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© 2008 Plant Management Network. Abundance of Predaceous Arthropods and Lygus spp. (Hemiptera: Miridae) in Response to Different Nitrogen Fertilizer Rates in Acala 1517-99 Cotton Tracey Carrillo, Department of Extension Plant Sciences, Jeff Drake, Department of Agriculture, Animal Plant Health Inspection Service, and Joe Ellington and Jane Pierce, Department of Entomology, Plant Pathology, and Weed Science, Skeen Hall, MSC 3BE, College and Knox, New Mexico State University, Las Cruces, NM 88003 Corresponding author: Tracey Carrillo. tcarrill@nmsu.edu Carrillo, T., Drake, J., Ellington, J., and Pierce, J. 2008. Abundance of predaceous arthropods and Lygus spp. (Hemiptera: Miridae) in response to different nitrogen fertilizer rates in Acala 1517-99 Cotton. Online. Crop Management doi:10.1094/CM-2008-1212-01-RS. Abstract A three-year study evaluated the density of predaceous arthropods and Lygus spp. in response to different nitrogen fertilizer rates in Acala 1517-99 cotton [Gossypium hirsutum (L)]. The most abundant predator genera in the samples were Nabis spp., Chrysoperla spp., and Hippodamia spp. There were no significant increases in densities of predaceous arthropods with increased nitrogen rates. There were, however, significant increases in densities of adult Lygus spp. as nitrogen fertilizer rates were increased. Densities of Lygus spp. nymphs were much lower among years and treatments than adult densities. The highest Lygus spp./predator ratio (41.5:1) occurred at the lowest nitrogen rate (50.6 kg/ha) with no insecticide treatments. Introduction Nitrogen is a key element required by all living cells to build amino acids and proteins. All plant and animal life ultimately is limited by biologically available nitrogen (11). Several studies have indicated that densities of plant-feeding insects are related to the nitrogen content of plants fed upon [e.g., (4,7,10)]. The general importance of nitrogen for the development of plant-eating insects is well known [e.g., (6,8)], yet most field studies in cotton have ignored the nutritional quality of plants when evaluating the cause of phytophagous insect build-ups. Lygus spp. can be an important pest in some western states (3), as well as the mid-South, because they feed on cotton squares, which subsequently abort or have pollination problems, and their feeding can stain lint on larger bolls. Phytophagous insects such as Lygus spp. have been found in greater abundance in vigorously growing parts of cotton fields, which generally have faster fruiting rates, taller plants, and larger canopies (12), suggesting that Lygus spp. are attracted to plants with higher nitrogen levels. The objectives of this study were to determine whether applications of nitrogen fertilizer use in cotton production in the Southwest influenced densities of Lygus spp. and the beneficial insect complex. The importance of these effects may influence management styles and may increase or reduce cost associated with inputs. Field Management Acala 1517.99 cotton seed was planted at the rate of 16 kg/ha using a precision planter on furrow irrigated, uniform Harkey clay loam soil. Plots were planted on ground on which Sudan grass had been planted as the preceding annual crop. The Sudan grass had been bailed and removed to assist in the depletion of nitrogen carried over from previous crops and no additional nitrogen fertilizer was applied during this period. Originally plots were arranged in a randomized split plot design, replicated four times, with three irrigation rates as whole plots and nitrogen fertilizer rates as subplots. Later in the study a constant irrigation rate was used and the design used was a randomized complete block (RCB) split-block with irrigation as the whole block and varying nitrogen rates randomized within each block. Nitrogen fertilizer was applied as a split application — 50.4 kg/ha as a preplant broadcast application for all plots prior to bed formation and the remaining designated rate of nitrogen applied as a side-dress application at squaring. On average it is common practice for growers to make N fertilizer applications of about 128.8 kg N per ha. For those years in which irrigation rates varied each replicated sub-plot consisted of eight rows, each row 1 m wide by 30 m long, and for the year where irrigation rates remained constant, the block dimensions consisted of 40 rows with each row 1 m wide by 30 m long. The plot design was developed from earlier research conducted by Ellington et al. (1). The outer two rows of each plot were used as buffer rows, and the interior four rows were sampled for insects. The same area was not re-sampled within a 2-week period. Irrigation and fertilizer regimes were adjusted each year to accommodate for precipitation events that prevented access with equipment. One consistent data set which included a five irrigation regime for all three years was analyzed separately from those years which had varying irrigation treatments. There is also mention of insecticide Chlorpyrifos applications which were not planned for this study but reported due to incidentals which may have had an effect on arthropod density. In 2003, water was applied as four (48 cm/ha), five (60 cm/ha), and six (72 cm/ha) irrigations. There were two nitrogen side-dress application treatments of 50.4 kg/ha and 151.2 kg/ha post initial preplant broadcast application. Chlorpyrifos was applied three times to control pink bollworm. It was not the intent of this study for irrigations to be a separate treatment but to mention the results found with plot design optimization for irrigation strategies. For this reason it was determined that five irrigations were optimal to provide adequate soil moisture from planting to maturity for this soil type. Fewer than five irrigations for this particular soil type would increase stress to physiological processes, whereas more than five irrigations would provide no production benefit and could delay harvest. Therefore data from four and six irrigations were collected for years 2003 and 2004 but not 2005. In 2004, nitrogen rates were the same as they were in 2003, but one less irrigation per treatment was applied in which rainfall was sufficient enough to be counted as a single irrigation. As with irrigations, insecticide use was mentioned to bring awareness of potential interactions that may have occurred from applications of chlorpyrifos which were applied five times for control pink bollworm control. In 2005, agronomic processes were the same as 2003 and 2004 and nitrogen application treatments consisted of 56.0, 89.6, 128.8, 159.0, and 262.0 kg/ha. Five irrigations (60 cm/ha) were made for each fertilizer treatment, and no insecticides were applied. Vacuum Sampling of Insects To sample diverse cotton insect populations, a self-propelled platform (the Insectavac), which produced a 17.5-cm/sec vacuum through a 30-cm polyvinyl tube, was designed and built to take samples of representative cotton insects (1). The vacuum collector estimated the densities of 24 genera of cotton insects to within an average of 32.6% (range 0.2 to 47.4%) of their true mean values, depending on genera, densities, clumping patterns, and plant height. In previous work (2), estimates of densities of 24 genera of insects collected by a 38.1-cm diameter sweep net were within only 8.7% (range 0.07 to 19.0%) of their true mean value. A single, random 30-m sample over one row in each plot was taken at approximately 10:00 a.m. MST every two weeks from first bloom to physiological cutout which is considered to be about 5 nodes above white flower (NAWF). The insects in field-collected samples were killed by placing them in 1-gal sealable plastic bags and storing them in a freezer at 10°C. Samples were later removed from the freezer, and all leaf material and insects were manually separated with #3 and #7 sieves. Material was then meticulously hand-separated and data recorded. Processed samples were retained and placed back in freezer in case recounts needed to be made. Arthropod groups evaluated included Reduviidae adults and nymphs, Geocoris spp. adults and nymphs, Collops spp. adults and larvae, Chrysoperla spp. adults and larvae, Hippodamia spp. adults and larvae, Nabis spp. adults and nymphs, Orius spp. adults and nymphs, Lygus spp. adults and nymphs, and arachnids. Incidental arthropods such as Diptera, Hymenoptera, and other orders were excluded from dataset due to the vast numbers (thousands), minute size, or lack of importance. Statistical Design and Analyses The optimal plot size and sampling method for this study were predetermined from previous research conducted by Ellington et al. (1). Nitrogen rates were the variable split plot factor and or irrigation split block factor. Seasonal means for arthropod densities were analyzed due to clumping, seasonal occurrence, and or effects of insecticide treatments. Data were analyzed using mixed model analysis (PROC MIXED 1999, SAS Institute Inc., Cary, NC) to detect differences in seasonal means of densities of predaceous arthropods and Lygus spp. Data were not pooled across irrigations because if nitrogen was available but water was limited, the full potential of nitrogen uptake could not be realized. Means were separated using Fisher's least significant differences (LSD). Treatment effects with P values ≤ 0.05 were considered to be significant. Beneficial Arthropod Densities with Five Irrigations and Different Nitrogen Rates Chrysoperla spp. adults, Nabis spp. adults, and Hippodamia spp. adults were the most numerous predators collected from vacuum samples for year 2003. Of the 8 beneficial arthropod genera collected as adults and immature stages only two were statistically significantly different among different nitrogen rates, these included Geocoris spp. adults and Nabis spp. nymphs (Table 1). Mean seasonal total predators collected per 30 m of row (85.2, 93.0) between fertilizer rates throughout the season were not significantly different among nitrogen fertilizer treatments (50.4, 151.2 kg N per ha), respectively (Table 1). Densities of very mobile predators such as adult Chrysoperla spp., Nabis spp., and Hippodamia spp. were affected very little by higher nitrogen rates. Densities of predators remained relatively stable throughout the season even though chlorpyrifos was applied twice. Past research in New Mexico has shown that after about two weeks post insecticide applications, there is often a resurgence of migratory predaceous arthropods back into treated fields after insecticide residue subsides. Reservoirs of beneficial arthropods taking refuge in neighboring alfalfa fields are often the major contributing source (9). Table 1. Mean densities of beneficial arthropods from 30 linear m of row of Acala 1517-99 cotton with different nitrogen fertilizer rates with five irrigations for 2003, 2004, and 2005.
Means followed by like letters within the same year and taxa are not statistically different (P > 0.05) among different fertilizer rates. All treatments received five irrigations. In 2004 the most numerous beneficial arthropod genera were Nabis spp. adults and Collops spp. adults with no significant differences among seasonal means for total predators per 30 m of row (21.4, 21.9), with respective fertilizer rates (50.4, 151.2 kg N per ha). Lower seasonal means for total predaceous arthropods can be attributed to five repeated applications of chlorpyrifos insecticide applications made for pink bollworm control. Overall densities of all genera were substantially lower for this year. In 2005 the most abundant genera were Nabis spp. adults, Hippodamia spp. adults, and Arachnids with no significant differences among seasonal means of total predators collected (70.5, 61.0, 64.5, 67.6, 66.4) and nitrogen fertilizer rates (56.0, 89.6, 128.8, 159.0, 262.0 kg N per ha), respectively. With regards to individual genera some significant differences between beneficial arthropods and nitrogen fertilizer rates occurred but with no linearity towards increases in nitrogen fertilizer rates. This may due to the occurrence of random clumping throughout the field. Pest Arthropod (Lygus spp.) Densities and Different Nitrogen Rates Three year’s data (Table 1) comparing the effects of different fertilizer rates and five irrigations on Lygus spp. densities generally revealed that larger numbers of adults occurred in plots with higher rates of nitrogen. Mean densities of Lygus spp. adults were statistically higher (19.0, 14.1) in plots with a higher nitrogen rate (151.2 kg/ha) than in plots with a lower nitrogen rate (50.4 kg/ha) in 2003, respectively. Repeated insecticide applications probably reduced Lygus spp. densities in 2004; however, the mean density of Lygus spp. was also low in 2005, compared with the density of Lygus spp. in 2003, possibly attributable to large densities of predators. Densities of Lygus spp. nymphs did not seem to be affected by nitrogen rate for all years and all nitrogen rates. This might indicate that the adult population is primarily migratory selecting areas higher in nitrogen and that a native population of nymphs was kept in check by either insecticide applications or predation. Overall there was a general trend of increased Lygus spp. adult density as nitrogen rates increased and that both chlorpyrifos applications and low nitrogen levels contributed to a reduction in predator/Lygus spp. ratios (Fig. 1). Predator/Prey ratios (PPR) are an indication of the density dependant factors associated with nitrogen and insecticide. In 2005, 2004, and 2003 a higher PPR existed for those treatments that received less nitrogen. These data suggest an interaction between fertilizer rates and insecticide applications exist. It also shows that as nitrogen levels increased, numbers of Lygus spp. adults also increased, thus decreasing the predator/prey ratio. The highest predator/Lygus spp. ratio (41.5) occurred in the 56.0 kg N per ha treatments with no insecticide applications. These treatments also had the lowest densities of Lygus spp. (1.7 Lygus spp. adults/30 m of row). In contrast, the lowest predator/Lygus spp. ratio (4.9) and largest density of Lygus spp. (19.7 Lygus spp. Adults/30 m of row) occurred in the 151.2 kg N per ha treatments with three insecticide applications. The larger densities of Lygus spp. in 2003 may have been attributed to incomplete control of Lygus spp. by insecticides and low numbers of predators. All predators counted were generalist predators that could have fed upon all stages of Lygus spp., from egg to adult. Lygus spp. themselves sometimes are considered to be predators.
Beneficial Arthropod Densities with Four and Six Irrigations and Different Nitrogen Rates In 2003 the most numerous beneficial arthropods were: Nabis spp. adults; Hippodamia spp. adults; and Chrysoperla spp. adults for both four and six irrigations with different nitrogen fertilizer rates (50.4, 151.2 kg/ha, respectively) (Table 2). There were significantly more Chrysoperla spp. adults and Hippodamia spp. adults at higher nitrogen rates for both four and six irrigation regimes. There were also significantly more total predators in the four irrigation regimes than the six irrigation regime at the higher fertilizer rates. Table 2. Mean densities of beneficial arthropods from 30 linear meters of row of Acala 1517-99 cotton with different nitrogen fertilizer and irrigation rates for 2003 and 2004.
Means followed by like letters within the same year, taxa, and number of irrigations are not statistically different (P > 0.05) among different fertilizer rates. In 2004 densities of all arthropods were substantially lower than in 2003 and 2005, mainly due to repeated (five) chlorpyrifos applications. Geocoris spp. adults, Collops spp. adults, and Nabis spp. adults were the more abundant for this year. There were no significant differences among fertilizer treatments and beneficial arthropods densities for this year. Adult Nabis spp., Geocoris spp. adults, and Chrysoperla spp. adults were the most abundant predators in each nitrogen treatment. Discussion Industrial nitrogen in the past had been a relatively inexpensive input, and the application of “luxury” amounts of nitrogen was generally accepted without regard to effects on other mechanisms of the cropping system. Today industrial nitrogen is very expensive and increased awareness of greenhouse gas emissions and climate change is at the forefront. A direct relationship exists between primary consumers such as Lygus spp. and nitrogen fertilizer rates but little effect on the beneficial arthropod systems was observed. This association should be further explored to determine the most profitable outcome of nitrogen fertilizer use while accounting for increases in pest densities and its determination of cost associated with controlling pest populations. The average application rate of nitrogen fertilizer for cotton is about 134 to 168 kg/ha in New Mexico. This has been a general rule of thumb and since nitrogen is a fleeting nutrient it often is difficult to measure accurately and should be monitored during the critical stages of crop production. Nitrogen fertilizer can be readily leached out of the soil profile under flood irrigation, converted to nitrous oxide as a greenhouse gas, and sequestered from removal of a crop which significantly reduces soil nitrogen. From a producer’s standpoint, it is important to know the level of available nitrogen prior to first bloom (fruiting initiation). Excessive nitrogen applications could reduce the bottom line for producers and nitrogen deficiencies could produce substandard yields and quality. Not only does excessive nitrogen increase primary insect consumer densities, but other research has shown that organisms (e.g., plant pathogens, nematodes, other microorganisms) respond to excessive nitrogen rates as well (11). Increases in nitrogen may provide higher yields, but with economic (e.g., increased costs of inputs) and environmental (e.g., contamination of water) consequences. Field personnel need to take added measures to evaluate soil and plant nitrogen, monitor use throughout the season, and make appropriate nitrogen applications. Economic thresholds for primary consumers currently do not include potential effects of nitrogen fertilizer rates and should be reevaluated to adjust for these variables. It is possible that thresholds would be much lower when the nitrogen rate is optimal, rather than excessive. Increases in pest densities from increases in nitrogen fertilizer also increase the need for other inputs such as insecticides. System balance and stability should be considered when choosing management strategies for inputs such as nitrogen. Acknowledgment Project funded by Waste-Management Education Research Consortium (WERC). Literature Cited 1. Ellington, J., Kiser, K., Cardenas, M., Duttle, J., and Lopez, Y. 1984. The insectavac: A high volume arthropod vacuuming platform for agricultural ecosystems. Environ. Entomol. 73:259-265. 2. Ellington, J., Kiser, K., Ferguson, G., and Cardenas, M. 1984. A comparison of swept-net, absolute, and insectavac sampling mechanisms in cotton ecosystems. J. Econ. Entomol. 77:599-605. 3. Gutierrez, A. P., Leigh, T. F., Wang, Y., and Cave, R. D. 1977. An analysis of cotton production in California: Lygus hesperus (Heteroptera: Miridae) injury – an evaluation. Can. Entomol. 109:1375-1836. 4. Letourneau, D. K. 1995. Associational susceptibility effects of cropping pattern and fertilizer on Malawian bean fly levels. J. Appl. Ecol. 5:823-829. 5. Loya, J. 2002. Preliminary studies on the integrated control of the pink bollworm in cotton. Ph.D. diss., New Mexico State University, Las Cruces, NM. 6. Mattson, Jr., W. J. 1980. Herbivory in relation to plant nitrogen content. Annu. Rev. Ecol. Syst. 1:119-161. 7. McClure, M. S. 1991. Nitrogen fertilization of hemlock increases susceptibility to hemlock woolly adelgid. J. Arboric. 17:227-230. 8. McNeill, S., and Southwood, T. R. E. 1978. The role of nitrogen in the development of insect/plant relationships. Page 77-98 in: Biochemical Aspects of Insect/Plant Interactions. J. B. Harborne and H. F. van Emden, eds. Academic Press, London, UK. 9. Ozkock, H. A. 1997. Effectiveness of unconventional and conventional insecticides on the density of the beneficial complex in cotton. M.S. thesis, New Mexico State University, Las Cruces, NM. 10. Rossi, A. M., and Strong, D. R. 1991. Effects of host-plant nitrogen on the preference and performance of laboratory populations of Carneocephala ordana (Homoptera: Cicadellidae). Environ. Entomol. 20:1349-1355. 11. White, T. C. R. 1993. The Inadequate Environment: Nitrogen and the Abundance of Animals. Springer, New York, NY. 12. Willers, J. L., Seal, M. R., and Luttrell, R. G. 1999. Remote sensing, line-intercept sampling for tarnished plant bugs (Heteroptera: Miridae) in Mid-south cotton. J. Cot. Sci. 3:160-170. |
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