Use of cover crop residues as organic mulches has a number of advantages to farming systems such as reducing soil erosion, conserving soil moisture, moderating soil temperature, improving infiltration of water, and providing a slow-release source of nutrients (Gruda 2008; Hatwig & Amnion 2002; Hatwig & Hoffman 1975; Powers & McSorley 2000; Snapp et al. 2005; Westerman & Bicudo 2005). Plant mulches can be an effective way to provide shelter for predatory insects (Johnson et al. 2004) and to control weeds (Reeleder et al. 2004; Teasdale et al. 2004). Mulches can help to maintain soil moisture required for plant vigor and to promote plant tolerance to the attack of insect pests (Johnson et al. 2004).

Cover crops and intercrops have been used as living mulches for managing some insect pests. Alfalfa (Medicago sativa L.) and kura clover (Trifolium ambiguum M. Bieb.) mulches increased predator populations to manage European corn borer (Ostrinia nubilalis Hübner) (Prasifka et al. 2006). Eggs and larval densities of pest caterpillars were higher in broccoli (Brassica oleracea L. var. botrytis) monoculture when compared to broccoli with undersown mulches like strawberry clover (Tribolium fragiferum L.), white clover (Tribolium repens L.), and yellow sweet clover (Melilotus officinalis L.) (Hooks & Johnson 2004). Alfalfa living mulch increased predators to manage outbreaks of the invasive soybean aphid, Aphis glycines Matsumura (Schmidt et al. 2007).

While these examples suggest that living mulches may offer resources to support predators, non-living mulches derived from killed cover crops, hay from cover crops, or composted waste products may offer benefits as well. In sweetpotato (Ipomoea batatas (L.) Lam.), higher numbers of fire ants, rove beetles, and carabid beetles were captured using pitfall traps in plots covered with killed-cover crop (Jackson & Harrison 2008). Also, the injury level from soil insect pests to roots of sweetpotato was lower in killed-cover crop plots than in conventional plots. In an apple (Malus domestica Borkh.) orchard, the dominance of several carabid species depended on different factors including sampling dates and different types of ground cover including plastic mulch and straw mulch (Miñarro & Dapena 2003). Predation of beet armyworm, Spodoptera exigua (Hübner), pupae was 33% greater in cover crop mulch as compared with conventional production plots (Pullaro et al. 2006). Mulch from sunn hemp (Crotalaria juncea L.) hay was effective in reducing incidence of lesser cornstalk borer, Elasmopalpus lignosellus (Zeller) on bean (Phaseolus vulgaris L.) (Gill et al. 2010).

Changes in cropping systems affect insect pests and their natural enemies (Hummel et al. 2002). Organic mulches might provide hiding places to harbor populations of natural enemies. Different types of cover crops harbor distinctive complexes of beneficial insects, pest arthropods, and their diverse trophic relationships (Bugg & Waddington 1994). Many previous studies that used mulches for the management of insect pests focused especially on flying insects moving into mulched areas (Brown & Tworkoski 2004; Gill et al. 2010; Hooks & Johnson 2004; Prasifka et al. 2006; Pullaro et al. 2006; Reeleder et al. 2004; Schmidt et al. 2007; Tremelling et al. 2002). The effects of mulches on insects and other soil arthropods living on the soil surface is a relatively less explored area.

More information is needed on arthropods that are active on the soil surface where the mulches occur, and how different materials on the soil surface affect these arthropods. To answer these questions, the present study was designed with main objective to determine the impact of mulches on the community of arthropods that live and move on the soil surface. The purpose was to obtain an overview of various arthropod groups that were active on the soil surface, rather than focusing on selected key species.

MATERIALS AND METHODS

Field experiments were conducted in fall 2007 and 2008 at the University of Florida Plant Science Research and Education Unit (29°24'N, 82°9'W), Citra, Florida. The soil at the experimental site was Arredondo sand (95% sand, 2% silt, 3% clay) with 1.5% organic matter (Thomas et al. 1979).

RESULTS

Fall 2007

Diptera were affected (P ≤ 0.05) by mulches, and were more common in pine bark mulch than in sunn hemp and sorghum-sudangrass (Table 1). Diptera consisted mainly of Dolichopodidae (43.9%, Asyndetus spp.) followed by Mycetophilidae (fungus gnats) and other micro-dipterans (37.1%) and other Diptera (19.0%). Cicadellidae and small plant-feeding insects were not significantly (P ≤ 0.05) affected by treatment, but Cicadellidae showed some an interesting trend (P ≤ 0.10) toward greater abundance in unmulched control plots. Small plant-feeding insects consisted of aphids (72.7%, Aphididae), whiteflies (24.3%, Aleyrodidae), and thrips (3.0%, Thripidae). The numbers of Formicidae (mixture of Pheidole spp., and Dorymyrmex spp.), Collembola (Isotomidae with a few Sminthuridae), Orthoptera (mixture of Melanoplus spp., Dichromorpha spp., and Gryllus spp.), Araneae, and Coleoptera (Staphylinidae, Carabidae, Elateridae, and Chrysomelidae) did not differ among treatments (Table 1). In addition, the few micro-Hymenoptera (mainly small parasitoid wasps) collected were also not affected by treatments (data not shown). Beetles collected were from the families Staphylinidae (23.4%), Carabidae (12.2%, Anisodactylus spp.), Elateridae (14.2%, Conoderus spp.), and Chrysomelidae (48.4%, Altica spp.), but none of these individual families were significantly (P ≤ 0.05) affected by treatments. A few specimens of other plant-feeding insects were occasionally recovered at low levels in pitfall traps, including cutworms (Noctuidae), plant hoppers (Fulgoridae), spittlebugs (Cercopidae), and stink bugs (Pentatomidae), but none were affected by treatments (P ≥ 0.10).

TABLE 1.

EFFECT OF MULCH TREATMENTS ON ARTHROPOD TAXA (NUMBERS/PITFALL TRAP) THROUGHOUT THE SEASON IN CITRA, FLORIDA, 2007.

DISCUSSION

The arthropods recovered during this study encompassed a variety of trophic groups and feeding habits (Table 3). Effects of treatments on different insect groups varied, but some interesting patterns were evident. Several insect groups, including ants, beetles, and small plant feeding insects (aphid, whiteflies, and thrips), were highest in unmulched control or cowpea plots in one season. It is possible that weeds (including nutsedges, grasses, and broadleaf) in unmulched control and cowpea plots may have led to the higher numbers of small plant-feeding insects in these plots. Cowpea mulch degraded quickly and allowed the emergence of weeds after 3–4 weeks. At this time, broadleaf weeds covered about 10% of the surface area in unmulched control and cowpea plots, but <5% in other plots. Broadleaf weeds consisted of Florida pusley (Richardia scabra L.), eveningprimrose (Oenothera laciniata Hill.), and cudweed (Gnaphalium spp.).

TABLE 2.

EFFECT OF MULCH TREATMENTS ON ARTHROPOD TAXA (NUMBERS/PITFALL TRAP) THROUGHOUT THE SEASON IN CITRA, FLORIDA, 2008.

TABLE 3.

ARTHROPODS AND THEIR FEEDING HABITS IN THE NATURAL ENVIRONMENT.

Beetles are the largest and most diverse group of insects, and varied in their response to treatment over the two seasons, reaching highest numbers in cowpea plots in 2008. The majority of these were Staphylinidae and Carabidae, which are predators, and the increased abundance of potential prey insects (Aphididae, Cicadellidae etc.) in unmulched control plots may have stimulated these predatory beetles as well (Table 3). Ants have been observed to feed on or tend sucking insects such as aphids and whiteflies (Borror et al. 1989), so their increased numbers may be related to the other insects in unmulched control and cowpea plots. This effect was observed by Pullaro et al. (2006) who recorded a greater number of fire ants in plots with killed-cover crop mulch compared with conventional plots. Flies were most common in pine bark plots in both years, possibly because pine bark was the only mulch that did not degrade as fast as others (C:N ratio = 208:1), and may have served as cover for these insects and their larvae. This mulch may have provided favorable habitat for long-legged flies (Dolichopodidae) that typically inhabit organic debris and feed on small invertebrates on the soil surface (Borror et al. 1989; Triplehorn & Johnson 2005; Ulrich & Schmelz 2001). Collembola were unaffected by treatments, with similar levels in mulched and unmulched plots. This was unexpected since the degradation of mulch could provide a continuous supply of organic matter. Generally, Collembola are cryptozoic and feed on fungi associated decaying organic matter (Coleman et al. 1996; Powers & McSorley 2000).

We were surprised to find a number of aphids, whiteflies, and thrips in pitfall traps. The pitfall trap is the one of the most commonly used methods to sample insects and other arthropods on the soil surface (Southwood & Henderson 2000). On the other hand, small plant feeders such as aphids, whiteflies, and thrips are typically sampled by other methods such as sticky cards rather than pitfall traps (Southwood & Henderson 2000). However, small numbers of them will fall from vegetation into pitfall traps as well (Tremelling et al. 2002). Future studies should anticipate presence of some small plant feeders in pitfall traps, which could probably be better explained by concurrent sampling of above-ground vegetation by other methods.

CONCLUSIONS

The present study suggests that insects varied in their responses to different mulches. During both years, flies (mainly Dolichopodidae) were found in highest numbers in pine bark plots throughout the season. Several other groups were affected indirectly due to the effects of mulches on weed growth. Weed growth in unmulched control and cowpea plots may have led to increased populations of small plant feeders such as aphids, thrips, and whiteflies. Ants that tend or feed on small plant feeders were more abundant in these plots as well, as were predatory beetles in 2008. Some groups, such as Collembola, spiders, and parasitoid wasps, were unaffected by mulches, while others such as leafhoppers showed only minimal trends.