controlled exposure of plants to nonlethal water