Effect of straw mulch on populations of Microtheca ochroloma (Coleoptera: Chrysomelidae) and ground predators in turnip Brassica rapa in Florida.
Microtheca ochroloma Stal (Coleoptera: Chrysomelidae), the yellow-margined leaf beetle, is a serious pest of crucifer crops in the southeastern USA (Chamberlin & Tippins 1949). The pest is a native of southern South America and was first found in Florida on watercress near Tampa in 1972 (Woodruff 1974). It is currently distributed along the Gulf Coast from Florida to Texas, and in Georgia, North Carolina, and Arizona. The biology, ecology, and feeding preferences of M. ochroloma were studied by Ameen & Story (1997a, b). Larvae and adults are defoliators that feed only on plants in the family Brassicaceae. Preferred hosts are turnips, mustard, radish, watercress, and Chinese cabbage, while less preferred hosts include cabbage and cauliflower (Chamberlin & Tippins 1949; Ameen & Story 1997b).
There are at least 497 organic farms in Florida involved in the production of crucifer vegetables (Florida Organic Grower 2008). However, many growers have reduced the proportion of crucifer crops as a consequence of damage by M. ochroloma (Bowers 2003). This pest is a serious problem from Oct to Apr, which corresponds to the primary growing season for organic farmers in many parts of Florida (Capinera 2001). Damage caused by M. ochroloma prevents the growers from selling high quality products, thus resulting in significant economic losses. Management of M. ochroloma is usually achieved with foliar insecticides (Capinera 2001), which are not an option for organic farmers. Management by cultural control has received limited study. According to Bowers (2003), intercropping mizuna (Brassica rapa L. var. Kyona) and oak leaf lettuce (Lactuca sativa L.) did not provide effective control of M. ochroloma. Therefore, organic farmers are desperately searching for alternative and more effective control measures against this pest.
Cultural practices have been used as a component of integrated pest management in several cropping systems. For example, the use of organic mulches has resulted in reduced pest populations, increased soil moisture, suppression of weeds, and increased crop yield (Greer & Dole 2003). In addition, the positive effect of mulches on ground predator populations has been reported in several cropping systems, including onion, potato, and soybeans (Halaj et al. 2000; Johnson et al. 2004). The objective of our study was to investigate the effect of straw mulch on the abundance of M. ochroloma and ground predators in turnip in Florida. In addition, measurements of leaf damage and yield were compared between treatment plots at the end of the experiment.
MATERIALS AND METHODS
This study was conducted during Mar and Apr 2009 at the University of Florida's Indian River Research & Education Center, Fort Pierce, FL (27[degrees]25'34.3"N, 80[degrees]24'20.31"W). The experimental plots had not been cultivated for at least 2 years prior to this study and were separated from adjacent crops (cucumber and tomato) by at least 200 m. The experimental design consisted of 2 treatments (straw mulch, no straw mulch) in a complete randomized block design, with 4 replications per treatment. Before planting, 2 fertilizers were incorporated into the plot beds, a root fertilizer 4-16-4 N-P-K (5 kg/30.5 m) and a plant growth fertilizer 8N-12[P.sub.2][O.sub.5]-20-[K.sub.2]O (10 kg/30.5 m) (Howard Fertilizer Co., Inc., Orlando, FL). Flood irrigation was provided between beds throughout the experimental plot. Turnip plants (Brassica rapa rapifera, Seven Top) (2 weeks old) were transplanted to the field on Mar 10, and an 8-10-cm thick layer of dried straw mulch was applied by hand 2 d later to half of the plots. The straw mulch (Bahia hay, Fair-Us Tractor Service, Okeechobee, FL) was 3 months old when the experiment started. Eight plots (5 m long x 9 m wide) separated by at least 10 m were arranged in a complete randomized block design. Each plot consisted of 5 beds separated by 1 m. Ten turnip plants were planted with a spacing of 50 cm in each bed (50 turnips per plot).
Sampling Insect Herbivores
Abundance of M. ochroloma and other insect herbivores was recorded twice weekly from Mar 13 to Apr 24. Six plants (2 plants in each of 3 inner beds) from each plot were selected randomly and visual observations of whole plants were conducted. Because M. ochroloma was not found on the experimental plots during the first 2 weeks of sampling, 10 pairs of adults were released in the outer beds of each plot on Mar 27.
Sampling Ground Predators
A pitfall trap consisting of a plastic cup (9 cm diameter at soil surface, l2 cm depth) was placed in each of the 3 inner beds of each plot (3 traps per plot). Traps were half-filled with soapy water and checked twice weekly from Mar 13 to Apr 24. The arthropods from each trap were preserved in separate vials with 75% EtOH. All predators were counted and identified to order or family.
Foliar Damage and Yield
At the end of the experiment (Apr 24), 6 randomly selected turnip plants from each plot were collected and brought to the laboratory. One new leaf and l older leaf were selected randomly from each plant and the leaf area consumed per leaf was determined by scanning each leaf and measuring the total area removed by herbivory with the software Image-J (http://rsb.info.nih.gov). New leaves were those that ranged from 10 to 15 cm in length, and older leaves were those ranging from 30 to 35 cm. Fresh weight of the tuber and dry weight of all leaves were recorded for each plant.
Repeated measure analysis of variance was used to compare the abundance of M. ochroloma, total herbivores, predators per taxon, and total predators between treatments (mulch versus no mulch) over time. Data for each predator were combined for all dates because there were no interactions (treatment x time effect), except for earwigs and spiders, which were analyzed by single factor analysis of variance for each date. Leaf area consumed and yield were compared between treatments by single factor analysis of variance. Means were separated with the Student-Newman-Keuls (SNK) test (SAS Institute, 1999). A significance level of [alpha] = 0.05 was used for all statistical analyses. Means are reported with their standard error.
Differences were detected in the number of M. ochroloma found between treatments over time, with a significant interaction between treatment and time (F = 3.65, df = 11, 95, P = 0.0005). When analyses were conducted separately for each date, higher numbers of M. ochroloma were found in plots with mulch compared to plots with no mulch (Fig. 1). Other insect herbivores found on turnip plants included aphids, several lepidopteran species, and other leaf beetles. However, no significant differences between treatments were detected for total number of herbivores per plant (~28 per plant) (treatment x time: F = 1.24, df = 11, 95, P = 0.27; treatment: F = 1.48, df = 1, 95, P = 0.22). Aphids were the most abundant insect herbivore in Mar, whereas M. ochroloma was the most abundant herbivore in Apr. The high numbers of M. ochroloma (20-28 per plant) recorded in Apr may not correspond to natural infestations since adults were released in each plot at the beginning of the experiment.
[FIGURE 1 OMITTED]
Higher numbers of predators (each predator taxon and total predators) were recorded in the no-mulch treatment compared with the mulch treatment (Table l). Fewer predatory beetles (ground beetles, Carabidae; rove beetles, Staphylinidae; and lady beetles, Coccinellidae) were captured compared with the higher numbers of ants (Formicidae), earwigs (Dermaptera), and spiders (Aranaeae). Overall, significantly more ground beetles and rove beetles were captured in traps in the no mulch treatment versus the mulch treatment. No significant difference in abundance of lady beetles between treatments was detected on any date. Significant treatment x time effect interactions were observed for earwigs and spiders; therefore, data were analyzed separately for each date. Significantly more earwigs were recorded in the no-mulch treatment on all but 2 of the 13 sampling dates (P < 0.05). For spiders, significantly higher numbers were recorded from the traps in the no-mulch treatment on 5 of the sampling dates (P < 0.05); on all but 2 dates for which there was no significant difference between treatments, more spiders were noted in the no-mulch treatment.
Foliar Damage and Yield
Leaf area consumed from new turnip leaves was greater in plants from plots with straw mulch (mean = 4.1 [+ or -] 0.4 [cm.sup.2]) compared to those from plots without mulch (mean = 2.0 [+ or -] 0.5 [cm.sup.2]) (F = 9.09; df = 1, 7; P = 0.02). However, no differences were detected between treatments for damage to older leaves (no-mulch treatment mean = 5.6 [+ or -] 1.5 [cm.sup.2], mulch treatment mean = 8.4 [+ or -] 1.4 [cm.sup.2]) (F = 1.88; df = 1, 7; P = 0.22). Similar mean tuber weights were recorded for turnips in the no-mulch (mean = 130.7 [+ or -] 24.9 g) and the straw mulch (mean = 145.9 [+ or -] 18.1 g) treatments (F = 0.25; df = 1, 47; P = 0.62). However, greater dry leaf weight was obtained for turnips in the straw mulch treatment (mean = 142.9 [+ or -] 16.4 g) compared with the no-mulch treatment (mean = 97.8 [+ or -] 9.5 g) (F = 5.66; df = 1, 47; P = 0.02).
Our study shows that the use of straw mulch does not provide a means for adequately controlling populations of M. ochroloma in turnips. Moreover, greater pest populations occur in plots with straw mulch compared to plots without mulch during Mar and Apr 2009. These data suggest that adults of M. ochroloma are able to successfully locate and recognize their host plants growing in straw mulch and that the pest population increases following colonization. Similarly, other pests, for example the squash bug, Anasa tristis (DeGeer), and the American palm cixiid, Myndus crudus Van Duzee, are favored by the use of organic mulches (Howard & Oropeza 1998; Cranshaw et al. 2001). In contrast, the use of organic mulches in other cropping systems has resulted in a decrease in pest populations. This has been observed for the onion thrips, Thrips tabaci Lindeman, in onions (Larentzaki et al. 2008), the silverleaf whitefly, Bemisia argentifolii Bellows & Perring, in zucchini squash (Summers et al. 2004), and the Colorado potato beetle, Stilodes decemlineata (Say), in potato (Johnson et al. 2004).
The use of straw mulch has been reported to increase the abundance of predators in the field, which in turn reduces pest populations (Halaj et al. 2000; Johnson et al. 2004). In our study, however, a greater abundance of ground predators was recorded in plots with no straw mulch compared to plots with straw mulch. The most abundant predators in pitfall traps were ants, earwigs, and spiders, while lower numbers of predatory beetles were recorded. The reduced numbers of M. ochroloma in the no-mulch treatment may be a direct effect of the greater ground predator population in that treatment. It could be that the absence of mulch facilitated the searching behavior of the predators on both the soil and the plant. Although M. ochroloma larvae and adults occur more frequently on the foliage, the larvae do move to the lower parts of the plant and to the leaf litter to pupate and the adult females move to the leaf litter and soil to deposit eggs (C. O. Montemayor, personal observation). This behavior in plots without straw mulch may make the prey more susceptible to predation by ground predators, whereas in plots with the structurally complex straw mulch the larvae and adults can easily hide and escape predation from the fewer ground predators. Other factors that may affect pest and predator populations are differences in plant quality and soil moisture. For example, turnip plants in the plots with straw mulch were larger (more leaf biomass), thus presenting higher quality host plants to the pest.
The commercially valuable part of a turnip crop is the tubers and leaves that are sold in fresh markets. Although greater leaf biomass was obtained from turnip plants grown in plots with straw mulch, those plants suffered greater damage since the leaf area removed by herbivory was greater in plots with straw mulch compared to plots without mulch. Thus, the damaged leaves cannot be marketed and must be discarded, thereby resulting in lower yield from a turnip crop grown in straw mulch. The increase in leaf damage was correlated with the increase of M. ochroloma population in the straw mulch treatment, which highlights the importance of this pest as an economically important defoliator of turnips and other leafy green crucifer crops. Although our study was conducted during l growing season in 2009, the trends observed in the abundance of insect populations between treatment plots were quite significant. For example, there was a two-fold increase in M. ochroloma populations in the plots with straw mulch compared to the plots without mulch.
In conclusion, the use of straw mulch as a pest management option for M. ochroloma in crucifer crops on organic farms is not recommended. More appropriate management tactics that are environmentally-friendly and effective in reducing pest populations (e.g., biological control) should be evaluated to help organic farmers in Florida and elsewhere.
The authors are grateful to Daniel Mancero Castillo and Jose A. Castillo Altamirano for providing field assistance during this study. In addition, we appreciate the help of other colleagues from the Indian River Research and Education Center in Fort Pierce: Janet Dawson, Rodrigo Diaz, and Randy Burton. This research project was supported by grants from the Florida Specialty Crop Foundation and Florida Department of Agriculture and Consumer Services.
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VERONICA MANRIQUE (1), CECIL O. MONTEMAYOR (1), RONALD D. CAVE (1), EDWARD A. SKVARCH (2) AND BRADLEY W. SMITH (1)
(1) University of Florida, Indian River Research and Education Center, 2199 South Rock Road, Fort Pierce, FL 34945
(2) St. Lucie Co. Cooperative Extension Service, University of Florida, 8400 Picos Rd., Fort Pierce, FL 34945
TABLE 1. MEAN NUMBERS ([+ or -] SE) OF PREDATORS RECOVERED PER PITFALL TRAP IN STRAW MULCH AND NO STRAW MULCH PLOTS OF TURNIPS. Straw No straw Predator mulch mulch Carabidae 0.12 0.08 b 0.41 0.17 a (ground beetles) Staphylinidae 0.07 0.04 b 0.19 0.11 a (rove beetles) Coccinellidae 0.05 0.04 a 0.11 0.07 a (lady beetles) Formicidae 2.81 0.82 b 4.12 1.18 a (ants) Dermaptera 0.43 0.23 b 2.45 0.62 a (earwigs) Araneae 1.07 0.3 b 2.01 0.40 a (spiders) Total 4.69 1.51 b 10.02 2.55 a Predator Treatment effect Time effect Carabidae F = 19.40, P < 0.0001 F = 1.64, P = 0.09 (ground beetles) Staphylinidae F = 7.74, P = 0.006 F = 1.25, P = 0.26 (rove beetles) Coccinellidae F = 2.89, P = 0.09 F = 2.19, P = 0.02 (lady beetles) Formicidae F = 7.88, P = 0.006 F = 1.97, P = 0.03 (ants) Dermaptera F = 100.06, P < 0.0001 F = 2.58, P = 0.006 (earwigs) Araneae F = 39.57, P < 0.0001 F = 2.81, P = 0.003 (spiders) Total F = 48.45, P < 0.0001 F = 1.26, P = 0.26 Treatment x Predator time effect Carabidae F = 1.57, P = 0.11 (ground beetles) Staphylinidae F = 0.73, P = 0.72 (rove beetles) Coccinellidae F = 1.45, P = 0.16 (lady beetles) Formicidae F = 0.48, P = 0.92 (ants) Dermaptera F = 2.03, P = 0.03 (earwigs) Araneae F = 2.18, P = 0.02 (spiders) Total F = 0.68, P = 0.76 Different letters within each predator group indicate a significant difference between straw mulch and no straw mulch treatments (P < 0.05).