Field experiments revealed that presence of spiders increased mortality of butterfly eggs and improved brocolli yield

Hooks, Cerruti R2

Johnson, Marshall W.

Pandey, Raju R.

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Insect PestsMay 2007IP-25Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in coopera-tion with the U.S. Department of Agriculture. Andrew G. Hashimoto, Director/Dean, Cooperative Extension Service/CTAHR, University of Hawai‘i at Mänoa, Honolulu, Hawai‘i 96822. An equal opportunity/affirmative action institution providing programs and services to the people of Hawai‘i without regard to race, sex, age, religion, color, national origin, ancestry, dis-ability, marital status, arrest and court record, sexual orientation, or status as a covered veteran. CTAHR publications can be found on the Web site <http://www.ctahr.hawaii.edu/freepubs>.Evaluating Spiders for Their PotentialTo Control Cabbage White Butterflies (Pieris rapae)Cerruti R2 Hooks,a Raju R. Pandey,b and Marshall W. JohnsoncaCTAHR Department of Plant and Environmental Protection Sciences; bHimalayan College of Agricultural Sciences and Technology, Kathmandu, Nepal; cDepartment of Entomology, University of California, RiversideSummaryA field experiment was conducted three times during two seasons (twice in winter and once in spring) to evaluate the impact of spiders on the survival of the cab-bage white butterfly, Pieris rapae (= Artogeia rapae). The proportion of P. rapae eggs surviving to the first caterpillar stage was significantly reduced on spider treatment plants compared to check treatment plants. During the three experiments, the percentage of P. rapae eggs surviving to the fifth caterpillar stage was increased 1.7-, 2.7-, and 1.3-fold, respectively, on check plants compared to spider plants. Additionally, by completion of the the experiment, above-ground plant biomass of spider-“protected” plants was increased by 80, 121, and 28 percent compared to check plants.IntroductionAlthough several studies have shown that spiders can significantly reduce insect pest populations and the as-sociated crop damage (Agnew and Smith 1989, Hooks et al. 2003), their ability to suppress insect pest popula-tions and enhance plant productivity has received limited attention in cropping systems. On several occasions, spiders were observed feeding on eggs and caterpillars of lepidoptera prey inhabiting broccoli (Brassica olearacea L.) plants, and although their densities were recorded, no attempt was made to quantify their predatory impact (Hooks and Johnson 2002).  Hooks et al. (2003) found significantly fewer large P. rapae caterpillars on plants where spiders were allowed to forage freely, compared to control plants in which spiders were removed daily. However, during that study the amount of mortality spiders inflicted upon P. rapae was not estimated. There-fore, field experiments reported here were conducted to quantify the impact of spiders on P. rapae’s survivor-ship and broccoli plant biomass. The objective of this study was to address two questions: (1) Does caterpil-lar survival differ on plants containing spiders? (2) Do spiders indirectly increase broccoli plant size through suppression of caterpillars?ProceduresExperimental designThree field trials were conducted to assess the impact of spiders on P. rapae’s survival. Experiments were conducted during 2003 and 2004 at the University of Hawai‘i at Mänoa’s Poamoho Research Station. For each trial, 5-week-old greenhouse-grown broccoli plantlets were transplanted and randomly assigned to two treat-ments: (1) spiders present, and (2) a check (spiders re-moved). Twelve plants were assigned to each treatment during each trial period.  Spiders were removed daily from check treatment plants at 10:00 and 13:30 during the duration of each trial. Immediately after the 13:30 removal, a sleeve cage constructed of a transparent fabric was gently placed over each check plant to prevent spiders from foraging them. During the initial 16 days after planting (DAP), the cages were removed from the plants daily from 09:00 until 13:30 to allow oviposition on the test plants by P. rapae.UH–CTAHR  IP-25 — May 20072SamplingThe proportion of P. rapae eggs reaching the first caterpillar stage was assessed during spring 2003 and winter 2004. Twelve plants were randomly selected and assigned to the check or spider treatment for each trial. All P. rapae eggs found on these plants during the initial 16 DAP were counted and recorded, and their location was marked with a permanent marker. Each egg was checked daily to determine if it reached the caterpillar stage. At the experiment’s completion, the percentages of the 16-day egg cohort reaching the caterpillar stage were calculated for each treatment. The percentage of P. rapae eggs reaching their final (fifth) caterpillar stage was measured on 12 additional randomly selected check and spider plants during three field trials (i.e., winter and spring 2003, and winter 2004), respectively. Similarly, as mentioned above, P. rapae eggs laid during the initial 16 DAP were monitored to determine the percentage reaching their final caterpillar stage. To determine if whole-plant biomass differed between check and spider plants, upon the experiment’s comple-tion test plants from each treatment were cut at soil level, transported to the laboratory, oven-dried, and weighed to measure above-ground dry vegetative biomass.Statistical analysisTreatment effects on the number of eggs oviposited, plant weight, and percentage of eggs reaching the first and final caterpillar stages were assessed using analysis of variance (Proc GLM, SAS Institute, Cary, NC 1990). To fulfill assumptions regarding normality and equal variances, data were transformed when necessary.ResultsSpidersOver the course of the trial, spiders removed from the check plants included Nesticodes rufipes, Oxyopes sp., Cheiracanthium mordax Koch, Neoscona oaxacensis Keyserling, and an unidentified linyphiid. The average number of spiders found per broccoli leaf during the three trials ranged from 0.25 to 0.69, 0.17 to 0.60, and 0.17 to 0.46 for the winter 2003, spring 2003, and winter 2004 trials, respectively. During each trial the number of spiders found per plant increased during the broccoli growth cycle. Pieris rapaeDuring the spring egg mortality experiment, the pro-portion of P. rapae eggs reaching their first caterpillar stage was significantly higher on check (83%) com-pared to spider (56.5%) treatment plants. However, no significant differences were found during the winter experiment (Fig. 1). For all three trials, the percentage of P. rapae eggs reaching their final caterpillar stage was significantly reduced on spider plants compared to check plants (Fig. 2).Whole plant biomassDuring the 2003 trials, the average plant weight was sig-nificantly greater for spider plants than for check plants (Fig. 3). During the 2004 winter experiment, spider plants were larger than check plants, but the difference was not statistically significant.DiscussionDuring one of the two field trials, the percentage of eggs reaching the first caterpillar stage was significantly lower on spider treatment plants compared to check plants. Furthermore, the proportion of eggs reaching the fifth instar stage was significantly lower on spider plants compared to check plants during all three field trials. Broccoli whole-plant biomass of spider plants was significantly greater than that of check plants during the first two field experiments.  Spiders were rarely observed feeding on P. rapae eggs, but the results suggest that spiders had a significant impact on P. rapae egg mortality. A number of cabbage looper (Trichoplusia ni) eggs were also encountered on spider plants during the spring, but no larva of this species was observed during the trial, suggesting that spiders also fed on T. ni eggs. The wandering spider (Oxyopes sp.) appears to be the most important spider for suppressing populations of P. rapae and T. ni. Sup-pression of P. rapae was greatest when populations of Oxyopes sp. were high and least when they were low.AcknowledgmentThe authors wish to thank the crew at the Poamoho Re-search Station for their valuable help in the field. This research was partially funded by the USDA/CSREES, Special Grant for Tropical and Subtropical Agriculture Research (T-STAR).3UH–CTAHR  IP-25 — May 2007Figure 3. Average (± S.E.) dry whole-plant biomass on spider-removed and spider-present treatment plants during three field trials. Different letters above a bar indicate that treatments are significantly different at the 5% level (P < 0.05).Figure 2. Percentage (± S.E.) of Pieris rapae eggs to reach the fifth caterpillar stage on spider-removed and spider-present treatment plants during three field trials. Different letters above a bar for each trial indicate that treatments are significantly different at the 5% level (P < 0.05).Figure 1. Percentage (± S.E.) of Pieris rapae eggs reaching the first caterpillar stage on spider-removed and spider-present treatment plants during two field trials. Different letters above a bar for each trial indicate that treatments are significantly different at the 5% level (P < 0.05).Fig. 3. Winter 2003 Spring 2003 Winter 2004Dry weight/broccoli plant (g)010203040506070Spider removalSpider present31.256.321.015.39.5 11.9ababBrocolli dry weight per plant (g)Fig. 1. Spring 2003 Winter 2004Percentage of A. rapaeeggs to reach 1st instar020406080100Spider removalSpider presentab83.056.589.179.3Fig. 2.Winter 2003 Spring 2003 Winter 2004Percentage of A. rapaeeggs to reach 5th instar020406080Spider removalSpider presentababab66.338.872.352.419.256.2Percentage of P. rapae eggs to reach 5th instarFig. 1. Spring 2003 Winter 2004Percentage of A. rapaeeggs to reach 1st instar020406080100i r removalSpider presentab83.056.589.179.3Fig. 2.inter 2003 Spring 2003 inter 2004Percentage of A. rapaeeggs to reach 5th instar020406080Spider removalSpider presentababab66.338.872.352.419.256.2Percentage of P. rapae eggs to reach 1st instarUH–CTAHR  IP-25 — May 20074References and further readingsAgnew, C.W., and J.W. Smith Jr. 1989. Ecology of spi-ders (Araneae) in a peanut agroecosystem. Environ. Entomol. 7: 402–404.Hooks, C.R.R., and M.W. Johnson. 2002. Lepidopteran pest populations and crop yields in row intercropped broccoli. Agric. For. Entomol. 4: 117–125.Hooks, C.R.R., R.R. Pandey, and M.W. Johnson. 2003. Impact of avian and arthropod predation on lepi-dopteran caterpillar densities and plant  productivity in an ephemeral agroecosystem. Ecol. Entomol. 28: 522–532.Hooks, C.R.R., R.R. Pandey, and M.W. Johnson. 2006. Effects of spider presence on Artogeia rapae and host plant biomass. Agri. Ecosys.  Environ. 112: 73–77.

Evaluating Spiders for their Potential to Control Cabbage White Butterflies (Pieris rapae)

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Evaluating Spiders for their Potential to Control Cabbage White Butterflies (Pieris rapae)

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