Control of Cabbage Lepidoptera by Naturally Occurring Arthropod Predators
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Control of cabbage Lepidoptera by naturally occurring arthropod predators M. A. Schmaedick1, A. M. Shelton1, and M. P. Hoffmann2 1Department of Entomology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456, U.S.A. 2Department of Entomology, Cornell University, Ithaca, NY 14853, U.S.A. Abstract In recent years, pesticide resistance in some lepidopteran species, along with a general desire to reduce dependence on chemical pesticides, have led to increased interest in evaluating and enhancing the effects of naturally occurring biological control agents on the crucifer Lepidoptera. Due to the relative ease of documenting the impact of parasitoids on pest populations, however, virtually all the research on natural enemies have focused on parasitoids, while the role of predators has remained largely unexplored. Our work evaluating the effects of predatory arthropods on Pieris rapae in cabbage shows that generalist predators are responsible for much of the mortality of this pest in our area. We describe predator exclusion experiments and laboratory predation assays that demonstrate the important role of these natural enemies in reducing populations of P. rapae. As generalist feeders, these predators undoubtedly affect populations of other crucifer Lepidoptera as well and have the potential to survive in the crop at low pest densities by feeding on alternative prey. These attributes, along with their propensity to destroy the early stages of pests, make these predators potential key players in efforts to increase reliance on non chemical methods for managing crucifer Lepidoptera. Key words: Pieris rapae, predation, cabbage Introduction Materials and Methods Occurrence of insecticide resistance in populations of Estimation of mortality from arthropod predation lepidopteran pests of cabbage, along with a desire to Mortality of P. rapae eggs and larvae due to arthropod reduce use of broad spectrum insecticides, have led to predators in cabbage fields was estimated by increased interest in ecologically based management comparing survivorship on protected and exposed strategies. Improved understanding of the natural plants infested with known numbers of P. rapae eggs. mortality factors affecting populations of Lepidoptera The experiment was conducted in two 0.2 ha cabbage in cabbage is critical to development of such strategies. fields planted with “Vantage Point” seedlings on 2–5 While numerous studies have addressed the role of June 1995. An additional 120 seedlings were planted insect parasitoids in controlling crucifer Lepidoptera, into 30.5 cm diam. plastic pots and placed in an much less is known about the effects of predators outdoor screenhouse. The seedlings were entirely (Jones, 1981; Talekar and Shelton, 1993). Pieris rapae enclosed by no-see-um mesh bags (Balson-Hercules L.(Lepidoptera: Pieridae) is one of the most important Group, Ltd., Providence, RI, U.S.A.). The mesh bags pests of crucifer vegetables (e.g., cabbage, broccoli, lined the pots and were supported above the plant by cauliflower) in New York State and many other 46 cm high (2.5 cm mesh) wire net cylinders resting crucifer growing areas worldwide. We describe initial on the soil surface in the pots and anchored by two results of our efforts to estimate the overall mortality short bamboo stakes. The bags were tied at the top to of P. rapae eggs and larvae due to arthropod predation exclude arthropods yet allow access for sampling. and to determine the predator species causing the On 11 July the potted plants were placed in an mortality. Overall mortality was estimated by enclosed screenhouse, the bags opened, and adult P. comparing P. rapae survivorship on plants from which rapae from a laboratory culture released and allowed predators were excluded to that on plants to which to oviposit for 24 h. Sixty of the plants were then arthropod predators were allowed access. Species placed in each of the two cabbage fields, replacing preying on P. rapae were determined by use of sticky every fifteenth plant in every third row, forming a 30.5 traps to ascertain predator species that occur on x 24.7 m grid in the plot center. The pots were cabbage plants followed by laboratory predation assays embedded to just below the soil surface in the plots. to learn which of these species feed on P. rapae early On each plant the locations of five well-spaced eggs stages. on the leaf undersides were marked by drawing a circle on the leaf’s upper surface with a permanent marker. Any additional eggs were removed. Plants at alternate points of the grid were designated as sham cage plants. On 14 July the lower 15 cm of the above-ground 308 Proceedings: The Management of Diamondback Moth and Other Crucifer Pests portion of the mesh bags on these plants was cut away cm length of dental wick. The arenas containing and the upper portion left in place and fastened to the predators were placed in an environmental chamber wire netting cylinder using straight pins. The pot rims at 22:17 (L:D) °C, 60% RH, and 15:9 (L:D) h and and soil inside the pots were then covered with field starved for 24 h before adding either 10 or 20 P. rapae soil to form a continuous surface with the surrounding eggs or five first instars. Eggs and first instars were soil. P. rapae on the plants were counted and any newly used because the exclusion cage experiments indicated oviposited eggs or colonizing aphids were removed that virtually all predation occurred during these stages. on 16, 22, and 26 July. By 26 July most larvae had The eggs were presented on pieces of parafilm cut from reached fifth instar. egg sheets used for oviposition in our P. rapae The plants and P. rapae were removed from the laboratory culture. The larvae were transferred shortly pots on 26 July and replaced later with new plants after hatching from egg sheets to 3.2 cm cabbage leaf (variety “Bravo”) in 10 cm diam. pots bearing five P. disks which were then placed with the predators. After rapae eggs per plant. The five eggs /plantwere obtained an additional 24 h in the environmental chamber, by placing the plants in our lab culture’s oviposition remaining eggs and larvae were counted. A minimum cage for a few minutes apiece until >5 eggs were laid of five control arenas identical to the test arenas except then removing the excess eggs. The smaller pots were without predators were included with each batch of embedded in the soil inside the larger pots that were predators tested. No eggs or larvae were damaged or already in the field such that the soil in the pots formed disappeared in any of the control arenas. a continuous surface. The second set of plants was placed in one of the fields on 2 Aug. and in the other Predation on cabbage plants in the laboratory on 8 Aug. P. rapae were counted and any new P. rapae Three of the four most abundant predator species eggs or aphids were removed from the plants every captured on sticky traps were further tested for 2–3 d until most of the larvae had reached the fifth predation ability on cabbage plants in the laboratory. instar. For each of the four experiments survivorship Predators were collected by hand or by dry pitfall traps curves for the two treatments were compared visually and starved for 24 h as in the small arena assays. to determine stages at which most mortality occurred. Groups of predators were then placed in plastic In addition, 95% confidence intervals for total cylinder cages with screen tops covering small (4–6 mortality due to treatment effect were constructed leaf) cabbage plants that had been transplanted into using Elston’s (1969) method as described by 25 cm diam. plastic pots and infested with ten first Rosenheim and Hoy (1989). instar P. rapae. A moist 2.5 cm length of dental wick was placed on the surface in each pot. Three control Monitoring predators on cabbage plants with P. rapae but no predators were included in Cabbage (variety ‘Cheers’) was transplanted into 0.2 each trial. After 24 h at 22 °C, 40% RH, and 16:8 (L:D) ha plots at three locations ≥1 km apart on 22–26 May h, remaining P. rapae larvae were counted. No larvae 1995. Sticky traps were placed on 36 plants in a 6 x 6 disappeared from any of the control plants in the three grid in the center of each plot on 10 July when most experiments. of the plants were in precupping stage (13–19 leaves). The outermost trap plants were ≥6.4 m from the plot Results edges. The traps consisted of two 8.6 cm rings of insect Estimation of mortality from arthropod predation trap coating (Tanglefoot Co., Grand Rapids, MI, The survivorship curves for the four experiments U.S.A.) pressed onto the upper and lower surfaces showed a reduction in survival of the larvae on plants respectively of two opposite frame leaves of each trap in the sham cages versus those in the predator plant. A plastic “deli container” (32T, Fabri-Kal Corp., exclusion cages. In all cases almost all of the difference Kalamazoo, MI, U.S.A.) with a hole cut out of the arose during the egg or first instar. The bulk of the bottom was used to apply the trap coating to the leaf mortality (52–84%) was common to both cage types, surfaces. The traps were checked every 3–4 d and all apparently arising from factors that acted equally on predaceous arthropods removed and preserved for P. rapae in both treatments. The 95% confidence identification. Each week the coated leaves were intervals for total mortality attributable to the treatment removed and new trap coating applied to leaves of the were 58 ± 22% and 51 ± 21% for the two fields in the same or nearby plants.