Effects of Insecticides on Gas Exchange, Vegetative and Floral Development, and Overall Quality of Gerbera

This study evaluated the influence of insecticides on gas exchange, chlorophyll content, vegetative and floral development, and plant quality of gerbera (Gerbera jamesonii Bolus ‘Festival Salmon’). Insecticides from five chemical classes were applied weekly at 1× or 4× their respective recommended concentration. The insecticides used were abamectin (Avid), acephate (Orthene), bifenthrin (Talstar), clarified hydrophobic extract of neem oil (Triact), and spinosad (Conserve). Photosynthesis and stomatal conductance were reduced in plants treated with neem oil. Plants treated with neem oil flowered later—and at 4× the recommended label concentration had reduced growth, based on lower vegetative dry mass (DM) and total aboveground DM, reduced leaf area, thicker leaves (lower specific leaf area), higher chlorophyll content (basal leaves), and reduced flower production. Plants treated with acephate at 4× the recommended label concentration were of the lowest quality due to extensive phytotoxicity (leaf chlorosis). Plants treated with 1× or 4× abamectin or spinosad were of the highest quality due to no phytotoxicity and no thrips damage (thrips naturally migrated into the greenhouse). The control plants and plants treated with 1× bifenthrin had reduced quality because of thrips feeding damage; however gas exchange was not negatively affected. Gerbera (Gerbera jamesonii Bolus) is an economically important greenhouse crop produced and sold for cut flowers, potted plants, and bedding plants. Gerbera is highly susceptible to a variety of insect pests including aphids, spider mites, and its most important insect pest—western flower thrips [(WFT) Frankliniella occidentalis Pergande]. WFT are difficult to control due to their biology and feeding behavior (Robb, 1989). WFT prefer to feed within flowers and buds, which protect them from harmful insecticides and natural enemies. Hence, insecticides are often applied Biorational insecticides are becoming increasingly popular in the management of greenhouse insect pests to minimize reliance on more toxic chemicals. “Biorational” refers to pesticides of natural origin that have limited or no effects on the environment or beneficial organisms. Miller and Uetz (1998) evaluated several biorational insecticides (horticultural oil, insecticidal soap, and neem extract) for phytotoxicity on a variety of greenhouse crops. No phytotoxicity was observed on flowers or foliage of bedding plants and plant height did not appear to be affected by the treatments. However, horticultural oil (SunSpray UF spray oil, Mycogen Corp., San Diego, Cal.) and insecticidal soap (M-Pede, Mycogen, Corp., San Diego, Cal.) caused leaf burn and reduced growth in potted poinsettia (Euphorbia pulcherrima Willd.); while neem extract (Azatin, AgriDyne, Salt Lake City, Utah) had no adverse affect (Miller and Uetz, 1998). Only obvious phytotoxic effects were evaluated, while plant gas exchange, biomass, and development were not measured. The effects of biorational insecticides on gerbera have not been reported. Some insecticides have translaminar activity, in which the material enters the leaf to form a reservoir of active ingredient. When these materials enter the leaf cuticle, they may impact plant gas exchange processes, and alter plant growth and development. Acephate, a commonly used organophosphate insecticide with translaminar activity, has been reported to be phytotoxic to greenhouse crops such as chrysanthemum (Dendranthema grandifl oraTzvelev ‘Charm’) (Spiers et al., 2004) and peace lily (Spathiphyllum Schott ‘Clevelandii’) (Chase and Poole, 1984). Organophosphate insecticides have also been reported to reduce photosynthesis in strawberries (LaPré et al., 1982), oranges (Jones et al., 1983), and lettuce (Haile et al., 2000). Studies on the effects of insecticides on gas exchange have been conducted primarily on agronomic crops with varying results (Abdel-Reheem et al., 1991; Godfrey and Holtzer, 1992; Veeraswamy et al., 1993). Many of these studies were conducted using insecticides that are no longer commercially available or are not registered for use on ornamental greenhouse crops. The effect of commonly used insecticides, as well as newly introduced compounds, needs to be addressed in determining how they affect plant gas exchange, and the subsequent effects on plant development, quality, and production. Pesticides have varying recommendations for application rates based on the crop, pest, and at times application methods. Method and number of applications as well as combination with non-pesticides occasionally influence photosynthesis (Ferree, 1979). Haile et al. (2000) reported that the surfactant (Kinetic; Helena Chem. Co., Memphis, Tenn.), alone reduced photosynthetic rates in seedling lettuce. Several pesticides, surfactants, or carriers are oil-based. Oil-based formulations may mechanically interfere with gas exchange by blocking plant stomates, thus reducing stomatal conductance and photosynthesis (Ferree, 1979). While insecticides may adversely affect plants, few studies have reported insecticidal effects on host plant physiology in addition to at frequent intervals to prevent damage from occurring. However, WFT populations may rapidly develop resistance to insecticides when they are used on a continual basis (Immaraju et al., 1992; Jensen 2000). Insecticides from various insecticide classes with diverse modes of action are frequently used on gerbera to control insect pests and reduce the risk of insecticide resistance. Due to increasing concerns over the use of conventional insecticides in greenhouse production, reduced-risk insecticides are used because they are less toxic to workers, have short residual properties, and minimal adverse environmental impact (Lowery and Isman, 1995; Miller and Uetz, 1998). These insecticides also may have a narrower spectrum of activity against pest species and may require frequent applications to achieve production goals. The continual presence of these insecticides on plant foliage may adversely affect plant growth processes. Insecticides are typically evaluated for visible phytotoxicity prior to their registration, but subtle impacts on plant physiology, growth, and development are not often tested. JuneBook 701 4/4/06 10:59:57 AM HORTSCIENCE VOL. 41(3) JUNE 2006 702 plant growth and development. In this study, five insecticides that are commonly used in greenhouse crop production were chosen to represent various classes of insecticides: organophosphates, pyrethroids, spinosyn, macrocyclic lactone, and botanical. Each of the selected insecticides is used for thrips control in gerbera greenhouse production. The objective of this study was to determine the effects of selected insecticides, when applied 1× or 4× the recommended label concentration, on the physiology and growth of gerbera. Materials and Methods Plant cultural conditions. In total, 84 Gerbera jamesonii Bolus ‘Festival Salmon’ seedlings were transplanted into 6-inch standard pots (15.5 × 11.5 cm; 2050 cm) on 11 June 2004. Each pot with one established seedling plug was a single replication. Sunshine Mix #1 (Sun Gro Horticulture Inc., Pine Bluff, Ark.) was used as the growing media. Gerberas were grown according to recommended cultural practices for pot plant production (Benke, 1991; Kessler, 1999). Plants were initially placed pot-to-pot for 5 d, then were evenly spaced (about 33 × 33 cm apart) on two benches (1.2 m × 3.7 m). Plants were fertilized as needed with 150 mg·L N for 3 weeks, then with 250 mg·L N for the remainder of the experiment. At each irrigation, plants were fertilized with a 15N–7P–14K fertilizer (Peters Professional Peat-lite special 15–16–17, Scotts-Sierra Horticultural Products Co., Marysville, Ohio). The greenhouse temperature and relative humidity (RH) was recorded hourly using data loggers (Watchdog Data Logger model 150; Spectrum Technologies, Inc., Plainfield, Ill.). Data loggers were placed at canopy level (about 20 cm high) in the center of each bench. The average day and night temperature were, respectively, 28.4 ± 0.1 °C and 23.5 ± 0.2 °C. The average day and night RH were, respectively, 74% and 91%. A line quantum sensor was placed at canopy level in the center of each bench to determine photosynthetic photon flux (PPF). PPF was recorded hourly with an Apogee nanologger model MCQI (Apogee Instruments Inc., Logan, Utah). Daily peak light intensity averaged 421 μmol·m·s. There were no significant differences among benches. Etridiazole/thiophanate-methyl [Banrot Broad Spectrum Fungicide 40% WP (ScottsSierra Crop Protection Co., Marysville, Ohio)] was applied at a rate of 600 mg·L to all pots 4 d after planting. The plant growth regulator, daminozide, (B-Nine, Uniroyal Chemical Company Inc., Middlebury, Conn.) was sprayed on all plants at 2500 mg·L to the point of runoff, 19 d after transplanting to reduce leaf expansion and encourage compact growth. The experiment was terminated after 57 d on 6 Aug. 2004. Insecticidal treatments. Treatments consisted of five insecticides and one control (control plants were sprayed with de-ionized water). Insecticides were applied weekly at 1× or 4× the recommended concentration. The 4× concentration was included to maximize the potential phytotoxic response. There were seven plants or replicates for each treatment. The recommended concentrations and rates varied for the selected insecticides. The recommended concentrations and rate (weekly) used in this study were based on insecticide label instructions for thrips control on greenhouse floral crops. The insecticides and recommended concentrations (in g a.i./100 L) were: acephate Fig. 1. Effect of insecticides and concentration on (A) flower number; (B) days to flower, determined by number of days from transplanting to pollen shed of the most mature flower; and (C) overall plant quality. Gerberas were rated on a quality rating scale of 1 to 5 on day 57, based on the following scale: 1 = extensive damage on leaves and/or flowers due to thrips feeding or phytotoxicity; 2 = excessive to moderate injury; including distorted flowers and/or leaves due to thrips feeding or phytotoxicity; 3 = moderate injury from thrips feeding or phytotoxicity; 4 = acceptable quality plant with slight thrips feeding damage or phytotoxicity; 5 = high quality plant with healthy flowers and foliage, and no thrips feeding injury or phytotoxicity. Means were separated using Fisher’s protected least significant difference (LSD) test (P 0.05); treatments followed by the same letter are not significantly different; 1× = recommended concentration, 4× = 4 times the recommended concentration; see Table 3. JuneBook 702 4/4/06 11:00:00 AM 703 HORTSCIENCE VOL. 41(3) JUNE 2006 (Orthene 97; Valent U.S.A. Corp., Walnut Creek, Calif.), 58; bifenthrin (Talstar; FMC Corp., Philadelphia, Pa.), 27; spinosad (Conserve SC; Dow AgroSciences LLC, Indianapolis, Ind.), 21; clarified hydrophobic extract of neem oil (Triact 70; Olympic Horticultural Products Co., Mainland, Pa.), 654; and abamectin (Avid 0.15 EC; Syngenta Crop Protection Inc., Greensboro, N.C.), 1. Each plant was sprayed using a handheld 700-mL sprayer (Spraymaster; Delta Industries, King of Prussia, Pa.) until runoff (about 90 mL per plant). Spray applications started 6 d after transplanting and continued weekly for the duration of the experiment (total = 8 applications). Insecticides were applied between the hours of 08:00 to 10:30 HR. Plant growth measurements. Vegetative dry mass (DM), leaf area, specific leaf area (SLA, cm·g DM of leaves), and total aboveground plant DM were determined at the termination of the experiment (day 57) to characterize plant growth. Both vegetative and reproductive plant parts were included in the total aboveground DM measurements. Plants were cut at the soil line, dried at 70 °C for 96 h, and weighed to determine DM. Flower development. Flower number, flower DM, and reproductive DM (flowers, flower buds, and peduncles) were determined at the termination of the experiment to assess the effect of the insecticide treatments on flower production. The rate of flower development was determined by recording the number of days from transplanting to pollen shed of the most mature flower. Plant quality characteristics. Plant quality was determined by rating each plant (1 to 5, with 5 = optimal) (Fig. 1). The quality rating was based on overall aesthetic quality and was determined using the following scale: 1 = extensive damage on leaves and/or flowers due to thrips feeding or phytotoxicity; 2 = excessive to moderate injury; including distorted flowers and/ or leaves due to thrips feeding or phytotoxicity; 3 = moderate injury from thrips feeding damage or phytotoxicity; 4 = quality plant with slight thrips feeding damage or phytotoxicity; 5 = high quality plant with no thrips feeding damage or phytotoxicity. Statistical significance for the quality ratings was determined with a two-way ANOVA design for ranked data—the ScheirerRay-Hare extension of the Kruskal–Wallis Test (Sokal and Rohlf, 1995). Chlorophyll determination. Leaf chlorophyll concentration was determined with a portable chlorophyll meter (SPAD-502; Minolta Camera Co., LTD, Japan). The SPAD-502 meter readings were correlated with a chlorophyll content prediction equation: y = 1.4929x – 12.979, where y = chlorophyll content (μg·cm), x = meter reading (R = 0.9683). This equation was obtained by running a linear regression analysis between the SPAD-502 readings obtained in a separate fertility study with physiologically mature leaves from five pots per fertility treatment (six treatments); and the total chlorophyll concentration of the same leaves (J.D. Spiers, unpublished data). Leaf chlorophyll was extracted with N, N-dimethylformamide (DMF) and the total concentration was determined by the optical density of filtered aqueous supernatant, which was measured at 647 nm and 664 nm with a spectrophotometer (Moran, 1982). Each leaf was a single replication and one physiologically mature leaf, and one older, basal leaf was randomly selected from each plant and measured at the end of the experiment with the SPAD-502 meter (n = 7). Plant gas exchange.Net photosynthesis (Pn) and stomatal conductance (g s ) measurements of individual leaves were taken one day after each insecticide application between 11:00 and 15:30HRusing a portable photosynthesis system (LI-6400; LI-COR Inc., Lincoln, Neb.). A fixed substrate level of 360 μL·L CO 2 was provided with a 12-g cartridge, and the light source was a LED 6400 R/B at 600 μmol·m·s. Each leaf was a single replication, and there were 3 replications per treatment (n = 3). Experimental design and statistical data analysis. The experiment was arranged as a six × two factorial design. Each pot had one established seedling plug as a replicate. There were seven replications per treatment arranged in a completely randomized design. Data were compared using a two-way analysis of variance (ANOVA) (SAS Institute Inc., 2000), with insecticide, insecticide concentration, and insecticide × concentration as main effects. When appropriate, mean separations were performed using Fisher’s protected least significant difference test to determine treatment differences (P 0.05).