Influence of Light Intensity, Photoperiod, and Temperature on the ...

PHYSIOLOGICAL ECOLOGY

Influence of Light Intensity, Photoperiod, and Temperature on the Efficacy of Two Aphelinid Parasitoids of the Greenhouse Whitefly G.M.G. ZILAHI-BALOGH,1 J. L. SHIPP,1, 2 C. CLOUTIER,3

AND

J. BRODEUR4

Environ. Entomol. 35(3): 581Ð589 (2006)

ABSTRACT In short-term laboratory experiments, we studied the inßuence of light intensity, photoperiod, and temperature on the feeding and oviposition activity of two aphelinid parasitoids, Encarsia formosa Gahan and Eretmocerus eremicus Rose and Zolnerowich, on the greenhouse whiteßy, Trialeurodes vaporariorum (Westwood). E. eremicus parasitized signiÞcantly more whiteßy hosts than E. formosa in all treatment combinations of light intensity and photoperiod at 24 and 20⬚C. At 24⬚C, both E. formosa and E. eremicus parasitized approximately twice as many whiteßy hosts at the high light intensity (112Ð114 W/m2)Ðlong daylength (L 16:D 8 h) treatment than at the low light intensity (12Ð14 W/m2)Ðshort daylength (L 8:D 16 h) treatment. In most instances at 20 and 24⬚C, signiÞcantly more dead whiteßy hosts were recorded in leaf cages introduced with parasitoids than leaf cages with no parasitoids, indicating that host feeding occurred. Both E. formosa and E. eremicus parasitized signiÞcantly more whiteßy hosts under the simulated summer (i.e., high light intensity [82.0 Ð 83.6 W/m2]; long daylength [L 16:D 8 h], 24⬚C) treatment than the winter (i.e., low light intensity [10.8 Ð11.1 W/m2]; short daylength [L 8:D 16 h], 20⬚C) treatment. In addition, signiÞcantly more dead whiteßy hosts were observed in leaf cages introduced with E. eremicus than E. formosa under the winter treatment, suggesting that E. eremicus killed more whiteßy hosts through host feeding than E. formosa. This was not the case for the other treatment combinations. E. eremicus contributes more to greenhouse whiteßy mortality than E. formosa by a combination of greater parasitism and host feeding and therefore is recommended for use in winter months with day temperatures ⱖ 20⬚C, in addition to later in the season, where it is currently being used. KEY WORDS biological control, Trialeurodes vaporariorum, Encarsia formosa, Eretmocerus eremicus, greenhouse climate

In biological control programs, it is important to understand how environmental factors inßuence the interaction between the pest and the biological control agent. Greenhouses are unique in that climatic conditions can be manipulated. Temperature and humidity and their effects on pestÐnatural enemy interactions have been the most studied environmental factors to date in greenhouse environments (Kajita 1983, Nihoul 1993, Shipp and Gillespie 1993, Shipp et al. 1996, 2003, Shipp and van Houten 1997. However, only a few studies have investigated the inßuence of light on the foraging behavior of natural enemies (Smith and Rutz 1991, Maeda et al. 2000, Gu and Dorn 2001, VanLaerhoven et al. 2003, Peridikis et al. 2004). The trend in northern temperature regions, especially in Scandinavian countries, is toward the use of supplemental lighting in the winter (Va¨nninen and Johansen 2005) for greenhouse production. For the 1 Agriculture and Agri-Food Canada, Greenhouse and Processing Crops Research Centre, Harrow, Ontario, Canada N0R 1G0. 2 Corresponding author, e-mail: [email protected]. 3 Universite ´ Laval, Department de Biologie, Ste Foy, Quebec, Canada G1K 7P4. 4 Institut de Recherche en Biologie Ve ´ ge´ tale, Universite´ de Montre´ al, Montre´ al, Quebec, Canada H1X 2B2.

development and improvement of pest management strategies under supplemental lighting conditions, we need a better understanding of arthropod response to light intensity (Chu et al. 1998, Gu and Dorn 2001) and spectral quality (Vaishampayan et al. 1975, Gillespie and Quiring 1992, Mellor et al. 1997, Antignus et al. 2001). The greenhouse whiteßy, Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae) is an economically important pest of greenhouse vegetable crops worldwide (Avilla et al. 2004). Whiteßy adults and nymphs are phloem feeders. At high population densities, they can signiÞcantly reduce crop yields by reducing plant vigor. In addition, honeydew produced by whiteßy feeding results in the growth of sooty mold that can decrease the photosynthetic rate of the crop. Finally, whiteßies are known to vector plant viruses (Byrne et al. 1990). Encarsia formosa Gahan (Hymenoptera: Aphelinidae) and Eretmocerus eremicus Rose and Zolnerowich (Hymenoptera: Aphelinidae) are commercially available parasitoids used for inundative releases to suppress whiteßies in greenhouses. E. formosa has been used widely in greenhouse crops since the 1970s when the Þrst problems with insecticide resistance occurred

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(van Lenteren et al. 1996, Avilla et al. 2004), whereas E. eremicus, described in 1997 from the Bemisia tabaci complex in the United States (Rose and Zolnerowich 1997), is a relatively new commercialized biological control agent. In Canada, both parasitoids are used in ⬇93% of greenhouse tomato production area (⬇444 ha) for management of greenhouse whiteßy (Murphy et al. 2002). Despite the commercial success of seasonal inoculative releases of E. formosa to control greenhouse whiteßy over the last 30 yr (Gerling et al. 2001), it has long been recognized that parasitism by E. formosa is reduced during the winter months compared with the rest of the year in northern greenhouses where no supplemental lighting is used (Parr et al. 1976, Gerling et al. 2001, J.L.S., personal observation). In Ontario, it is recommended that vegetable growers avoid using E. formosa between December and February without adjusting light and temperature conditions to suit the parasitoid (OMAF 2005). The use of supplemental lighting is still very limited in Ontario because the increased production beneÞts with tomatoes do not yet exceed the increased energy costs associated with the use of supplemental lighting. Short daylength and low light intensity conditions in greenhouses might be an explanation for reduced performance of E. formosa, rather than cooler temperatures, because dispersal (van der Laan et al. 1982) and oviposition (van Lenteren and Hulspas-Jordaan 1983) by E. formosa can occur at temperatures as low as 13⬚C. Studies by van Lenteren et al. (1992) found that emerging E. formosa have the capacity to disperse at low (500 lux) and high (8,000 lux) light intensities but that they cover longer distances at high light intensities. Additionally, oviposition by E. formosa was found to occur at light intensities ⬎7,300 lux (L 16:D 8 h), whereas no oviposition occurred at 4,200 lux (McDevitt 1973, Parr et al. 1976). This seems to be contradictory to results of work by van Lenteren et al. (1992), where dispersal and oviposition by E. formosa occurred with light intensities of ⬇500 lux. Despite reduced efÞcacy of E. formosa in the winter, greenhouse vegetable growers in Ontario use E. formosa early in the year (i.e., winter months) and later in the season in combination with E. eremicus (J.L.S., personal communication), a species that performs well under high temperatures (Gerling et al. 2001). We observed that E. eremicus achieved higher parasitization rates on greenhouse whiteßy than E. formosa during the winter months and were interested in testing the hypothesis that E. eremicus had a greater parasitization rate on greenhouse whiteßy in the winter than E. formosa (J.L.S., unpublished data). To our knowledge, no studies have directly compared parasitism rates of E. formosa and E. eremicus in response to varying light intensities, photoperiods, and temperatures that are typical of summer and winter conditions in commercial greenhouse operations in northern temperate regions. We report results from short-term controlled growth chamber experiments where the inßuence of light intensity, photoperiod, and temperature were determined on parasitism and

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host feeding by E. formosa and E. eremicus on greenhouse whiteßy in conÞned arenas. Materials and Methods Plants. Tomato plants, Lycopersicon esculentum Mill. (cultivar Rhapsody) (Solanaceae) grown in 7.5-cm2 rock wool blocks under commercial greenhouse conditions were used in all trials. Once the plants reached the fourth to sixth leaf stage, they were transplanted into 15-cm-diameter plastic pots using sterilized orchard soil (sandy loam) and moved into an environmental chamber (Conviron model PGW 36; Controlled Environments Ltd., Winnipeg, Manitoba, Canada) at 24 ⫾ 1⬚C and L 16:D 8-h photoperiod. Potted plants (n ⫽ 12) were then exposed to adult greenhouse whiteßies (⬇300 whiteßies per plant) for 1 d to allow for oviposition. Once the eggs hatched and the immatures settled on the lower leaf surfaces, leaf cages (n ⫽ 6) were attached to the lower leaf surface of each of six tomato leaßets per plant. Leaf cages (2.5 cm diameter by 2 cm height) were made of clear plastic styrene and covered with translucent Þne mesh (53-␮m pore size) screening (B.E.H. Thompson, Scarborough, Canada). A 6-mm hole on the wall of the leaf cage was used to introduce an individual parasitoid. Leaf cages were held onto the leaßet with a clip. All but ⬇40 whiteßy immatures were removed from the lower leaßet surface enclosed within the leaf cage with a Þne brush. After ⬇14 d at 24⬚C, immature whiteßies molted to the third instar (⬇225 DD; Dth ⫽ 8.3⬚C) (Osborne 1982) and were used in experiments. Parasitoids. Parasitoids (E. formosa and E. eremicus) were obtained as pupae on cards from a commercial insectary (Biobest Canada, Leamington, Canada). Pupae were placed in clear plastic vials with a streak of honey as a nutrient source and held in an environmental chamber at 24⬚C and a 16:8-h (L:D) photoperiod with standard ßuorescent lighting (⬇12 W/m2 light intensity). Pupae were inspected daily for adult emergence. Parthenogenic E. formosa females ⬍24 h were used in experiments, whereas bi-parental E. eremicus used in experiments were 24 to ⬍48 h to allow for mating. Female E. eremicus used in experiments were assumed to be mated. E. eremicus was sexed using a ⫻20 ocular microscope. Eretmocerus females can be easily distinguished from males because female antennae have two funicular segments, whereas antennae of males have no funicular segments (Rose and Zolnerowich 1997). Females of both species were individually isolated in gelatin capsules shortly before being introduced into the leaf cages. Light, Photoperiod, and Temperature Treatments Applied. Before conducting the laboratory experiments, we obtained baseline light intensity readings, measured in Watts per square meter, from research glasshouses that are structurally similar to those used by commercial vegetable growers, located at the Agriculture and Agri-Food Canada, Greenhouse and Processing Crops Research Centre, Harrow, Ontario, Canada (latitude: 42.02⬚ N; longitude: 82.93⬚ W), using a CM3 pyranometer (spectral range 305Ð2,800 nm;

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ZILAHI-BALOGH ET AL.: SEASONAL INFLUENCES ON BIOLOGICAL CONTROL OF T. vaporariorum

Kipp & Zonen, Delft, The Netherlands). The pyranometer was placed within a full crop canopy (canopy height ⬇ 2.9 m) below the eighth leaf node from the plant apex. We selected the eighth leaf node (approximately one-third below the plant apex) because we previously determined that the frequency distribution of second- and third-instar whiteßies is the greatest at this stratum. Second- and third-instar whiteßies are the preferred stages for parasitism by E. eremicus (Headrick et al. 1995) and E. formosa, respectively (Nell et al. 1976, Nechols and Tauber 1977). Tomato plants used in commercial greenhouse production are indeterminate, and because whiteßy adults prefer to oviposit on the youngest leaves at the plant apex, there is a differential vertical distribution of whiteßy immature stages, with age/stage increasing as one moves away from the plant apex. Instantaneous light intensity levels were recorded every minute, and the average was logged over each 15-min interval. Light intensity was averaged for daylight hours over the 5- to 7-d period that the data were logged. Summer daytime light intensity was recorded between 20 and 24 August 2003, and a mean of 61.5 W/m2 was obtained. Winter daytime light intensity was logged between 17 and 23 December 2003. and a mean of 9.86 W/m2 was obtained. For calibration of light intensity in the growth chambers, the pyranometer was set at 35 cm height above the ßoor, which corresponded to the average potted tomato plant mid-canopy. We determined that light intensity 5 cm below a mid-canopy leaf was ⬇60% of the ambient light intensity in a chamber. This approximated the light intensity at the leaf cage level where the parasitoids were introduced. Light intensity in the chamber was therefore adjusted to reßect transmission of light through a leaf (i.e., chamber light intensity ⫻ 0.60 ⬇ light intensity at leaf cage). High light intensity was achieved by a combination of coolwhite ßuorescent (160 W, F72T12/D/VHO; Philips Electronics Ltd., Marham, Ontario, Canada) tubes and incandescent (60 W) light bulbs. Low light intensity was provided by cool-white ßuorescent tubes. We measured spectral photon ßux for low light and high light intensity treatments from 300 to 1,100 nm in 1-nm increments with a spectroradiometer (LI-1800; LICOR, Lincoln, NE), to verify that the spectral range of low and high light intensity chambers were similar. The movable light panel was raised or lowered to achieve the desired light intensity level in the chamber and adjusted before the beginning of each trial. Light intensity in the chambers was measured using the CM3 pyranometer in experiment 1 and a LI-COR LI200S pyranometer (spectral range, 400 Ð1,100 nm) in experiment 2, because the CM3 pyranometer was not available for use. Before experiment 2, the LI-COR pyranometer was calibrated against the CM3 pyranometer under the low and high experimental light conditions in each of the four environmental chambers used. This was achieved by placing the two pyranometers side-by-side in each chamber for 24 h at both low light and high light intensities. Light intensity data were recorded every minute, and the average was logged

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at each 15-min interval. Irradiance (light intensity) recordings of the LI-COR pyranometer (dependent variable) were regressed against CM3 irradiance recordings (independent variable). A signiÞcant positive linear regression (LI-COR ⫽ 0.253 ⫹ 0.714CM3) was determined (r2 ⫽ 0.99; df ⫽ 1,1168; F ⫽ 10,875; P ⬍ 0.0001). Irradiance levels used in experiment 2, measured with the LI-COR pyranometer, were estimated by the regression equation to closely approximate the irradiance levels used in experiment 1. Photoperiod treatments selected represent summer (16-h photophase) and winter (8-h photophase) in northern temperate climates at the summer and winter solstices at a latitude of ⬇45⬚ N. Optimal greenhouse temperatures for growth and fruit production of greenhouse tomatoes grown in soilless media are 21: 18⬚C (day:night) under winter low-light conditions, and 24:20⬚C under summer high-light conditions (Papadopoulos 1991). Therefore, 20 and 24⬚C were selected to represent average winter and summer temperatures, respectively. Experiment 1: Effect of Light Intensity and Photoperiod on Parasitism and Host Feeding. This experiment studied the inßuence of light intensity and photoperiod on parasitism and host feeding of E. formosa and E. eremicus. Four environmental chambers (Conviron models E-15, EY-15, PGW 36; Controlled Environments Ltd., Winnipeg, Canada) were set at one of the following four combinations of light intensity (high: 112Ð114 W/m2; low: 12Ð14 W/m2; photoperiod: 16:8; 8:16-h L:D) at 24 ⫾ 1⬚C and 75% RH ⫾ 10% in a randomized complete block factorial design. Parasitoids (E. formosa, E. eremicus, no-parasitoid control) were individually introduced into leaf cages (experimental unit) on tomato plants, for a total of 12 parasitoid ⫻ light intensity ⫻ photoperiod treatment combinations. Leaf cages (n ⫽ 6 per plant) on each plant were randomly assigned a single naõ¨ve female E. formosa, E. eremicus, or no parasitoid (control), with two observations per parasitoid treatment per plant, for a total of 12 plants. Plants were randomly assigned to an environmental chamber (three plants per chamber) with one of the four light treatment combinations. There were a total of (n ⫽ 6 replicates: 2 parasitoids per plant ⫻ 3 plants) of each parasitoid treatment for each light intensity ⫻ photoperiod treatment per trial. Parasitoids were removed after 24 h, and all plants were moved into the one environmental chamber (24 ⫾ 1⬚C and 16:8 h [L:D] photoperiod) to allow for parasitoid progeny development under similar conditions. If a parasitoid was not recovered alive, the observation was not used in the analysis. After ⬇2 wk, the number of wasp pupae (parasitized whiteßies), dead whiteßies, and unparasitized whiteßies (whiteßy exuviae) were recorded. Dead whiteßies were observed as ßattened, dried, and discolored. Host feeding was measured indirectly by taking the difference between the number of dead whiteßies in the control (no parasitoid) leaf cages and the leaf cages where an individual parasitoid was introduced. The experiment was replicated four times on different occasions to allow each light treatment combination to occur once

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Table 1. ANOVA for the no. of parasitized whitefly pupae and dead whiteflies after either E. formosa or E. eremicus were introduced for 24 h into leaf cages containing third-instar T. vaporariorum under different light intensity (high: 112–114 W/m2, low: 12–14 W/m2) and photoperiod (long: 16-h photophase, short: 8-h photophase) regimens at 24°C Source of variation Encarsia formosa Block Light Photoperiod Light ⫻ photoperiod Error Eretmocerus eremicus Block Light Photoperiod Light ⫻ photoperiod Error

df

Number of parasitized whiteßy pupae

Number of dead whiteßies

3 1 1 1 75

0.62 21.97c 12.03b 0.54

5.41a 0.05 0.48 0.49

3 1 1 1 74

1.41 17.95c 1.99 0.34

7.54b 4.08a 0.04 0.87

F values with indicated level of signiÞcance are given for each effect tested. The block effect examines for different growth chambers and parasitoid batches being used in trials. SigniÞcant at P ⱕ a0.05, b0.01, or c0.001.

in each of the four chambers to avoid possible chamber effects. Experiment 2: Effect of Simulated Seasonal Light Conditions and Temperature on Parasitism and Host Feeding. In a second laboratory experiment, we studied the effect of simulated seasonal light conditions (summer versus winter) in combination with seasonal variation of temperature (20 and 24 ⫾ 1⬚C) on parasitism and host feeding by E. formosa and E. eremicus using similar experimental conditions and protocols as in experiment 1. We deÞned our simulated seasonal summer conditions to be 16:8-h (L:D) photoperiod and high light intensity (82.0 Ð 83.6 W/m2) and simulated seasonal winter conditions to be 8:16-h (L:D) and low light intensity (10.8 Ð11.1 W/m2). Simulated seasonal light conditions in the chambers were tested at both 20 and 24⬚C. The temperatures 20 and 24⬚C were selected to test because they reßect average winter and summer temperatures, respectively, in a commercial vegetable greenhouse. Data Analysis. When necessary to correct for heterogeneity of variance and non-normality (Zar 1984), data were transformed by log(x ⫹ 1) before analysis.

Analysis of variance (ANOVA) was performed using PROC GLM (SAS Institute 1989). The main effects of block, light intensity, photoperiod, parasitoid species (experiment 1) and block, season, temperature, parasitoid species (experiment 2), and all two- and threeway interactions other than those including block were assessed for parasitism and whiteßy mortality (associated with host feeding). Also tested were the main effects and two-way interactions of light ⫻ daylength (experiment 1) and season ⫻ temperature (experiment 2) (Tables 1 and 2) for each parasitoid species separately. Least signiÞcant difference (LSD) tests were applied to determine differences in the number of parasitized pupae and dead whiteßies between and within each species of parasitoids at each treatment combination. In all cases, ␣ ⫽ 0.05. Results Effect of Light and Photoperiod on Parasitism and Host Feeding. The number of whiteßy hosts parasitized (by either parasitoid species) was signiÞcantly affected by the main effects of light intensity (F ⫽ 38.4,

Table 2. ANOVA for the no. of parasitized whitefly pupae and dead whiteflies after either E. formosa or E. eremicus were introduced for 24 h into leaf cages containing third-instar T. vaporariorum under summer light conditions (high light intensity: 82– 83.6 W/m2; long day: 16-h photophase) and winter light conditions (low light intensity: 10.8 –11.2 W/m2; short day: 8-h photophase) at 20 and 24°C Source of variation Encarsia formosa Block Season Temperature Season ⫻ temperature Error Eretmocerus eremicus Block Season Temperature Season ⫻ temperature Error

df

Number of parasitized whiteßy pupae

Number of dead whiteßies

3 1 1 1 81

8.83b 6.43a 9.82b 0.02

2.45 0.56 3.89 6.66a

3 1 1 1 75

2.94a 8.35b 9.28b 0.26

2.12 1.71 0.01 2.28

F values with indicated level of signiÞcance are given for each effect tested. The block effect examines for different growth chambers and parasitoid batches being used in trials. SigniÞcant at P ⱕ a0.05 or b0.01.

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Fig. 1. Mean ⫾ SE number of parasitized T. vaporariorum pupae after exposure to a single naõ¨ve individual E. formosa or E. eremicus for 24 h under different combinations of light intensity (high: 112Ð114 W/m2; low: 12Ð14 W/m2) and photoperiod (long: 16-h photophase; short: 8-h photophase) at 24⬚C. Means followed by the same lowercase letter, within a species, are not signiÞcantly different among treatments (LSD test, P ⬍ 0.05). Means followed by the same capital letter, between species, are not signiÞcantly different within treatments (LSD test, P ⬍ 0.05).

df ⫽ 1,244, P ⬍ 0.0001), photoperiod (F ⫽ 20.86, df ⫽ 1,244, P ⬍ 0.0001) and parasitoid species (F ⫽ 944.61, df ⫽ 2,244, P ⬍ 0.0001), but light ⫻ species (F ⫽ 11.99, df ⫽ 2,244, P ⬍ 0.0001) and photoperiod ⫻ species (F ⫽ 5.44, df ⫽ 2,244, P ⫽ 0.005) interactions indicate these effects varied among species. When the effect of light intensity was examined separately for each species, signiÞcantly more whiteßy hosts were parasitized at high light intensity than low light intensity for both E. formosa and E. eremicus (Table 1; Fig. 1). Photoperiod signiÞcantly affected the number of whiteßy hosts parasitized by E. formosa, but not E. eremicus (Table 1; Fig. 1). E. formosa parasitized signiÞcantly more whiteßy hosts at the long daylength (16:8 h) than short (8:16 h) daylength (Table 1; Fig. 1). E. eremicus parasitized approximately twice as many whiteßy hosts than E. formosa at 24⬚C in all treatment combinations (Fig. 1). Each species parasitized approxi-

Fig. 2. Mean ⫾ SE number of dead immature T. vaporariorum after exposure to a single naõ¨ve individual E. formosa, E. eremicus, or no parasitoid (control) for 24 h under different combinations of light intensity (high: 112Ð114 W/m2; low: 12Ð14 W/m2) and photoperiod (long: 16-h photophase; short: 8-h photophase at 24⬚C. Means followed by the same letter, between species, are not signiÞcantly different within treatments (LSD test, P ⬍ 0.05).

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mately twice as many whiteßy hosts at the high light intensityÐlong daylength treatment than the low light intensityÐshort daylength treatment (Fig. 1). The number of dead whiteßy hosts was signiÞcantly affected by species (F ⫽ 14.21, df ⫽ 2,244, P ⬍ 0.0001) but not by light intensity (F ⫽ 0.01, df ⫽ 1,244, P ⫽ 0.91) and photoperiod (F ⫽ 0.88, df ⫽ 1,244, P ⫽ 0.35). There were no interactions involving main effects. E. eremicus killed more whiteßy hosts at high light intensity than low light intensity (Table 1; Fig. 2). This was not the case for E. formosa. Photoperiod did not inßuence the number of whiteßies killed for either species of parasitoid (Table 1). In most treatment combinations, signiÞcantly more dead whiteßy hosts were found in leaf cages introduced with parasitoids than the control (no parasitoid) (Fig. 2), indicating that host feeding occurred in general. A chamber effect was apparent with a signiÞcant block effect for dead whiteßies but not for parasitized whiteßy hosts (Table 1). Chamber effects are expected and might be explained by slight differences among chambers in controlling temperature (⫾1⬚C), humidity (⫾10% RH), or light intensity (⫾2 W/m2) or caused by different batches of parasitoids used among trials. Effect of Simulated Seasonal Light Conditions and Temperature on Parasitism and Host Feeding. The number of parasitized pupae was signiÞcantly affected by simulated season effects (F ⫽ 5.08, df ⫽ 1,251, P ⫽ 0.0251), temperature (F ⫽ 11.37, df ⫽ 1,251, P ⫽ 0.0009), and parasitoid species (F ⫽ 559.89, df ⫽ 2,251, P ⬍ 0.0001), but simulated season ⫻ species (F ⫽ 4.63, df ⫽ 2,251, P ⫽ 0.01) and temperature ⫻ species (F ⫽ 5.14, df ⫽ 2,251, P ⫽ 0.007) interactions indicate these effects varied among species. When the effect of season was examined for each parasitoid species separately, both E. formosa and E. eremicus parasitized more whiteßy hosts under the simulated summer than winter light treatments (Table 2; Fig. 3). Similarly, when the effect of temperature was examined, the number of parasitized whiteßies increased signiÞcantly from 20 to 24⬚C for both species of parasitoids (Table 2). E. eremicus parasitized approximately twice as many whiteßy hosts than E. formosa in all treatment combinations (Fig. 3). The number of dead whiteßy hosts was signiÞcantly affected by parasitoid species (F ⫽ 55.41, df ⫽ 2,250, P ⬍ 0.0001) but not by season (F ⫽ 0.44, df ⫽ 1,250, P ⫽ 0.51) and temperature (F ⫽ 0.04, df ⫽ 1,250, P ⫽ 0.85) (Table 2). There was a season ⫻ temperature interaction for E. formosa (Table 2). This can be explained by the number of dead whiteßy hosts increasing as the temperature increased from 20 to 24⬚C under high light intensity and 16:8-h photophase summer treatment but not at the low light intensity and 8:16-h photophase winter treatment (Fig. 4). SigniÞcantly more dead whiteßies were found in the leaf cages introduced with parasitoids than the control, indicating that host feeding occurred (Fig. 4). Significantly more dead whiteßies were found in leaf cages introduced with E. eremicus than E. formosa only at the 20⬚C winter treatment (Fig. 4). In the other treatment

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Fig. 3. Mean ⫾ SE number of parasitized T. vaporariorum pupae after exposure to a single naõ¨ve individual E. formosa or E. eremicus for 24 h under simulated summer (high light intensity: 82Ð83.6 W/m2; long day: 16:8-h [L:D] photoperiod) and simulated winter (low light intensity: 10.8Ð11.2 W/m2 and short day 8:16-h [L:D] photoperiod) conditions at 20 and 24⬚C. Means followed by the same lowercase letter, within a species, are not signiÞcantly different among treatments (LSD test, P ⬍ 0.05). Means followed by the same capital letter, between species, are not signiÞcantly different within treatments (LSD test, P ⬍ 0.05).

combinations, no signiÞcant difference was found in the number of dead whiteßies between the two parasitoids (Fig. 4). A chamber effect was observed with a signiÞcant block effect for both E. formosa and E. eremicus with the number of parasitized whiteßy pupae (Table 2). Discussion In this study, we made direct comparisons between E. formosa and E. eremicus in terms of their response to light intensity, photoperiod, and temperature conditions that would be typical of both summer and winter conditions in northern temperate regions. Light clearly seems to be a stimulus for activity by E. formosa and E. eremicus (Figs. 1Ð 4). Both parasitoids responded positively to increasing light intensity and daylength in terms of their foraging activity, suggesting that these parasitoids are mainly active during daylight hours. However, because E. formosa parasitized signiÞcantly more whiteßy hosts at the long day than short day treatment (Table 1), whereas E. eremicus did not, suggests that E. formosa is sensitive to changes in daylength, whereas E. eremicus is not under these experimental conditions. Blackmer and Cross (2001) reported that E. eremicus did not initiate ßight in ßight chambers unless the chamber was illuminated, and van Lenteren et al. (1992) reported that, in complete darkness, E. formosa hardly moves and does not oviposit. Increased parasitism activity occurred for both species of parasitoids as temperature increased from 20 to 24⬚C. This Þnding was expected as these temperatures are within the range of activity for both E. formosa and E. eremicus (Qiu et al. 2004). Qiu et al. (2004) reported

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that in studies comparing life history parameters of various aphelinids on B. argentifolii, E. formosa (Beltsville strain) had the shortest development time, the longest life span, and the highest fecundity at temperatures ⬍20⬚C, whereas at temperatures ⬎20⬚C, E. eremicus and E. mundis performed better. Commercial biological control producers typically recommend E. formosa during the winter months (December to February) because E. eremicus is not very active at temperatures ⬍20⬚C (Qiu et al. 2004). In Ontario, recommended air temperatures for tomatoes for maximum fruit production are 18⬚C at night and 20⬚C in the day. Minimum day and night temperatures under low light conditions typical of the winter are 19 and 17⬚C, respectively (Papadopoulos 1991). Because these parasitoids are diurnal in their activity, the night temperatures should not affect their foraging activity. The fact that E. eremicus parasitized more whiteßy immatures than E. formosa over all light, photoperiod, and temperature treatments (Figs. 1 and 3) and killed more whiteßy hosts under the winter treatment (Fig. 4) suggests that E. eremicus is more efÞcacious at controlling whiteßies over all seasons than E. formosa. These Þndings are particularly useful for the winter months in northern latitudes where E. formosa has been reported to have reduced efÞcacy (Gerling et al. 2001, J.L.S., personal observation). Based on these Þndings, it is recommended that growers use E. eremicus over E. formosa, especially when natural light is limiting in the winter months and where daytime temperatures are ⱖ20⬚C. We measured parasitism by recording the number of parasitized greenhouse whiteßy pupae ⬇2 wk after female wasps had access to hosts for 24 h. Other studies dissected the parasitoid eggs immediately after 24-h exposure of whiteßy hosts. Our methods do not account for parasitoid or whiteßy mortality before visible parasitism is recorded and therefore may underestimate the parasitization rate. However, parasit-

Fig. 4. Mean ⫾ SE number of dead immature T. vaporariorum after exposure to a single naõ¨ve individual E. formosa, E. eremicus, or no parasitoid (control) for 24 h under summer (high light intensity: 82Ð83.6 W/m2 and long day: 16:8-h [L:D] photoperiod) and winter (low light intensity: 10.8Ð11.2 W/m2 and short day 8:16-h [L:D] photoperiod) at 20 and 24⬚C. Means followed by the same letter, between species, are not signiÞcantly different within treatments (LSD test, P ⬍ 0.05).

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ization rates for both E. formosa (4 Ð10 parasitized hosts/d) and E. eremicus (10 Ð21 parasitized hosts/d) at 20 Ð24⬚C are comparable with other studies. Christochowitz et al. (1981) reported parasitization rates for E. formosa of ⬇5 eggs per female per day at 18⬚C and 10:14 (L:D) h, whereas Arakawa (1982), reported ⬇12 eggs laid per day at 25⬚C and 16:8 (L:D) h. For E. eremicus, McAuslane and Nguyen (1996) reported ⬇20 eggs per day using a photoperiod of 14:10 (L:D) h at 27.5:24⬚C (L:D) with an Eretmocerus sp. reared on B. argentifolii on hibiscus, whereas Greenberg et al. (2002) reported ⬇15 parasitized hosts per day on greenhouse whiteßies at 25⬚C and 16:8 (L:D) h. Host feeding was estimated by taking the difference between the number of dead whiteßies in the control and in the parasitoid treatments. If host feeding occurred, we expected more dead whiteßies in the leaf cages exposed to parasitoids than the control leaf cages. This was the case in most instances (Figs. 2 and 4). In our study, only under the simulated winter treatment at 20⬚C was host feeding signiÞcantly higher by E. eremicus than E. formosa (Fig. 4). Host feeding by E. formosa (0.6 Ð2.5 hosts/d) is lower than what was reported by Arakawa (1982) (⬇ 4 hosts/d) but approximates the values of Nell et al. (1976) (2.8 hosts/ d). Host feeding by E. eremicus (0.8 Ð3.8 hosts/d) is lower than what was reported by McAuslane and Nguyen (1996) (4.5 ⫾ 1.9 hosts/d) by Eretmocerus sp. reared on B. argentifolii on hibiscus for the Þrst 3 d of life. Success in host location by parasitoids is dependent on their tendency to initiate ßight and orient to herbivore-infested plants and host-searching efÞciency on the plant (Gu and Dorn 2001). Light intensity may inßuence ßight initiation and orientation in parasitoids. Gu and Dorn (2001) showed that ßight initiation and host plant orientation by Cotesia glomerata (Marshall) (Hymenoptera: Braconidae) is favored by a higher (1,600 lux) light intensity than a lower (600 lux) light intensity, at an optimal wind velocity (⬇30 cm/s). Similarly, van Lenteren et al. (1992) showed that emerging E. formosa dispersed further under high (8,000 lux) than low (500 lux) light intensities. In our study, despite minimal search area, parasitism and host feeding increased with higher intensity and duration of light. In northern temperate regions, the major limiting factor for winter greenhouse crop production is the low natural light level (Dorais and Gosselin 2002). The light environment (i.e., light intensity, spectral range, and photoperiod) in greenhouses is increasingly being manipulated for photosynthesis enhancement, regulation of plant growth, ßowering, and increased production (Dorais and Gosselin 2002, Runkle et al. 2002, Va¨nninen and Johansen 2005). Knowledge of the visual behavior in natural enemies can be manipulated to enhance biological control. Arthropod response to light (i.e., visual orientation behavior) has long been exploited in greenhouse integrated pest management (IPM) with the use of color sticky traps (Gillespie and Vernon 1990, Vernon and Gillespie 1990, 1995, Stack and Drummond 1997) and UV absorbent screens (An-

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tignus et al. 1996, 1998) for the management of pests. This study increases our understanding of the foraging activity of two aphelinid parasitoids (E. formosa and E. eremicus) under variable light intensity, photoperiod, and temperatures typical of summer and winter conditions and provides baseline information for future studies that address the inßuence of supplemental lighting on the population dynamics of natural enemies. On-going greenhouse studies comparing the efÞcacy of E. formosa and E. eremicus in season-long summer and winter trials are currently being conducted to conÞrm our Þndings in environmental chamber studies. These Þndings should provide useful information to growers, as to the most appropriate parasitoid to use in varying seasons (i.e., summer versus winter). Further research is required to determine the appropriate release rates of these biological control agents under varying seasons, and the determination of threshold and optimal levels of light intensity for parasitoid activity and effective biological control, respectively.

Acknowledgments We thank E. Armstrong, A. Freeman, G. Mosey, K. Wang, K. Wharram, M. WhitÞeld, and Y. Zhang for technical assistance; G. Bergman, E. Berecz, J. Goerzen, S. Johnson, and J. Myer for crop production assistance; R. Buitenhuis and R. Van Driesche for reviewing an earlier version of the manuscript; and the subject editor and two anonymous reviewers for valuable comments that improved the quality of the manuscript. Y. Zhang provided technical expertise on light intensity and spectral photon ßux measurements. E. eremicus and E. formosa were obtained from BioBest Canada, Leamington, Canada. Research was carried out at the Agriculture and Agri-Food Canada, Greenhouse and Processing Crops Research Centre in Harrow, Canada. This research was supported by the NSERC Biocontrol Network, Ontario Greenhouse Vegetable Growers, and the Matching Investment Initiative of Agriculture and Agri-Food Canada.

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