Pests May 2007 IP-25

Evaluating Spiders for Their Potential To Control White ( rapae)

Cerruti R2 Hooks,a Raju R. Pandey,b and Marshall W. Johnsonc aCTAHR Department of Plant and Environmental Protection Sciences; bHimalayan College of Agricultural Sciences and Technology, Kathmandu, Nepal; cDepartment of Entomology, University of California, Riverside

Summary However, during that study the amount of mortality A field experiment was conducted three times during spiders inflicted upon P. rapae was not estimated. There- two seasons (twice in winter and once in spring) to fore, field experiments reported here were conducted to evaluate the impact of spiders on the survival of the cab- quantify the impact of spiders on P. rapae’s survivor- bage white , Pieris rapae (= Artogeia rapae). ship and plant biomass. The objective of this The proportion of P. rapae surviving to the first study was to address two questions: (1) Does caterpil- caterpillar stage was significantly reduced on spider lar survival differ on plants containing spiders? (2) Do treatment plants compared to check treatment plants. spiders indirectly increase broccoli plant size through During the three experiments, the percentage of P. rapae suppression of caterpillars? eggs surviving to the fifth caterpillar stage was increased 1.7-, 2.7-, and 1.3-fold, respectively, on check plants Procedures compared to spider plants. Additionally, by completion of the the experiment, above-ground plant biomass of Experimental design spider-“protected” plants was increased by 80, 121, and Three field trials were conducted to assess the impact 28 percent compared to check plants. of spiders on P. rapae’s survival. Experiments were conducted during 2003 and 2004 at the University of Introduction Hawai‘i at Mänoa’s Poamoho Research Station. For each Although several studies have shown that spiders can trial, 5-week-old greenhouse-grown broccoli plantlets significantly reduce insect pest populations and the as- were transplanted and randomly assigned to two treat- sociated crop damage (Agnew and Smith 1989, Hooks ments: (1) spiders present, and (2) a check (spiders re- et al. 2003), their ability to suppress insect pest popula- moved). Twelve plants were assigned to each treatment tions and enhance plant productivity has received limited during each trial period. attention in cropping systems. On several occasions, Spiders were removed daily from check treatment spiders were observed feeding on eggs and caterpillars of plants at 10:00 and 13:30 during the duration of each prey inhabiting broccoli (Brassica olearacea trial. Immediately after the 13:30 removal, a sleeve cage L.) plants, and although their densities were recorded, constructed of a transparent fabric was gently placed no attempt was made to quantify their predatory impact over each check plant to prevent spiders from foraging (Hooks and Johnson 2002). Hooks et al. (2003) found them. During the initial 16 days after planting (DAP), significantly fewer large P. rapae caterpillars on plants the cages were removed from the plants daily from 09:00 where spiders were allowed to forage freely, compared until 13:30 to allow oviposition on the test plants by P. to control plants in which spiders were removed daily. rapae.

Published 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 . UH–CTAHR IP-25 — May 2007

Sampling Pieris rapae The proportion of P. rapae eggs reaching the first During the spring mortality experiment, the pro- caterpillar stage was assessed during spring 2003 and portion of P. rapae eggs reaching their first caterpillar winter 2004. Twelve plants were randomly selected and stage was significantly higher on check (83%) com- assigned to the check or spider treatment for each trial. pared to spider (56.5%) treatment plants. However, no All P. rapae eggs found on these plants during the initial significant differences were found during the winter 16 DAP were counted and recorded, and their location experiment (Fig. 1). For all three trials, the percentage was marked with a permanent marker. Each egg was of P. rapae eggs reaching their final caterpillar stage checked daily to determine if it reached the caterpillar was significantly reduced on spider plants compared to stage. At the experiment’s completion, the percentages check plants (Fig. 2). of the 16-day egg cohort reaching the caterpillar stage were calculated for each treatment. Whole plant biomass The percentage of P. rapae eggs reaching their final During the 2003 trials, the average plant weight was sig- (fifth) caterpillar stage was measured on 12 additional nificantly greater for spider plants than for check plants randomly selected check and spider plants during three (Fig. 3). During the 2004 winter experiment, spider field trials (i.e., winter and spring 2003, and winter 2004), plants were larger than check plants, but the difference respectively. Similarly, as mentioned above, P. rapae was not statistically significant. eggs laid during the initial 16 DAP were monitored to determine the percentage reaching their final caterpillar Discussion stage. During one of the two field trials, the percentage of To determine if whole-plant biomass differed between eggs reaching the first caterpillar stage was significantly check and spider plants, upon the experiment’s comple- lower on spider treatment plants compared to check tion test plants from each treatment were cut at soil level, plants. Furthermore, the proportion of eggs reaching transported to the laboratory, oven-dried, and weighed the fifth instar stage was significantly lower on spider to measure above-ground dry vegetative biomass. plants compared to check plants during all three field trials. Broccoli whole-plant biomass of spider plants was Statistical analysis significantly greater than that of check plants during the Treatment effects on the number of eggs oviposited, first two field experiments. plant weight, and percentage of eggs reaching the first Spiders were rarely observed feeding on P. rapae and final caterpillar stages were assessed using analysis eggs, but the results suggest that spiders had a significant of variance (Proc GLM, SAS Institute, Cary, NC 1990). impact on P. rapae egg mortality. A number of cabbage To fulfill assumptions regarding normality and equal looper (Trichoplusia ni) eggs were also encountered variances, data were transformed when necessary. on spider plants during the spring, but no of this was observed during the trial, suggesting that Results spiders also fed on T. ni eggs. The wandering spider (Oxyopes sp.) appears to be the most important spider Spiders for suppressing populations of P. rapae and T. ni. Sup- Over the course of the trial, spiders removed from the pression of P. rapae was greatest when populations of check plants included Nesticodes rufipes, Oxyopes sp., Oxyopes sp. were high and least when they were low. Cheiracanthium mordax Koch, Neoscona oaxacensis Keyserling, and an unidentified linyphiid. The average Acknowledgment number of spiders found per broccoli leaf during the The authors wish to thank the crew at the Poamoho Re- three trials ranged from 0.25 to 0.69, 0.17 to 0.60, and search Station for their valuable help in the field. This 0.17 to 0.46 for the winter 2003, spring 2003, and winter research was partially funded by the USDA/CSREES, 2004 trials, respectively. During each trial the number Special Grant for Tropical and Subtropical Agriculture of spiders found per plant increased during the broccoli Research (T-STAR). growth cycle.

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Fig. 1. Figure 1. Percentage (± S.E.) of Pieris rapae eggs reaching the first caterpillar stage on spider-removed and spider- Fig. 1. 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). lavomerredipS Spider removal 100 Spi der present Spider present 100 a a

instar 80 st

instar 80

st b 60 instar b

st 60 83.0 89.1 40 83.0 89.1 40 79.3 56.5 79.3 20 56.5

to reach 1 to reach 20 Percentage of A. rapae eggs to reach 1 Percentage of A. rapae eggs to reach 1 0

P. rapae eggs of P. Percentage 0 Spring 2003 Winter 2004 Spring 2003 Winter 2004

Figure 2. Percentage (± S.E.) of Pieris rapae eggs to reach the fifth caterpillar stage on spider-removed and spider- present treatmentFig. 2. plants during three field trials. Different letters above a bar for each trial indicate that treatments are Fig. 2. significantly different at the 5% level (P < 0.05).

Spider removal 80 Spider removal a 80 a Spider present a a Spider present instar

b instar

a b th 60 a th 60 A. rapae A. instar b th

A. rapae of A. b of 4040 72.3 66.366.3 72.3 52.4 b 52.4 b 56.256.2 2020 38.8 to reach 5 to reach 38.8 19.2

Percentage eggs to reach 5 0 Percentage eggs to reach 5 P. rapae eggs of P. Percentage 0 WinterWinter 20032003 Sprin g 2003 WWinterinter 20042004

Figure 3. AverageFig. (± 3. 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% levelP ( < 0.05).

70 b Spider removal 60 Spider present 50

40 a 30 56.3 b 20 31.2 a 10 21.0 9.5 11.9 15.3 Dry weight/broccoli plant (g) plant weight/broccoli Dry Brocolli dry weight per plant (g) Brocolli 0 Winter 2003 Spring 2003 Winter 2004

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References and further readings Agnew, 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 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.

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