Download Running title: Effects of intercropping on arthropod community

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Running title: Effects of intercropping on arthropod community
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Effects of intercropping systems on community composition and diversity of
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predatory arthropods in vegetable fields
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Cai Hongjiao1, You Minsheng2*, Lin Cui2
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Fisheries College of Jimei University, Xiamen, Fujian 361021, PR China, Phone:
+86-13365921679, Email: [email protected]
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Institute of Applied Ecology, Research Centre for Biodiversity and Eco-Safety, Fujian
Agriculture and Forestry University, Fuzhou 350002, PR China; Phone: (591) 8378-9396;
Fax: (591) 8376-8251; E-mail: [email protected]
*Corresponding author
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Abstract: Field trials were carried out on Langqi Island, Fujian, P. R. China in 2004, to
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determine the effects of intercropping Chinese cabbage (Brassica chinensis) with green
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cabbage (Brassica oleracea), garlic (Allium sativum) and lettuce (Lactuca sativa), on
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community composition and diversity of predatory arthropods in vegetable fields. Two
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intercropping plots were designed and used in this study. In plot 1, two ridges of Chinese
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cabbage were intercropped with one ridge of garlic (CG1), lettuce (CL1) or green cabbage
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(CB1). In plot 2, the Chinese cabbage was planted in the center (100 cm wide) of the ridge,
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and under-sown with garlic (CG2), lettuce (CL2) or green cabbage (CB2) on both edges (25
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cm wide) of the same ridge. A monoculture plot of the Chinese cabbage (CK) was arranged
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for comparison with plots 1 and 2. The highest species richness was found in CG1, and the
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lowest in CK. The highest abundance was found in CL1 (141.67 predators/plot), whereas the
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lowest was in CB1 (97.67 predators/plot). With the exception of CL1, significantly higher
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diversity indices were found in intercropping treatments than in CK. The majority of spiders
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sampled from fields were from families Theridiidae (34.04%) and Lycosidae (30.57%).
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These findings suggest that Chinese cabbage intercropped with non-cruciferous crops might
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increase species richness, abundance and diversity of the arthropod community in general
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and predators in particular.
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Key words: intercropping, Chinese cabbage, arthropod community, predators
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Intercropping as one form of polyculture is commonly used in tropical parts of the
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world and by indigenous peoples throughout the world[1]. Many findings suggest that
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intercropping encourages biodiversity or abundance of natural enemies, such as spiders or
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parasitoids
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improves nitrogen fixation[8]. Therefore, many ecologists and entomologists advocate
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intercropping in integrated pest management systems for suppression of insect pests [2, 10, 11].
[2, 3, 4]
, increases the crop yield and quality[5, 6], reduces soil erosion[7] and
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Studies on natural enemy dispersal and colonization influenced by cropping systems
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indicate that entomophagous insects depend on ground cover such as that provided by
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intercropping systems. For example, increased botanical diversity generally enhances
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abundance of ground predators, such as carabids, staphylinidae and lycosid spiders [12, 13].
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Higher populations of predators were found in both cotton-maize and peanut-corn
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intercropping systems [2, 10]. Neighboring crops provide alternative foods, prey and refuges
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for predators and parasitoids, thereby increasing natural enemy abundance and colonization
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[2, 10]
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appears to have a potential application for producing commercially acceptable vegetables
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while reducing insecticide uses [14, 15]. However, good documentation is lacking on the effects
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of intercropping on the arthropod community in vegetable fields.
. Therefore, under-sowing or intercropping Brassicas with non-cruciferous crops
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This paper deals mainly with the effect of intercropping of Chinese cabbage with green
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cabbage, garlic and lettuce on community composition and predators of arthropods in
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vegetable fields, and is aimed at elucidating manipulation of biodiversity to support
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conservation
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agroecosystems.
of
natural
enemies
and
ecologically-based
pest
management
in
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1 Materials and methods
1.1 Study site
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This study was conducted on Langqi Island in Fuzhou, Fujian, P.R. China from October
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to December, 2004. Lanqi Island (E119 ° 18′, N26 ° 05′), with an area of 92 km2 and 26 km2
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of arable land, is located in the mouth of Min River where the river feeds into the Pacific
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Ocean (Fig. 1). Forest covers approximately 20% of the island, which possesses a typically
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subtropical climate. The area receives an annual rainfall of 900-2100 mm, and has an
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average annual temperature of 19℃. It is one of the largest vegetable production areas in
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Fujian province owing to the mild climate, fertile soil and abundance of fresh water.
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1.2 Experimental design
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Garlic (Allium sativum L.) and green cabbage (Brassica oleracea L. var. capitata) seeds
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were obtained from the Vegetable Research Institute of Fuzhou and lettuce (Lactuca sativa L.)
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seeds from Choi Hing Lee Seed Company Limited as the accompanying plants for
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intercropping with Chinese cabbage (Brassica. chinensis L.); the seeds for the Chinese
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cabbage were from Choi Hing Lee Seed Company Limited. The plant density was
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approximately 30 plants /m2.
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The experimental design for the intercropping experiments was a split-plot with three
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replicates on a 14x18m plot. Each plot was divided into eight ridges of raised beds
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(14x1.5m2) with a 50cm space between ridges. Seven Chinese cabbage cropping systems
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were examined and evaluated (Table 1). Since the Chinese cabbage grows faster with a
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shorter growing period than the accompanying plants, the garlic, lettuce and green cabbages
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for the intercropping sites were sown two weeks earlier than the Chinese cabbage.
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1.3 Sampling methods
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Ten samples (0.11 m2 /sample) were randomly selected in each plot of Chinese cabbage at
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an interval of four days, and a total of eight samplings from Oct 27 to Nov 24, 2004 were
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conducted during the experimentation beginning 7 days after the Chinese cabbage was sown.
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Arthropods were captured using a vacuum suction sampler [16], which collects all insects,
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including tiny acarids. The arthropods collected were placed in a 75% ethanol solution in
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glass vials and brought to the laboratory, where they were identified and counted with the
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aid of a microscope.
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1.4 Data analysis
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To quantitatively analyze the effects of intercropping on predatory spiders, some currently
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used indices of community ecology were used. They include the diversity index of
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Shannon-Wiener (H’) [17], evenness index (E’) [18], and Simpson's dominant index (C’) [19]. The
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standard dominance degrees (D’) in each group or habitat were also determined [20].
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Community indicators involve the total number of orders, families, genera, species and
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individuals, and were calculated for each plot. The richness, abundance, diversity, evenness
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and dominance indices calculated for each plot were subjected to analysis of variance
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(ANOVA). Each group of herbivores, predators, parasitoids and neutral insects were
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independently compared among the different plots based on the community indicators. The
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dynamics of species richness and abundance, as well as the population abundance of
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dominant species, were also presented. All analyses were conducted using the DPS software
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[21]
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2 Results
2.1 Structure and composition of the community
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A total of 53,392 individuals were collected, belonging to the Insecta, Arachnida and
Millipedes, and representing 98 families, 111 genera and a total of 175 species.
Based on the ecological functions and feeding habits in the communities, the species
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were classified into four groups: predators, parasitoids, herbivores and neutral insects that
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are neither pests nor beneficial species [20].
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More predatory species (59 species, accounting for 33.71% of the total species) were
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found than any other group. Neutral insects accounted for 27 species (15.43%); herbivores
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were 45 species (25.71%); parasitoids were 44 species (15.14%) (Table 2). Hence, the
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percentage of natural enemies, consisting of predators and parasitoids, accounted for 48.85%
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of the total species. However, neutral insects were more abundant than the other groups. The
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abundance of different groups can be ranked by neutral insects (accounting for 62.13% of
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the total number of individuals), herbivores (27.24%), predators (6.79%), and parasitoids
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(3.84%) (Table 2).
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2.2 Species diversity of the predatory group
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The species richness in the predatory group was the highest in CG1 and lowest in CK.
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The other treatments of intercropping had similar numbers of species, ranging from 23 to 29
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in CG2, CL1, CL2, CB1, and CB2 (Table 3).
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The highest number of predatory individuals was found in CL1 (141.67 predators/plot),
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followed by CG1, whereas the lowest was in CB1 (97.67 predators/plot). There was no
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significant difference (P>0.05) among CG2, CL2, CB1, CB2 and CK; however, a
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significantly (P≤0.05) higher predatory population was found in CL1 than in other treatments,
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except for CG1 (Table 3).
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In general, for the predatory group, significantly (P≤0.05) higher diversity indices were
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observed in the intercropping systems, except for CL1, than in the monoculture system (Table
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3). In particular, the highest value was obtained in CB1, whereas the lowest was in CK. No
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significant (P>0.05) difference was observed between CL1 and CK (Table 3).
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The evenness indices of the predatory group of all cropping systems were very similar,
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with a value of approximately 0.80. Only CL1 presented a value below 0.74. Statistical
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analysis revealed a significantly (P≤0.05) lower evenness index in CL1 than in the other
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systems. No significant difference (P>0.05) was found between CB1 and CB2. The evenness
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indices in CG1, CG2, CL2 and CK were the same at 0.80 (Table 3).
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The index of dominance, estimated for the predatory group by Simpson’s method (1949),
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was highest in CL1 and CK treatments. The lowest occurred in CB1 and CB2 (Table 3). The
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ANOVA indicated that CG1, CG2 and CL2 had significantly (P≤0.05) lower dominance
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indices than CL1 and CK (Table 3).
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Overall, higher species richness was observed in the intercropping systems than in
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monoculture (Fig. 2). The peak occurred from day 20 to day 36 varying from 16 to 19
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predatory species (Fig. 2). A similar trend occurred in CG1 and CL2, where the seasonal
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trend was characterized by a slow increase and a subsequent sharp drop (Fig. 2). The species
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richness of CK consistently increased throughout the growing season (Fig. 2).
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On day 12, a distinctly higher abundance of predators was observed in CL1, as compared
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to CK (Fig. 3). The seasonal trends were generally similar in CG1 and CG2, and peaked on
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day 24. During day 16 and day 32, higher abundance was found in the intercropping systems
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CG1, CG2 and CB2 than CK. A continuous increase was observed in CK over the duration of
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the growing season (Fig 3).
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2.3 Seasonal trends of major predators
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The most abundant predators found in our study were spiders of the family Theridiidae,
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followed by the wolf spider of the family Lycosidae. Of the total complex of predators,
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34.04% was from the family Theridiidae, and 30.57% from Lycosidae.
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Despite a peak population of wolf spider collected in CL1 during the seedling period,
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similar trends occurred in all cropping systems from day 20 to day 36 (Fig.4). During the
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growing season, the population levels of spiders in CG1, CL1, and CB1 were higher than in
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CK with a consistent increase in abundance (Fig. 4). Meanwhile, a similar trend with slight
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increases in abundance over the season occurred in other trials, including CG2, CL2, CB2
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and CK (Fig. 4).
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Few spiders from the family of Theridiidae were collected from day 8 to day 16, and
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the population levels started to increase after day 20 (Fig. 5). Similar trends occurred in CG1,
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CG2, CB1 and CL2 with the characteristic initial increase followed by a sharp drop and an
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increase (Fig. 5). On the other hand, consistent growing trends were found in CL1 and CK.
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Both CL1 and CK populations of spiders peaked on day 36, and their seasonal trends
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seemed to be synchronized throughout the season (Fig. 5).
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3 Discussion
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Higher species richness and diversity indices of predators were found in intercropping
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systems than in monocultures. Similar results were also found by Munyuli et al.[22, 23]. They
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reported that higher abundance of predators (e.g., Coccinellidae, Staphylinidae, Syrphidae,
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Anthocoridae, Mantidae, Dermaptera, ground beetle, predatory mite, lygaeid bugs,
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anthocoridae, dragonflies and spiders) and higher index of diversity were observed to be
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associated with cowpea/green gram systems. In light of the “nature enemy hypothesis”,
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intercropping can ensure the spatial and temporal availability of resources for predators and
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thereby increase predator diversity.
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In our study, spiders were the dominant predators, and most of them belonged to the
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family Theridiidae (34.04% of the total numbers of individuals in the predator group) and
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family Lycosidae (30.57%) in intercropping vegetable fields. Sunderland and Samu
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reviewed the literature regarding the influences of plant diversification on spider abundance.
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They found that diversification increased spider abundance by in 63% of the studies involving
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intercropping, non-crop strips, under-sowing or partial weediness. Similarly, spiders were
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dominant taxa in paddy fields [25], forest-floor systems [26], tea gardens [27] and cotton fields [28].
[24]
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Comparing spider abundance in different plots, we found that more wolf spiders were
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observed in the intercropping systems than in the monoculture, which implies that the
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increased abundance of spiders might be related to crop species composition and plant density.
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Moreover, predators might respond more to habitat type than prey density [29, 30]. Habitat types
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are related to the plant species. Van Emden stated that plant diversity provides sufficient
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alternative prey for the generalist predators to establish their populations within a crop before
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the arrival and seasonal increase of pests [31]. Crop diversification in terms of heterogeneity
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and weediness as well as by intercropping and the presence of the field boundaries could also
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enhance field predator assemblages [32]. For example, a significantly greater number of ground
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predators (e.g., Carabidae, Staphylinidae and spiders) was caught in the weedy and clover
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plots than in the clean cultivated plots [12].
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Although there have been many studies on the effects of botanical diversity on predators,
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current researchers focus more on the surrounding habitats (such as field margins, weed strips
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and no-tillage production systems[33, 34, 35, 36, 37] ) than on intercropping regimes. The difference
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in crop structure and culture practices, including habitat diversification and the provision of
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ground cover, could affect spider density and community composition [29, 38]. Future studies
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are required to test the underlying principles of predatory efficiency in intercropped systems.
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Acknowledgments
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The study was supported by the Fujian Provincial Science and Technology Foundation
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through project 2002N007, entitled “Bio-community Diversity in Vegetable Fields and
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Sustainable Control of Dominant Insect Pests”. We greatly thank Prof. Huang Jin-Zhi, Dr.
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Shiyou Li and Dr. Krista Ryall for their help in revising the manuscript.
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Fig. 1 Map of Langqi Island. The noted astral symbol is the trial site on the
island.
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1
A
Mean No.of Species
25
20
CG1
15
CG2
10
CK
5
0
0
8
12
16
20
24
No.Day after seeding
28
32
36
Mean No.of Species
25
20
B
CL1
15
CL2
10
CK
5
0
0
8
12
16
20
24
No.Day after seeding
28
32
36
Mean No.of Species
25
C
20
CB1
15
CB2
10
CK
5
0
0
8
12
16
20
24
No.Day after seeding
28
32
36
Fig. 2 Effects of Chinese cabbage monoculture (CK) and Chinese cabbage
intercropping with garlic (CG1), lettuce (CL) or green cabbage (CB) on seasonal
trends of species richness2 in predatory guild at Langqi in 2004.
1.
The plot planting design for the intercropping applied 2 ridges planted with Chinese
cabbage and 1 ridge of accompanying plants, either garlic (CG1), lettuce (CL1) or green
cabbage (CB1). The second design under-sowed Chinese cabbage in the middle (100 cm
wide) and planted garlic (CG2), lettuce (CL2) or green cabbage (CB2) on both edges (25
cm wide) in a ridge.
2. Means calculated from a total 30 samples per treatment and the control with 3 replicates
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1
Mean number of individual
2
120
A
100
80
CG1
60
CG2
CK
40
20
0
Mean number of individual
0
8
16
20
24
28
No.Day after seeding
32
36
120
100
80
CL1
60
CL2
CK
40
20
0
0
Mean number of individual
12
8
12
16
20
24
28
No.Day after seeding
32
36
120
100
B
80
CB1
CB2
60
CK
40
20
0
0
8
12
16
20
24
28
No.Day after seeding
32
36
Fig.3 Effects of Chinese cabbage monoculture (CK) and Chinese cabbage
intercropping with garlic (CG1), lettuce (CL) or green cabbage (CB) on seasonal
trends of species abundance2 in predatory guild at Langqi in 2004.
1.
The plot planting design for the intercropping applied 2 ridges planted with Chinese cabbage
and 1 ridge of accompanying plants, either garlic (CG1), lettuce (CL1) or green cabbage
(CB1). The second design under-sowed Chinese cabbage in the middle (100 cm wide) and
planted garlic (CG2), lettuce (CL2) or green cabbage (CB2) on both edges (25 cm wide) in a
ridge.
2. Means calculated from a total 30 samples per treatment and the control with 3 replicates.
C
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Mean No. of Individuals
1
80
70
60
50
40
30
20
10
0
A
CG1
CG2
CK
0
8
16
20
24
28
No.Day after seeding
32
36
B
100
Mean No. of Individuals
12
80
CL1
60
CL2
40
CK
20
0
0
8
12
16
20
24
28
No.Day after seeding
32
36
Mean No. of Individuals
60
C
50
40
CB1
30
CB2
CK
20
10
0
0
8
12
16
20
24
28
No.Day after seeding
32
36
Fig. 4 Fluctuation of population abundance2 in wolf spiders (Lycosida family) as
affected by Chinese cabbage monoculture (CK) and Chinese cabbage intercropping
with garlic (CG1), lettuce (CL) or green cabbage (CB) at Langqi in 2004.
1.
The plot planting design for the intercropping applied 2 ridges planted with Chinese cabbage and
1 ridge of accompanying plants, either garlic (CG1), lettuce (CL1) or green cabbage (CB1). The
second design under-sowed Chinese cabbage in the middle (100 cm wide) and planted garlic
(CG2), lettuce (CL2) or green cabbage (CB2) on both edges (25 cm wide) in a ridge.
2. Means calculated from a total 30 samples per treatment and the control with 3 replicates.
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1
Mean No. of individual
60
A
50
40
CG1
30
CG2
20
CK
10
0
0
8
Mean No. of individual
70
12
16
20
24
28
No.Day after seeding
32
36
B
60
50
CL1
40
CL2
30
CK
20
10
0
0
8
12
16
20
24
No.Day after seeding
28
32
36
Mean No. of individual
60
C
50
40
CB1
30
CB2
CK
20
10
0
0
8
12
16
20
24
No.Day after seeding
28
32
36
Fig. 5 Fluctuation of population abundance2 in spiders from Theridiidae family as
affected by Chinese cabbage monoculture (CK) and Chinese cabbage intercropping
with garlic (CG1), lettuce (CL) or green cabbage (CB) at Langqi in 2004.
1.
The plot planting design for the intercropping applied 2 ridges planted with Chinese cabbage and
1 ridge of accompanying plants, either garlic (CG1), lettuce (CL1) or green cabbage (CB1). The
second design under-sowed Chinese cabbage in the middle (100 cm wide) and planted garlic
(CG2), lettuce (CL2) or green cabbage (CB2) on both edges (25 cm wide) in a ridge.
2. Means calculated from a total 30 samples per treatment and the control with 3 replicates.
2
18
1
Table 1 Plot design of the intercropping experimentation in Langqi.
Replicate 1
Replicate 2
Replicate 3
CG1
CK
CL1
CG2
CK
CL2
CB1
CG2
CB2
CB2
CG1
CB1
CK
CL1
CK
CK
CL2
CK
CL2
CB2
CG2
CL1
CB1
CG1
2
3
Habitat
Control (CK)
Chinese cabbage-garlic system 1
(CG1)
Chinese cabbage-garlic system 2
(CG2)
Chinese cabbage-lettuce system
1(CL1)
Chinese cabbage-lettuce system
2(CL2)
Chinese cabbage-green cabbage
system 1 (CB1)
Planting model
All ridges planted with the Chinese cabbages
2 ridges planted with the Chinese cabbages and intercropped
with 1 ridge of garlic in-between
Garlic planted on both edges (25 cm wide) and the Chinese
cabbages under-sowed in the middle (100 cm wide) of a ridge
2 ridges planted with the Chinese cabbages and intercropped
with 1 ridge of lettuce in-between
Lettuce planted on both edges (25 cm wide) and the Chinese
cabbages under-sowed in the middle (100 cm wide) of a ridge
2 ridges planted with the Chinese cabbages and intercropped
with 1 ridge of green cabbages in-between
Green cabbages planted on both edges (25 cm wide) and the
Chinese cabbage- green cabbage
Chinese cabbages under-sowed in the middle (100 cm wide) of
system 2 (CB2)
a ridge
4
19
1
Table 2 Species richness and abundance for each insect guild including neutral
2
insects, herbivores, predators and parasitoids in Chinese cabbage field at
3
Langqi, China, 2004.
Total species
Total abundance
Numbers of
Percentage
Numbers of
Percentage
species
(%)
individuals
(%)
Neutral insects
27
15.43
33174
62.13
Herbivores
45
25.71
14543
27.24
Predators
59
33.71
3627
6.79
Parasitoids
44
25.14
2048
3.84
Total
175
100.00
53392
100.00
4
5
20
1
2
Table 3 Species richness, abundance, diversity index, evenness index and
3
dominant degree calculated for the predatory guild in Chinese cabbage fields at
4
Langqi in 2004.
Total
Treatment
Mean No. of
Mean Diversity
Mean Evenness
Mean Simpson's
individuals
index (H’)
index (E)
dominance index (C)
number of
Species
CG11
33
132.67+8.02ab3
3.38+0.04 bc
0.80+0.03b
0.13+0.01b
CG2
25
100.00+3.61c
3.37+0.05 bc
0.80+0.02b
0.14+0.01b
CL1
29
141.67+4.73a
3.30+0.05 cd
0.74+0.03c
0.17+0.01a
CL2
23
105.33+3.61bc
3.35+0.09 bc
0.80+0.02b
0.14+0.01b
CB1
25
97.67+20.21c
3.78+0.38 a
0.89+0.02a
0.09+0.01c
CB2
24
116.00+23.00abc
3.65+0.13 ab
0.88+0.01a
0.10+0.01c
CK2
19
101.33+7.51bc
2.99+0.23 d
0.80+0.03b
0.17+0.02a
5
1. The plot planting design for the intercropping applied 2 ridges planted with
6
Chinese cabbage and 1 ridge of accompanying plants, either garlic (CG1), lettuce
7
(CL1) or green cabbage (CB1). The second design under-sowed Chinese cabbage
8
in the middle (100 cm wide) and planted garlic (CG2), lettuce (CL2) or green
9
cabbage (CB2) on both edges (25 cm wide) in a ridge.
10
2. The monoculture of Chinese cabbage was as the control (CK) with 3 replicates.
11
3. Means calculated from a total 30 samples per treatment with 3 replicates. Within
12
each column, means +SD marked by the same letters are not significantly different
13
at 5% level of significance, as determined by ANOVA Duncan’s test.
14
15
21