Abstract
Simple SummaryIn agroecosystems, predatory ladybeetles play an important role in suppressing aphid populations. How ladybeetles make use of host plant diversity in multicropping landscapes has rarely been documented in China. In this study, we examined the relationship between aphid densities and ladybeetle densities at both the local field and landscape scales. Overall, we found that there was a positive correlation between aphid densities and ladybeetle densities. However, plant taxa had no significant influence on predatory ladybeetle abundance at the local field scale. In addition, the effect of aphids on ladybeetles abundance was influenced by the crop type and growing season at the regional landscape scale. There was a significant positive correlation between aphid and ladybeetle populations on cotton only in July and August, whereas the correlation was significant for maize throughout the whole growing season. The δ13C value indicated that most prey aphids for ladybeetles originated from crops where aphids are abundant (cotton in June and July; both maize and cotton in August). These findings improved our understanding of the migration and dispersal of ladybeetles among different habitats and plant species and provided insight into the promotion of regional conservation and pest control of natural enemies in Northern China.In agroecosystems, predatory ladybeetles play an important role in restraining aphid population growth and suppressing aphid populations. They can adapt to various habitats and make use of various aphid species associated with multiple host plants during their life cycle. Agricultural landscapes in China are composed of a mosaic of small fields with a diverse range of crops, and how ladybeetles make use of host plant diversity in such landscapes has rarely been documented. In this study, we examined the relationship between aphid densities and ladybeetle densities in two different settings: (i) on the majority of plant species (including crops, trees, and weeds) at a local field scale in 2013 and 2014, and (ii) in paired cotton and maize crop fields at a regional landscape scale in 2013. Overall, we found that aphid abundance determined predatory ladybeetle abundance at both the local field and landscape scales, and there was a positive correlation between aphid densities and ladybeetle densities. However, plant taxa had no significant influence on the predatory ladybeetle abundance at the local field scale. In addition, the effect of aphids on ladybeetles abundance was influenced by the crop type and growing season at the regional landscape scale. There was a significant positive correlation between aphids and ladybeetles populations on cotton only in July and August, whereas the correlation was significant for maize throughout the whole growing season. We also conducted an analysis of the stable carbon isotope ratios of the adult ladybeetles caught in cotton and maize fields (C3 and C4 crops, respectively) in a regional landscape-scale survey in 2013. The δ13Cvalue indicated that most prey aphids for ladybeetles originated from crops where aphids are abundant (cotton in June and July; both maize and cotton in August).These findings improved our understanding of the migration and dispersal of ladybeetles among different habitats and plant species and provided insight into the promotion of the regional conservation and pest control of natural enemies in northern China.
Highlights
We found P. japonica was the dominant ladybeetle specieson these two main crops—cotton (97.6%) and maize (86.2%)—at the regional landscape scale in 2013, and significant differences were found for the species compositions of aphidophagous ladybeetles in different sampling months on cotton (χ2 = 8.96, df = 2, p = 0.011, Figure S2c) and maize (χ2 = 22.26, df = 2, p < 0.001, Figure S2d)
The ladybeetle metapopulation density (LMD) wassignificantly positively related to AD across all plants in both years (2013: Wald Z = 4.14, p < 0.001, Figure 2a; 2014: Z = 5.62, p < 0.001, Figure 2b). These results indicated that aphid density was the main determinant of aphidophagouspredatory ladybeetle abundances at the local field scale
The ladybeetles responded differently depending on the host plant type: they responded to high aphid densities in maize fields but had a less obvious response to those in cotton fields at the regional landscape scale, which was similar to the results of Ouyang et al [38], and this response possibly occurred through a phenology effect/host shift, which is discussed below
Summary
Both adult and larvalladybeetles are important predators of various aphid speciesin agroecosystems [1,2].Combinations of laboratory, greenhouse, and field studies carried out in multiple agricultural systems have reported the contributions and importance of predatory ladybeetles to both decrease aphid population growth and lower peak aphid densities, making them essential biological control agents [3,4,5,6].For example, they can cause strong, season-long suppression of aphid populations in soybean fields [7].Many factors could affect the population occurrence of predatory ladybeetles and their potential ecosystem services through pest suppression, but prey abundance is the most important and direct component [1,8,9]. Combinations of laboratory, greenhouse, and field studies carried out in multiple agricultural systems have reported the contributions and importance of predatory ladybeetles to both decrease aphid population growth and lower peak aphid densities, making them essential biological control agents [3,4,5,6]. They can cause strong, season-long suppression of aphid populations in soybean fields [7]. Compared with the monoculture landscape, the incidence for natural enemies’ access to high quality habitat appear much higher within a complex one, which is beneficial for the biocontrol services contributed by natural enemies [32,33]
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