Current Economic Threshold Research Articles

Re-evaluating the Economic Injury Level for Alfalfa Weevil (Coleoptera: Curculionidae) Control in Low Desert Irrigated Alfalfa.

Alfalfa (Medicago sativa L.) dominates cropping systems in the Western United States and is first in terms of acreage planted in Arizona. The alfalfa weevil, Hypera postica (Gylenhall) and/or Hypera brunneipennis (Boheman), respectively, is the most destructive pest in terms of yield loss in low desert-grown alfalfa hay. The current economic threshold of 15–20 larvae per sweep, established in California in 1975, is currently not suitable or adopted by growers in the western U.S. low desert. Here, we conducted 4 yr of field trials to re-evaluate this economic threshold. Supporting observations of agricultural growers and professionals in the region, our results indicate that the economic threshold established in 1975 is too high. Specifically, one to three large larvae often cause a significant decrease in yield justifying weevil control based on current hay prices and costs of insecticide application. These results are discussed in the context of sustainable alfalfa production in the western U.S. low desert.

Economic Injury Levels for Bt-resistant Helicoverpa zea (Lepidoptera: Noctuidae) in Cotton.

Thresholds for Helicoverpa zea (Boddie) in cotton Gossypium hirsutum L. have been understudied since the widespread adoption of Bt cotton in the United States. Our study was possible due to the widespread presence of H. zea populations with Cry-toxin resistance. We initiated progressive spray timing experiments using three Bt cotton brands (Deltapine, Stoneville, and Phytogen) widely planted across the U.S. Cotton Belt expressing pyramided toxins in the Cry1A, Cry2, and Vip3Aa19 families. We timed foliar insecticide treatments based on week of bloom to manipulate H. zea populations in tandem with crop development during 2017 and 2018. We hypothesized that non-Bt cotton, cotton expressing Cry toxins alone, and cotton expressing Cry and Vip3Aa19 toxins would respond differently to H. zea feeding. We calculated economic injury levels to support the development of economic thresholds from significant responses. Pressure from H. zea was high during both years. Squares and bolls damaged by H. zea had the strongest negative yield associations, followed by larval number on squares. There were fewer yield associations with larval number on bolls and with number of H. zea eggs on the plant. Larval population levels were very low on varieties expressing Vip3Aa19. Yield response varied across experiments and varieties, suggesting that it is difficult to pinpoint precise economic injury levels. Nonetheless, our results generally suggest that current economic thresholds for H. zea in cotton are too high. Economic injury levels from comparisons between non-Bt varieties and those expressing only Cry toxins could inform future thresholds once H. zea evolves resistance to Vip3Aa19.

Re-evaluation of leaf-lamina consumer thresholds for IPM decisions in short-season soybeans using artificial defoliation

The impact of plant defoliation on the yield of short-season soybean cultivars was evaluated in five trials carried out in two crop seasons (2008/2009 and 2009/2010) in three states in Brazil (Rio Grande do Sul, Goiás and Mato Grosso). In 2008/2009, defoliation was performed by removing one, two or three leaflets (33.3%, 66.6% and 100% defoliation, respectively) from all leaves at different plant developmental stages (V5, V8, R2, from V5 to R2, and from V5 to harvest). In 2009/2010, the defoliation levels studied were 16.7% (half leaflet) and 33.3% (one leaflet), performed weekly during the vegetative or reproductive period, and during the entire cycle in three trials in Goiás. All tested cultivars tolerated defoliation levels equal and sometimes even higher than current Economic Thresholds (ETs) (30% vegetative stage defoliation threshold and 15% reproductive stage defoliation threshold) without significant yield loss. This indicates that even for the new cultivars belonging to shorter maturity groups or of indeterminate growth habit, soybean plants showed high defoliation tolerance. Thus, we conclude that the currently recommended ET for leaf-lamina consumers in the soybean crop is still valid and should be used by growers, even for the newer cultivars belonging to the lower maturity groups with indeterminate growth habit.

Trophic interactions between three species of cereal aphid (Hemiptera: Aphididae) and spring wheat (Poaceae): implications for pest management

AbstractThe susceptibilities of genetically diverse Canadian spring wheats, Triticum aestivum L. and Triticum durum Desf., to three aphid species, Rhopalosiphum padi (L.), Sitobion avenae (Fabricius), and Schizaphis graminum (Rondani), were investigated. Trophic interactions measured as changes in biomass of aphids and wheat plants were used to quantify levels of resistance, components of resistance, and impact of aphids on yield. Plants in field cages were infested with small numbers of aphids for 21 days at heading. These plants were usually more suitable for the development of S. avenae and S. graminum than of R. padi. Partial resistance, measured as seed production by infested plants as a proportion of that by a control, varied from 11% to 59% for different aphid species and wheat classes when all wheat plants were infested at the same stage. Cultivars within wheat classes responded similarly to each of the aphid species. None of the wheat cultivars showed agriculturally effective levels of antibiosis. The specific impact of each aphid species and wheat class varied from 5 to 15 mg of plant biomass lost for each milligram of biomass gained by the aphids. Canadian Western Red Spring wheat had a lower specific impact and therefore was more tolerant to aphids than the other two classes, but not tolerant enough to avoid economic damage at the aphid densities observed. Plants did not compensate for feeding damage after aphid feeding ceased, based on the higher specific impacts observed for mature plants than for plants that were heading. The interactions between aphids and plants show that current economic thresholds probably underestimate the damage caused by cereal aphids to Canadian spring wheat.

Alfalfa Living Mulch Advances Biological Control of Soybean Aphid

Despite evidence for biological control in North America, outbreaks of the invasive soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), continue to occur on soybean (Glycine max L. Merr.). Our objectives were to determine whether natural enemies delay aphid establishment and limit subsequent population growth and whether biological control can be improved by altering the within-field habitat. We hypothesized that a living mulch would increase the abundance of the aphidophagous community in soybean and suppress A. glycines establishment and population growth. We measured natural enemy and A. glycines abundance in soybean grown with and without an alfalfa (Medicago sativa L.) living mulch. Soybean grown with an alfalfa living mulch had 45% more natural enemies and experienced a delay in A. glycines establishment that resulted in lower peak populations. From our experiments, we concluded that the current natural enemy community in Iowa can delay A. glycines establishment, and an increase in aphidophagous predator abundance lowered the rate of A. glycines population growth preventing economic populations (i.e., below the current economic threshold) from occurring. Incorporation of a living mulch had an unexpected impact on A. glycines population growth, lowering the aphids' intrinsic rate of growth, thus providing a bottom-up suppression of A. glycines. We suggest future studies of living mulches or cover crops for A. glycines management should address both potential sources of suppression. Furthermore, our experience suggests that more consistent biological control of A. glycines may be possible with even partial resistance that slows but does not prevent reproduction.

Alfalfa Living Mulch Advances Biological Control of Soybean Aphid

Despite evidence for biological control in North America, outbreaks of the invasive soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), continue to occur on soybean (Glycine max L. Merr.). Our objectives were to determine whether natural enemies delay aphid establishment and limit subsequent population growth and whether biological control can be improved by altering the within-field habitat. We hypothesized that a living mulch would increase the abundance of the aphidophagous community in soybean and suppress A. glycines establishment and population growth. We measured natural enemy and A. glycines abundance in soybean grown with and without an alfalfa (Medicago sativa L.) living mulch. Soybean grown with an alfalfa living mulch had 45% more natural enemies and experienced a delay in A. glycines establishment that resulted in lower peak populations. From our experiments, we concluded that the current natural enemy community in Iowa can delay A. glycines establishment, and an increase in aphidophagous predator abundance lowered the rate of A. glycines population growth preventing economic populations (i.e., below the current economic threshold) from occurring. Incorporation of a living mulch had an unexpected impact on A. glycines population growth, lowering the aphids’ intrinsic rate of growth, thus providing a bottom-up suppression of A. glycines. We suggest future studies of living mulches or cover crops for A. glycines management should address both potential sources of suppression. Furthermore, our experience suggests that more consistent biological control of A. glycines may be possible with even partial resistance that slows but does not prevent reproduction.

Experimental Treatment Threshold for the Cotton Aphid (Homoptera: Aphididae) Using Natural Enemies in Arkansas Cotton

The potential of an experimental threshold for reducing the number of insecticide applications for control of the cotton aphid, Aphis gossypii Glover, was demonstrated in cotton. A 3-yr field study at Clarkedale, AR, compared the current economic threshold for the cotton aphid to an experimental threshold that incorporates predaceous coccinellids, aphid parasitoids and the aphid fungus, Neozygites fresenii (Nowakowski) Batko. Treatments consisted of untreated plots, conventional treatment threshold plots, experimental threshold plots, and a fungicide treatment designed to disrupt aphid fungus epizootics. This fungicide treatment failed to influence fungal epizootics, so data for these plots were removed. Twice weekly, aphids were counted on 20 plants per plot and predators were counted using a dislodgment method. An application of 0.033 kg ai/ha of imidacloprid was made when aphids reached treatment level according to conventional or experimental threshold levels. Insecticide applications were triggered by the conventional threshold on 18 and 28 June 1999, on 28 June and 3 July 2000, and on 7 and 12 July 2001. The experimental threshold called for applications on 28 June 1999, 3 July 2000, and 19 July 2001. Aphid densities peaked in untreated plots at 142, 76, and 27 aphids per leaf and in treated plots at 49, 34, and 29 in 1999, 2000 and 2001, respectively. Larval coccinellid densities in untreated plots peaked at 9.0, 3.7, and 0.6 larvae per row-m and in treated plots 3.0, 1.4, and 0.5 in 1999, 2000 and 2001, respectively. Adult coccinellid densities peaked in untreated plots at 3.0, 2.7, and 0.5 adults/row-m and in treated plots at 0.8, 2.1, and 0.5 in 1999, 2000, and 2001, respectively. During each year, an epizootic of N. fresenii caused a rapid decrease in aphid numbers in mid to late July, eliminating the need for additional treatments. In 1999, significantly lower yield of lint occurred in the untreated plots than in conventional or experimental plots. No significant yield differences among treatments occurred in 2000 or 2001. Use of the experimental threshold delayed the initial insecticide application by 1 wk and eliminated one insecticide application on the experimental plots whereas maintaining cotton yields.

EVALUATION OF AN OVIPOSITION TRAP FOR MONITORING EGG POPULATIONS OF DIABROTICA SPP. (COLEOPTERA: CHRYSOMELIDAE) IN FIELD CORN

AbstractTraps consisting of open, cylindrical tins containing moistened clay aggregates were evaluated as field oviposition sites for corn rootworm, Diabrotica spp. Six trap features were investigated: clay aggregate size, water saturation level, trap opening, trap cover, trap volume, and trap position relative to corn plants. More eggs were recovered in traps containing clay aggregates ranging from 0.5 to 3.5 mm diameter compared with aggregates from 2.5 to 8.0 mm diameter. Saturation of the trap to 2.5 cm from the opening resulted in greater egg recovery compared with traps saturated to 6.0 cm from the opening. Covering the exposed surface of the trap with a metal ring and/or a corn leaf increased egg recovery in the field.Oviposition traps were placed in four commercial corn fields in 1990 and three in 1991 to monitor egg populations. Egg recovery from traps and estimates of the absolute egg population in the soil were compared with densities of adult corn rootworm to predict larval damage on roots of corn planted the next year. In five fields, adult populations were above the current economic threshold of one beetle per plant. However, economic damage to roots occurred only in the field in which the most eggs were recovered from traps (226.6 eggs per trap) and soil samples (30.2 eggs per litre). Mean egg recovery per trap per field was correlated with mean damage ratings from untreated corn.

Parasitism of Heliothis zea (Lepidoptera: Noctuidae) Eggs: Effect on Pest Management Decision Rules for Processing Tomatoes in the Sacramento Valley of California

Egg parasitism of Heliothis zea (Boddie) and other lepidopterous pests was monitored in a field trial of processing tomato cultivars in 1981 and in several late-season commercial fields of processing tomatoes in the Sacramento Valley of California in 1983, 1985, and 1986. Four species of Trichogramma were present: T. pretiosum Riley, T. thalense (Pinto & Oatman), T. deion Pinto & Oatman, and T. brevicapillum Pinto & Platner. Actual parasitism of H. zea eggs exceeded 83% in all fields monitored in 1985 and exceeded 62% in most fields monitored in 1986. Manduca spp. and Trichoplusia ni (Hübner) eggs were also commonly parasitized by Trichogramma spp. The high levels of H. zea egg parasitism recorded in these commercial fields indicates that the current economic threshold, which is based on the abundance of white H. zea eggs, can be increased to account for the mortality caused by Trichogramma. Procedures to incorporate H. zea egg parasitism into the current integrated pest management program for processing tomatoes are proposed. Acceptable fruit quality at harvest in fields that exceeded the current economic threshold during the growing season provides additional support for increasing the threshold. The effect of insecticide applications on H. zea egg parasitism is discussed.

Model Simulating the Use of Miticides to Control European Red Mite (Acarina: Tetranychidae) in Nova Scotia Apple Orchards

A model was developed to simulate chemical control of the European red mite, Panonychus ulmi (Koch), in Nova Scotia apple orchards. Simulated densities of summer eggs and motile P. ulmi were similar to densities observed in experimental plots that had been treated with miticide on different dates. The P. ulmi model was used to estimate the best dates to apply miticides with different biological half-lives (7 or 30 d) and specific toxicities (i.e., toxic to all eggs; toxic only to summer eggs; toxic only to motile forms). Criteria of effectiveness of a given application were cumulative mite-days to 15 July, yield per tree, and density of P. ulmi winter eggs on 31 August. The best dates to apply miticide (usually shortly before or after winter eggs hatched) differed according to the characteristics of the miticide and the criterion of effectiveness (e.g., the best dates for preserving yield were not necessarily the best dates for reducing winter eggs). The model also indicated problems in the use of the current economic threshold based on counts of summer eggs and motile P. ulmi on leaves. If the initial P. ulmi population is moderate or high and the choice of application date is based on mite density on leaves, then application of miticide may be too late to prevent significant yield lose sin the current year, a high population of winter eggs at the end of the season, and reduced return bloom and yield the following year. The model suggests the importance of early spring assessments of winter egg density and the necessity of considering age structure of the P. ulmi population and characteristics of the miticide in selecting dates for miticide application.

Testing the Interactions of Pest–Predator–Plant Components of the TEXCIM Model

By integrating the dynamics of cotton fruit production with insect pests and their natural enemies, the TEXCIM model predicted the timing and magnitude of pest and injured fruit pulses 7–14 d into the future. Biofixes from field counts of fruit, pests, and natural enemies provided a self-correcting feature for improving these predictions. Based on current economic thresholds, serious Type I and Type II errors occurred only 1.7% of the time for Heliothis species and 5.2% of the time for cotton fleahoppers. Thus, TEXCIM has considerable potential for developing dynamic decision rules for guilds of pests that feed on cotton and as a decision aid in pest management.

Value of Adult Corn Rootworm (Coleoptera: Chrysomelidae) Population Estimates in Pest Management Decision Making

Corn root worm adult densities were monitored in 31, 43, and 44 Iowa cornfields in 1979, 1980, and 1981, respectively. The season after monitoring, plots with no soil insecticide were left in most fields. Root worm larval damage was measured in untreated areas, and yield was estimated in both treated and untreated areas of each field. Linear regressions indicated significant relationships between adult densities and larval damage and yield loss, but much of the variation in damage or yield loss was not explained by the model. Current economic thresholds did not accurately predict economic damage or appreciable yield loss >50% of the time. Addition of edaphic and agronomic variables improved the models, but 60% of the variation in yield loss was still unexplained. Value of adult sample information was determined by Bayesian analysis to be zero. A 6-fold increase in ability to predict when economic loss would not occur would be necessary for sampling to be worthwhile. The optimal strategy for managing corn root worms in Iowa in our study was not to sample for adults and always to treat corn following corn with a soil insecticide at planting time.

Cereal leaf beetle ( Oulema melanopus (L.)) population densities and winter wheat yields

Grain yields of a soft white winter wheat cultivar, cv. Ionia, susceptible to cereal leaf beetle ( Oulema melanopus (L.)), were determined when the crop was subjected to four controlled infestation levels of the beetle in caged plots over a 3-year period in Michigan. Although identical infestation procedures and insect numbers were used each year, the intensity of larval infestation and feeding damage varied with the year. Significant ( P < 0·01) grain losses occurred in plots that had at least 9·0 larvae per stem. The greatest yield loss was 45% and occurred with 15 larvae per stem and 85% feeding damage. Beetle damage had a greater effect on kernel weight than on the other yield components. Although economic thresholds should be conservative to allow for environmental variations, it appears that soft white winter wheat can often withstand beetle infestation levels greater than the current economic threshold of three or more eggs and largae per stem before the boot stage or one or more large larvae per flag leaf during heading.