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1 Tillage, commonly used in agroecosystems, can influence the abundance of invertebrates through factors such as habitat change and food availability.
2 The effects of tillage on the composition and abundance of invertebrates were examined in a vineyard near Mildura in Victoria, Australia, focusing particularly on groups that might act as natural enemies in vineyards. We used pitfall traps at ground level and sticky traps in the canopy.
3 The collections were first sorted to order. Beetles (Coleoptera) were sorted to family and ants (Hymenoptera: Formicidae) to genus.
4 Ants were the only group to be affected by tillage when all months were considered. The same genera occurred in both treatments but the abundance of several genera was reduced by tillage. Families of several beetles, including predators, increased in tilled areas. Spider, millipede, centipede and earwig numbers were decreased by tillage. In the canopy, Trichogrammatidae and other parasitoids decreased in abundance after tillage.
5 These results indicate that tillage influences the composition of invertebrates and has the potential to negatively affect the abundance of beneficial groups.
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Soil tillage has long been an integral part of management of almost all agroecosystems. As well as having direct effects on soil structure, tillage can influence the abundance of invertebrates. By physically disturbing habitats and altering availability of food sources, tillage can affect invertebrates living in the soil (Stinner & House, 1990). Additionally, tillage may affect invertebrates living on the surface by influencing the litter layer, microclimate and the availability of shelters.
To investigate tillage impacts on invertebrates, comparisons are normally undertaken between conventional tillage and minimal tillage plots. In some cases, no differences between treatments have been found; for example, conventional deep ploughing and light surface tillage does not influence the number of carabid beetles (Huusela-Veistola, 1996). In other cases, tillage decreases the abundance of certain groups; for example, both spider (Linyphiidae and Lycosidae) and beetle (Carabidae and Staphylinidae) abundance can be reduced by tillage (Holland & Reynolds, 2003), as can ant numbers (Peck et al., 1998). Radford et al. (1995) found that, as tillage intensity decreased, ant colonies increased with the maximum number of colonies recorded in the no-till system. These types of comparisons also indicate that changes in the composition of particular invertebrate groups can occur such as species assemblages in the sub-family Aleocharinae (Krooss & Schaefer, 1998) and the family Staphylinidae (Kromp, 1999). Thus, tillage can affect the relative abundance of particular orders and can cause changes in their composition.
Changes in invertebrate numbers and composition may involve beneficial invertebrate groups (those important in pest control and soil conditioning) in agroecosystems such as spiders (Araneae), rove beetles (Staphylinidae), ground beetles (Carabidae) and ants (Formicidae) (House, 1989; House & Del Rosario Alzgaray, 1989; Weiss et al., 1990; Wilson-Rummenie, 1999; Holland & Reynolds, 2003). Although ants are widely used as indicators in ecosystem functioning, their importance is often overlooked when assessing the sustainability of agroecosystems, particularly because ants can affect pest control, soil processes and plant growth (Lobry de Bruyn, 1999). If tillage disrupts natural enemy groups, there may be consequences for pest control. Adequate ground cover may be needed in agricultural ecosystems for the conservation of spider populations and enhanced predatory control of prey species (Riechert, 1999). For example, the number of wolf spiders (Lycosidae) was reduced by tillage in rice paddies and this was associated with an increase in numbers of pest leaf hoppers (Ishijima et al., 2004).
There is very little information on the impact of tillage on invertebrate communities and pest control in vineyards, apart from some data on earthworms (Paoletti et al., 1998). Tillage within vineyards has in the past been necessary for furrow or flood irrigation. However, different irrigation options are now available to vineyard managers, including overhead sprays, drip and under-vine sprays. Tillage has therefore become optional, raising the issue of whether it should be maintained as a management technique for sustainable production of grapes.
In Australian vineyards, natural enemies play an important role in the control of pest populations (Thomson & Hoffmann, 2006a, 2007). Major insect pests in vineyards include light brown apple moth Epiphyas postvittana Walker (Lepidoptera) and a number of pest mite species, including rust mite Calepitrimerus vitis Nalepa (Acarina: Eriophyidea), bunch mite Brevipalpus spp. (Tenuilpalpidae), longtailed mealybug Pseudococcus longispinus (Targioni-Tozzetti) (Hemiptera: Pseudococcidae) and grapevine scale Parthenolecanium persicae (Fabricius) (Hemiptera: Coccidae). A variety of natural enemies can control pest populations through predation. Spiders are known to be predators of insects in vineyards (Isaia et al., 2006), where they can have considerable influence on the community structure of pest populations (Costello & Daane, 1998). Ants may act as predators of insect pests (Osborne et al., 1995), but they are also involved in mutualistic relationships with some pest species in vineyards (Way, 1963). Lacewings and a variety of beetles and predatory bugs are also known to be efficient predators of grape pests, including spider-mites (Tetranychidae), scale insects and light brown apple moth (Thomson & Hoffmann, 2006a).
In the present study, we evaluate the impact of tillage on arthropods and assess potential impacts on beneficial and pest insects. We also use indicator analysis to identify the groups most affected by tillage and explore how these indicators could be used in broad scale monitoring programs for sustainable production of grapes.
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A total of 39 995 individuals from ten orders were identified and recorded from the pitfall traps during the 16-week study period. Formicidae formed the dominant group of epigeic invertebrates, with 30 119 individuals collected, followed by earwigs (Dermaptera) (n = 3324), beetles (n = 1692), spiders (n = 1345), centipedes (n = 316) and millipedes (n = 404). Hymenoptera (excluding Formicidae), Diptera, Acarina and Isopoda were excluded from further analysis due to insufficient numbers collected. For the canopy traps, there were 815 organisms identified and recorded during the 8-week period, including Trichogrammatidae (n = 143), other parasitoid wasps (n = 602), spiders (n = 29) and beetles (n = 27). There was no concurrent decline in abundance after the single application of methomyl in December (Figs 1–4).
Figure 1. Mean number of (A) Formicidae (Hymenoptera), (B) Coleoptera, (C) Dermaptera, (D) Araneae, (E) Lithobiida (Chilopoda) and (F) Julida (Diplopoda) collected per plot with pitfall traps in the different treatments. Error bars represent standard errors. Stars indicate a significant difference between treatments per month (*P < 0.05, **P < 0.01). The checkerboard represents no tillage and dotted represents tillage.
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Figure 2. Mean number of ant genus (A) Iridomyrmex, (B) Pheidole, (C) Hypoponera and (D) Rhytidoponera collected per plot with pitfalls with the different treatments. Error bars represent standard errors. Stars indicate a significant difference between treatments per month (*P < 0.05, **P < 0.01). The checkerboard represents no tillage and dotted represents tillage.
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Figure 3. Mean number of beetle family (A) Anthicidae, (B) Tenebrionidae, (C) Nitulidae, (D) Byrrhidae and (E) Staphylinidae collected per plot with pitfalls with the different treatments. Error bars represent standard errors. Stars indicate a significant difference between treatments (*P < 0.05, **P < 0.01). The checkerboard represents no tillage and dotted represents tillage.
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Figure 4. Mean number of (A) parasitoids and (B) Trichogrammatidae collected per trap with yellow sticky traps with the different treatments. Error bars represent standard errors. Stars indicate a significant difference between treatments (*P < 0.05, **P < 0.01). The checkerboard represents no tillage and dotted represents tillage.
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In total, 12 genera of ants were recorded from the pitfall traps. Pheidole and Iridomyrmex were the two dominant ant genera, with 17 957 and 8436 individuals collected, respectively. The next most commonly collected genera were Rhytidoponera (n = 1539), Paratrechina (n = 1062), Cardiocondyla (n = 874) and Hypoponera (n = 456). Strumigenys, Notoncus, Monomorium, Solenopsis, Ochetellus and Tapinoma were rare genera and were excluded from analyses.
Twelve beetle families were recorded during the study period, with Staphylinidae the dominant family with 603 individuals collected. The next most commonly trapped family was Anthicidae (n = 295), Byrrhidae (n = 275), Tenebrionidae (n = 252), Curculionidae (n = 81), Carabidae (n = 51), Scarabidae (n = 48), Lathridiidae (n = 43) and Nitidulidae (n = 18; all collected in November).
For all orders, with the exception of Hymenoptera (Formicidae), we found a significant effect of month on numbers caught. There was a significant decrease in the abundance of most taxa from November to February (Fig. 1) including Coleoptera, (Wilcoxon signed rank test, Z = −2.803, P < 0.01), Lithobiida (Z = −2.803, P < 0.01), Julida, (Z = −2.803, P < 0.01) and Araneae (Z = −2.803, P < 0.01). However, average abundance for Dermaptera significantly increased in December (Z = −2.191, P < 0.05) and then significantly decreased in February (Z = −2.599, P < 0.01). Although there was no overall decrease in ants, there was significant monthly variation in numbers of ant genera caught (Fig. 2). Iridomyrmex abundance decreased significantly between November and December (Z = −2.803, P < 0.01), and then significantly increased again in January (Z = −2.803, P < 0.01) and February (Z = −2.191, P < 0.05) compared with December. The abundance of Pheidole increased significantly in December, January and February compared with November (Z = −2.803, P < 0.01). For the genus Hypoponera, abundance significantly increased in December (Z = −2.803, P < 0.01) and January (Z = −1.988, P < 0.05) compared with November. There was also a significant increase in the abundance of the genus Rhytidoponera in January (Z = −2.803, P < 0.01) and February (Z = −2.395, P < 0.05).
The number of beetles collected also varied between months (Fig. 3). For all beetles, we found a significant effect of month on numbers caught. There was a significant decrease in the abundance of beetle taxa from November to February including Anthicidae (Z = −2.805, P < 0.01), Tenebrionidae (Z = −2.607, P < 0.01) and Byrrhidae (Z = −2.807, P < 0.01), whereas Nitidulidae disappeared completely after November. Staphylinidae abundance remained stable from November to December, then significantly decreased in the months of January (Z = −2.805, P < 0.01) and February (Z = −2.805, P < 0.01).
Pitfall traps The MRPP analyses indicated differences in invertebrate abundance and composition between the tillage and no-tillage treatments when tested at the order, family (beetles) and genus (ants) levels (Table 1). At the order level, significant differences were detected for all months except for January and total numbers also differed significantly in all three comparisons when using the Sorensen distance measure. When we applied the relative Sorensen distance measure, both November and February differed significantly between treatments (Table 1). Indicator analyses (Table 2) showed that Formicidae was the only group affected by soil tillage across all months and, in each case, tillage decreased numbers (Fig. 1). When months were analysed separately, four taxa were significantly affected by soil tillage in November (Formicidae, Lithobiida, Dermaptera and Julida), three in December (Formicidae, Lithobiida and Araneae), none in January and only Formicidae in February (Table 2, Fig. 1).
Table 1. Multi-response permutation procedure tests of the null hypothesis of no difference in invertebrate abundance between tilled and untilled plots at different taxonomic levels
| ||Collection||Sorensen||Relative Sorensen|
|A (delta)||P||A (delta)||P|
| Order level||November||0.111||0.004||0.078||0.034|
| Coleoptera families||November||0.177||0.006||0.191||0.004|
| Formicidae genera||November||0.078||0.035||0.064||0.104|
|Yellow sticky traps|
| Parasitoid wasps||February||0.025||0.286||–||–|
Tillage significantly affected the abundance and composition of beetles at the family level in three of the 4 months, as well as across months, as is evident from the significant effects observed with both the Sorensen and relative Sorensen distance measures (Table 1). Overall, beetles were more abundant in the tillage treatment across all months and in specific collections (Fig. 3). Significantly more Anthicidae were present in all months, whereas there were more Byrrhidae and Nitidulidae in November. The abundance of Staphylinidae was significantly increased in the tillage plots in December (Fig. 3). Indicator analyses showed Staphylinidae to be a significant indicator of the absence of tillage in November (Table 2). The only family to show reduced abundance with tillage was Tenebrionidae, which was significantly reduced in November (Fig. 3) and showed a significant association with soil tillage (Table 2). Tillage did not significantly affect the abundance of beetle families in January or February.
For the ant genera, there were significant differences between the treatments for all months except January and also when pooled across months when the Sorensen distance measure was used. When we analysed the same data using the relative Sorensen distance measure, December and January were the only months that differed significantly (Table 1). Three ant genera were significant indicators of soil tillage across all months according to indicator analysis (Table 2), with the abundance for Pheidole, Hypoponera and Rhytidoponera being reduced under soil tillage (Fig. 2). Several genera showed significant reductions with soil tillage in specific months: Iridomyrmex in November, Pheidole in December and February, and Rhytidoponera in January (Table 2). Tillage did not significantly affect the abundance of Cardiocondyla or Paratrechina.
Yellow sticky traps The total invertebrate abundance and composition in sticky traps was affected by tillage in February as is evident from the MRPP analyses undertaken with Sorensen and relative Sorensen distance measures (Table 1). Invertebrate abundance was affected by tillage when both months were combined for analysis (Table 1). The abundance of Trichogrammatidae was significantly affected by tillage in February and when both months were combined for analysis, and the abundance of parasitoids other than Trichogrammatidae was only affected in March (Table 1). Indicator analysis showed Trichogrammatidae to be a significant indicator of tilled soil (Table 2). There were fewer Trichogrammatidae and fewer other parasitoids in the tillage treatment (Fig. 4).
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The vineyard where the present study was conducted supported a diverse epigeal invertebrate assemblage. Soil tillage significantly affected this assemblage and effects were evident at the order, family and genus levels. Tillage may affect soil invertebrates in many ways, including directly disturbing their distribution within the soil (Stinner & House, 1990) or by modifying the habitat where individual species may live (Tebrugge & During, 1999). Tillage had immediate effects on the soil biota, with invertebrates such as ants, earwigs, centipedes and millipedes all being reduced in numbers in the tillage plots. This sharp response to soil tillage on these taxa suggests that tillage may have directly caused mortality through mechanical damage or burying (Thorbek & Bilde, 2004). Few studies have looked at the effects of tillage on these groups, instead focusing on others such as Coleoptera and spiders (Thorbek & Bilde, 2004). These results also suggest that methomyl probably did not impact upon most taxa because there was a general decline in abundance across the 4 months and no sharp decreases after December, which would be expected if methomyl had an impact. However, Staphylinidae abundance may have been affected by methomyl, as reflected by the sharp decrease of this taxon in January.
The reduction in Formicidae abundance throughout the sampling period without a recovery contrasts with previous reports suggesting recovery of epigeal arthropods within 3–4 months after tillage (House & Del Rosario Alzgaray, 1989). Although few studies have investigated the effects of tillage on ants, reduction with tillage is commonly observed. In a survey of 90 agricultural locations in the U.S.A., Peck et al. (1998) found that the majority of the ant species collected were more common in conservation tilled sites compared with conventional tilled sites, suggesting that a reduction in tillage may allow for increased stability in nesting sites and enhancement of community structure. House (1989) however, found that ant abundance was only slightly reduced by tillage in maize crops.
The present study has shown that several ant genera are affected by tillage in vineyards. The effect of tillage on Pheidole was consistent with the study by Peck et al. (1998) who also found this genus to be indicative of tillage in agroecosystems. Ants have been used successfully as indicators in terrestrial ecosystems (Majer, 1984; Andersen & Sparling, 1997; Lobry de Bruyn, 1999) and conform to most of the attributes that are considered desirable as an indicator group, such as being important to the overall structure and function of agroecosystems, responsive to a range of environmental stresses, and easily measured. Epigeal ant species have a beneficial role through soil engineering (Lobry de Bruyn, 1999), whereas the role of ants as opportunistic or specialized predators is well known in Australia (Andersen, 1990, 1995) and elsewhere (Youngs, 1983). Ants are important predators of the larval sugarcane borer Diatraea saccharalis (Lepidoptera) (Rossi & Fowler, 2000) and two-spotted mite Tetranychus urticae (Acari: Tetranychidae), an important pest in grapevines (Osborne et al., 1995), whereas the anecdotal evidence suggests that ants may be important predators of lepidopteran eggs including E. postvittana (L. Thomson, unpublished observations). However, ants are also known to tend some pest species such as mealybugs and scale through trophobiosis.
Lithobiida and Julida were significantly affected by tillage in the first month of sampling but, in the latter 3 months, both groups were reduced regardless of treatment. Centipedes are effective predators in agroecosystems (Brust & House, 1990) and the results of the present study suggest that tillage may decrease potentially beneficial effects of predators early in the growing season. Centipedes and millipedes can also influence a number of soil processes, including the mixing of organic and mineral particles, the creation of biopores, the promotion of humification and redistribution of microorganisms throughout the soil (Hendrix et al., 1986). In addition, millipedes can directly influence the breakdown of soil litter and significantly contribute to earthworm nutrition through the production of faecal pellets that are often consumed by this group (Bonkowski et al., 1998).
Very few studies have specifically looked at the response of earwigs to tillage in agroecosystems. The present study showed that tillage influences earwig populations, although recovery was evident after the first month. Earwigs are often regarded by vineyard managers as a minor pest, but they may also be important predators of pests (Horne & Edward, 1995), including the light brown apple moth (Danthanarayana, 1983; Frank et al., 2007).
The benefits of maintaining large populations of spiders in vineyards for pest control is well known (Bolduc et al., 2005; Isaia et al., 2006). The reduction in spider abundance due to tillage might impact on pest control. Soil surface dwelling spiders such as Lycosidae, Clubionidae and Gnaphosidae are all considered excellent hunters and are very mobile predators and locomotion by hunting spiders is usually by cursorial dispersal (Samu et al., 1999).
Beetle abundance was increased by tillage throughout the sampling period. Other studies have found that tillage tended to reduce beetle populations. For example, House (1989) found that tillage in maize crops reduced beetle abundance, whereas Horne and Edwards (1998) found that conservation tillage benefited predatory carabid beetles and did not exacerbate pest populations in arable fields. As pitfall traps measure activity density as well as relative abundance, it is possible that the reduction in barriers in the tilled plots may have resulted in increased activity and higher numbers of beetles. In a study investigating pitfall trapping as a method for sampling Carabidae, Greenslade (1964) found that pitfall traps placed in bare ground consistently caught more carabids compared with pitfalls abutting vegetation. Carabids are known to be generalist predators, whereas anthicids are generally classified as scavengers, with some forms reported to feed on egg masses or fly puparia, and anthicids are usually found in decaying vegetation (Lawrence & Britton, 1994). Byrrhidae are mainly restricted to feeding on liverworts, mosses and fungi (Lawrence & Britton, 1994), whereas Nitidulidae include pests that may feed on fruit (James et al., 1995), although not on grapes. Tenebrionidae decreased with tillage; as young vines might be vulnerable to these beetles (Buchanan & Amos, 1992), tillage could be beneficial during vine establishment. Staphylinidae are one of the most ecologically important arthropod groups in agroecosystems. Our collection of staphylinids included several species of Aleocharinae, significantly the genus Oligota, which are known to be important predators of phytophagous mites (Paoletti & Lorenzoni, 1989) that are economically important in vineyards. In the present study, tillage increased Staphylinidae abundance in all months, and particularly in December, suggesting that tillage may enhance the abundance of this group and their potential as predators in the vineyard.
The results from the canopy traps suggest that parasitoid wasps in general and in particular Trichogrammatidae were significantly affected by the tillage within the vineyard. Loss of weedy vegetation from the interrow due to tillage may lead to a loss of ground cover (Thomson & Hoffmann, 2007) or floral resources. Weedy vegetation can provide nutritional resources important for parasitic wasps (Landis et al., 2000). A large number of parasitoids often use floral nectar as a food source (Jervis et al., 1993) and provision of floral resources can increase the abundance of Trichogramma in vineyards (Begum et al., 2006). Tillage may also decrease the presence of alternative hosts for the parasitoids. The importance of egg parasitoids such as the Trichogrammatidae to control of light brown apple moth (Glenn et al., 1997) should encourage further assessment of tillage effects on parasitoids. It seems prudent to avoid soil tillage, particularly when vineyards are engaged in mass-releasing Trichogrammatidae for biological control purposes (Glenn & Hoffmann, 1997) or when vineyards adopt management techniques associated with the maintenance of natural Trichogrammatidae. The role of parasitoids other than Trichogrammatidae is largely unknown. However, many other parasitoids have a role in control of mealybugs, scale and light brown apple moth (Thomson & Hoffmann, 2006a) and a recent study by Paull and Austin (2006) revealed 25 larval and pupal parasitoids of light brown apple moth.
In conclusion, our results suggest that tillage within vineyard systems may disrupt a number of beneficial invertebrate groups, including ants, centipedes and millipedes and Trichogrammatidae. Ant assemblages in vineyards may be particularly disrupted by tillage. Tillage implications for other groups are unclear. These results complement knowledge on the benefits of no-till or reduced till management in other systems (Radford et al., 1995; Tebrugge & During, 1999; Kendall, 2003) and this suggests that soil tillage in viticultural systems will often have a detrimental impact on important beneficial groups of invertebrates.