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Effect of Organic Mulches on Soil Surface Insects and Other Arthropods

ABSTRACT 
 Four different types of organic mulches were evaluated for their effects on soil surface insects and related arthropods. Field experiments were conducted in fall 2007 and 2008 near Citra, Florida. In both the years, five treatments were compared: cowpea (Vigna unguiculata (L.) Walp.) mulch, sunn hemp (Crotalaria juncea L.) mulch, sorghum-sudangrass (Sorghum bicolor Moench × S. sudanense ((Piper)] Stapf) mulch, pine bark nuggets, and unmulched control. Data were collected on insects and other arthropods using pitfall traps. Results indicate that organic mulches can affect a wide range of different insects. Diptera, dominated by Asyndetus spp. (Dolichopodidae), were most dense in pine bark plots in both years. Populations of small plant-feeding insects such as Aphididae, Thripidae, and Aleyrodidae were most dense in cowpea and unmulched control plots in one season. It is possible that these insects were affected by weed growth in cowpea and control plots. Ants, which tend or feed on small plant feeders, were fairly abundant in these plots as well, as were predatory beetles. Some groups, such as Collembola (mainly Isotomidae), spiders, and Orthoptera (Acrididae and Gryllidae) were unaffected by mulches.

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 & Ammon 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 ) 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 .
While these examples suggest that living mulches may offer resources to support preda-tors, 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).

Fall 2007
The experimental field was sprayed with glyphosate (Roundup®, Monsanto, St. Louis, Missouri) to kill weeds on Sep 26 followed by rototilling on Oct 3. Average soil moisture measured gravimetrically before planting was 6.1%. Five treatments compared were: cowpea (Vigna unguiculata (L.) Walp.) mulch; sunn hemp mulch; sorghum-sudangrass (Sorghum bicolor Moench × S. sudanense (Piper) Stapf) mulch; pine bark nuggets as mulch (HTC Hood Timber Co., Adel, GA); and unmulched control. Treatments were arranged in a randomized complete block design with five replications (total of 25 plots). Individual plots for each treatment were 3.0 m long and 2.4 m wide and the distance between plots was 3.0 m. All plots were planted with 'Roma II' bush beans (Phaseolus vulgaris L.) on Oct 4. Seeds were spaced 10 cm apart at a rate of 30 seeds per row, in two rows per plot.
The mulches used were readily available or easily supplied by cover crop residues. Cover crop mulches were obtained from crops of 'Iron and Clay' cowpea, 'Tropic Sun' sunn hemp, and 'Growers Choice' sorghum-sudangrass planted in early Jul. Mulches were obtained from these cover crops (prior to flowering) planted near the experimental site. To obtain mulches, these cover crops were harvested on Oct 11 by clipping plants at the base, removing above-ground biomass, and applying it to the plots The resulting mulches (3-5 cm deep) were a composite of leaves and stems and were applied by hand over the entire plot, next to the rows of bean plants. Therefore, except for the plant rows, the entire plot was covered with mulch. Mulches were applied only once at the start of experiment on Oct 11, using the following amounts of material: cowpea (18.1 kg fresh wt/ plot), sunn hemp (15.9 kg fresh wt/ plot), and sorghum-sudangrass (17.7 kg fresh wt/plot). The pine bark nuggets (29.8 kg fresh wt/plot) were not obtained from cover crops, but were purchased locally. Plots were irrigated as needed using drip irrigation, and no insecticides were applied during the course of the experiment.

Fall 2008
The experiment was repeated at the same site in the fall 2008, with the same treatments. Experimental procedures remained the same with a few minor changes. The experimental field was sprayed with glyphosate to kill weeds in the first week of Sep followed by rototilling on Sep 16. Average soil moisture measured gravimetrically at planting was 6.9%. Beans were planted on Oct 7. Cowpea (12.7 kg fresh wt/plot), sunn hemp (15.9 kg fresh wt/plot), sorghum-sudangrass (13.6 kg fresh wt/plot), and pine bark nuggets (29.8 kg fresh wt/plot) were applied on Oct 9. Early frost in each season caused severe damage to the bean plants, so that crop harvests were not possible.

Data collection
Insects were collected on several sampling dates in both seasons (Oct 24, Nov 6, Nov 20, Dec 3, and Dec 17 in 2007;Oct 13, Oct 28, Nov 9, and Nov 24 in 2008). Pitfall traps were used for capturing insects that run or move on the soil surface (Borror et al. 1989). A plastic sandwich container (14 cm × 14 cm × 4 cm) was used as a pitfall trap. One pitfall trap was placed in the middle of each plot, and buried so that the upper edge was flush with the soil surface. The traps were filled three quarters with water, along with 3 to 4 drops of dish detergent (Ultra Joy®, Procter and Gamble, Cincinnati, Ohio) to break surface tension, ensuring that the insects would remain in the trap. Pitfall traps were set out in the morning (9:00 am) and collected at approximately the same time (9:00 am) the next day (which was recorded as the sampling date). The traps were brought to the laboratory, kept in a cold room at 10°C, and contents transferred and stored in 70% ethanol in vials. Insects and related arthropods were identified to order and family levels using a dissecting microscope.

Data analysis
All statistical analyses were performed using the Statistical Analysis System (SAS) package (version 9.1; SAS Institute, Cary, North Carolina). Data for each dependent variable (insect groups) were analyzed across all sampling dates in each year using repeated measures (PROC MIXED procedure of SAS) to examine the effects of treatment, sampling date, and interactions between treatments and sampling dates. Since no interactions were found, data were pooled across sampling dates for calculations of means and standard errors of the means. When treatment effects were significant (P ≤ 0.05), least square means (LS) values were computed to compare means of mulch treatments.
Data are means ± standard error of 20 replications (data pooled across 4 sampling dates). Means in columns for each sampling date followed by the same letters do not differ significantly based on least square means (P ≤ 0.05).  sects (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.). 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.