Effectiveness of corn rootworm (Coleoptera: Chrysomelidae) areawide pest management in South Dakota

Diabrotica barberi Smith & Lawrence and Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) are serious pests of maize, Zea mays L. To reduce the amount of toxicants released into the environment, the Agricultural Research Service implemented a 5-yr (1997-2001) areawide pest management program in five geographic locations, including one in South Dakota. The objective was to use integrated pest management tactics to suppress adult Diabrotica populations over a broad geographic area by using aerially applied semiochemical-baited insecticides. Suppressed populations theoretically should reduce oviposition, limit larval feeding damage to maize roots, and result in fewer beetles emerging in subsequent years. We used emergence cages, sticky traps, and CRW lure traps to monitor adult D. barberi and D. v. virgifera populations. We sampled for Diabrotica eggs, and we determined damage to maize roots. We sampled in several maize fields (control) located near the areawide site. The baited insecticides were effective in reducing adult populations 1 and 2 wk after application, and most remained low for the duration of the maize growing season. Fewer beetles were captured in both sticky and lure traps in the areawide site than in the control site. With a few exceptions, egg counts, adult emergence, and maize root damage were similar between the areawide and control sites; however, maize roots had greater fresh weight in the control site. Although not all goals were accomplished, when considering the amount of toxicant released into the environment, using semiochemical-baited insecticides to suppress adult pest Diabrotica populations seems to be an effective areawide management tool.     

Comparison of adult corn rootworm (Coleoptera: Chrysomelidae) sampling methods

Studies were conducted in Kansas corn and soybean fields during 1997 to compare various sampling methods, traps, and trap components for capturing three species of adult corn rootworms: western (Diabrotica virgifera virgifera Leconte), southern (D. undecimpunctata howardi Barber), and northern (D. barberi Smith & Lawrence). Lure constituents affected the species of beetle attracted to the trap. Traps with a lure containing 4-methoxycinnamaldehyde attracted more western corn rootworms, those with a lure containing eugenol were more attractive to northern corn rootworms, and those containing trans-cinnamaldehyde were most attractive to southern corn rootworms. Multigard sticky traps caught more beetles than did Pherocon AM sticky traps. In corn, a newly designed lure trap caught more beetles than did sticky traps on most occasions. Also, lure-baited sticky traps caught more beetles than did nonbaited sticky traps. Varying the color of the lure trap bottom did not affect the number caught. In soybeans, the new lure traps captured more beetles than did the nonbaited Multigard or Pherocon AM sticky traps. Results of this study suggest the new lure trap may provide a more accurate assessment of corn rootworm populations than traditional monitoring techniques and may be more esthetically pleasing to growers and consultants.     

Monitoring western corn rootworm (Coleoptera: Chrysomelidae) susceptibility to carbaryl and cucurbitacin baits in the areawide management pilot program

Areawide pest management involves the uniform application of a pest control strategy over wide geographic areas. Therefore, these programs are likely to impose intense selective pressures, and the risk for resistance development among pest species for which areawide management programs are implemented is likely to be high. Pilot studies for areawide management of western corn rootworm, Diabrotica virgifera virgifera LeConte, were conducted from 1996 to 2002 at four different sites across the Corn Belt. This program used cucurbitacin baits to deliver high doses of a traditional neurotoxic insecticide (carbaryl) to individual insects while reducing the overall rate of insecticide use. Because of the concern and potential for resistance evolution, annual assessments of susceptibility to the active ingredient carbaryl were conducted both within the managed area as well as from untreated control areas. Significantly reduced susceptibility to carbaryl based on survival at a diagnostic concentration was detected in three of the four management sites (Kansas, Iowa, and Illinois/Indiana), whereas susceptibility of beetles collected outside the managed areas remained unchanged. Additionally, significantly reduced responsiveness to cucurbitacin baits was observed in beetles collected from the managed area relative to the control area at the same three sites. These results suggest strongly that areawide management has the potential to select for resistance and that a strategy for managing resistance and reducing selective pressure should be proactively implemented.     

Prairie grasses as hosts of the northern corn rootworm (Coleoptera: Chrysomelidae)

We evaluated 27 prairie grass species thought to be among those dominant 200 yr ago in the northern midwest as larval hosts of the northern corn rootworm, Diabrotica barberi Smith and Lawrence. Maize (Zea mays L.), spring wheat (Triticum aestivum L.), and grain sorghum (Sorghum bicolor L.) were included as controls for a total of 30 species. Twenty pots of each species were planted in a randomized complete block design. Each pot was infested 5 wk later with 20 neonate northern corn rootworm larvae. Two pots within each species and block were assigned an extraction date of 7 or 14 d after infestation. The remaining two pots from each block were used to monitor adult emergence. The percentage of larvae recovered, change in larval head capsule width, and change in average dry weights varied significantly among the grass species. The highest percentage of larvae was recovered from slender wheatgrass, Elymus trachycaulus (Link), and this was significantly greater than the percentage recovered from all other species including maize for the 14-d sample date. Several additional species were also relatively good hosts, in that the percentage of larvae recovered from these species was not significantly different from maize. The average dry weight of larvae recovered was significantly greater for larvae recovered from maize than for larvae recovered from all other species except slender wheatgrass, when the two samples dates were combined. Overall, adults were produced from only 6 of the 28 species evaluated, and no analysis was performed because of the low numbers. The results of this study are discussed in relation to the potential of alternate hosts of northern corn rootworm to serve as a bridge to survival on transgenic maize.     

Stochastic dynamic population model for northern corn rootworm (Coleoptera: Chrysomelidae).

A stochastic dynamic population model for the complete life cycle of northern corn rootworm, Diabrotica barberi Smith & Lawrence, is described. Adult population dynamics from emergence to oviposition are based on a published single-season model for which temperature-dependent development and age-dependent advancement determine adult population dynamics and oviposition. Randomly generated daily temperatures make this model component stochastic. Stochastic hatch is 50+/-8%. A stochastic nonlinear density-dependent larval survival model is estimated using field data from artificial infestation experiments. A regional model of corn phenology is estimated to incorporate the effect of dispersal on adult mortality. Random daily weather is generated using parameters for Brookings, SD. Model performance is evaluated with deterministic simulations, which show that the population converges to zero unless adult mortality is reduced by the availability of corn pollen from the regional model of corn phenology. Stochastic model performance is evaluated with stochastic daily weather, egg hatch, and larval survival in various combinations. Sensitivity analysis is conducted to evaluate model responsiveness to each parameter. Model results are generally consistent with published data.     

Foraging resources of adult Mexican corn rootworm (Coleoptera: Chrysomelidae) in Bell County, Texas

Pollen analyses were used to determine pollen foraging resources of adult Mexican corn rootworms, Diabrotica virgifera zeae Krysan & Smith, captured near Temple, Bell County, TX, in 1996 and 1997. In 1996, adult Mexican corn rootworms were captured in a corn, Zea mays L., field. In 1997, nine locations outside of cornfields were added. Overall, 92% of the beetles (n = 1,323) contained pollen. More than 142,000 pollen grains were counted, representing 45 families, 63 genera, and 27 species. Overall, in 1996, noncorn grass pollen (70%) occurred in the greatest percentage of total pollen followed by corn (17%), then nongrass pollen (13%). In 1997, noncorn grass pollen (76%) had the greatest percentage, then nongrass (18%), and finally corn pollen (6%). Corn pollen was found in 34% of the beetles in 1996 and 26% in 1997. Fifteen Asteraceae taxa were encountered including sunflower, Helianthus annuus L. Thirteen Fabaceae were found including soybean, Glycine max L. Mexican corn rootworm adults foraged mainly on noncorn grass pollen, but also foraged on pollen from a large diversity of plant species, indicating that noncorn pollen may play a role in the food habits of Mexican corn rootworm adults.     

Larval sampling and instar determination in field populations of northern and western corn rootworm (Coleoptera: Chrysomelidae)

Abundance and head capsule width were measured for northern (Diabrotica barberi Smith & Lawrence) and western corn rootworm (D. virgifera virgifera LeConte) larvae recovered primarily from maize root systems but also from large soil cores each centered around a root system. Larvae for measurement derived from field populations under infestation and rotation regimes that allowed most specimens to be assigned to species. A frequency distribution of head capsule widths indicated three separate peaks for western corn rootworm, presumably representing frequency of the three larval instars, with no larvae measuring 280 or 420 microm in the valleys between peaks. Multiple normal curves fit to similar but partially overlapping peaks generated by northern corn rootworm suggested that division of first to second and second to third instar can best be made for this species at 267 and 406 microm, respectively (270 and 410 when measurements are made to the nearest 20 microm). These results implied that instar of individuals from mixed northern and western corn rootworm populations can be accurately judged from head capsule width without having to determine species. The relative abundance of western corn rootworm instars was similar in root systems removed from the center of 19-cm diameter x 19-cm deep soil cores and in soil cores from which the root systems were removed. Furthermore, the number of larvae from root systems correlated significantly with that from the surrounding soil. These results indicated that the former and much more convenient sampling unit can be used to estimate population developmental stage and possibly density, at least early in the season when these tests were done and young larvae predominated.     

Baseline susceptibility of western corn rootworm (Coleoptera: Chrysomelidae) to clothianidin

Abstract:  Western corn rootworm, Diabrotica virgifera virgifera LeConte, neonate susceptibility to clothianidin, a contact and systemic neonicotinoid insecticide, was determined from both laboratory and field-collected populations. Neonates were exposed to filter paper treated with increasing clothianidin concentrations and mortality was evaluated after 24 h. Additionally, two populations were exposed to an artificial diet which was surface treated with clothianidin. Although larvae were five- to six-fold more sensitive to treated diet, results with treated filter paper were more reliable in terms of control mortality and required much less manipulation of rootworm larvae. Therefore, initial baseline comparisons were conducted using the filter paper assays. The variation among populations exposed to treated filter paper was generally low, 4.4-fold among laboratory populations tested; however, there was a 14.5-fold difference in susceptibility among all populations tested. In general, clothianidin was very toxic to rootworm neonates, with LC₅₀ values ranging from 1.5 to 21.9 ng/cm². These results indicate the practicability and sensitivity of the paper filter disc assay to establish baseline susceptibility levels, which is an essential first step in resistance management. A baseline response provides a reference for tracking shifts in susceptibility following commercialization of a control agent so that early changes in susceptibility can be detected.     

Multiplex polymerase chain reaction method for differentiating western and northern corn rootworm larvae (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte, and northern corn rootworm, D. barberi Smith and Lawrence, are sympatric species and serious pests of corn cultivation in North America. Comparison of nucleotide sequence of mitochondrial cytochrome oxidase I and II was used to design polymerase chain reaction (PCR) primers that discriminate immature stages of the two species based on differences in amplicon size. Multiplex PCR can be used to give a positive test for each species in a single amplification reaction. This provides a method to identify field caught larvae and facilitates investigations of larval interaction and competition between the species.     

Transect sampling to enhance efficiency of corn rootworm (Coleoptera: Chrysomelidae) monitoring in corn

Crop monitoring for adult corn rootworms, Diabrotica virgifera virgifera LeConte and Diabrotica barberi Smith and Lawrence, remains the best means to assess fields at risk from this pest if replanted to corn, Zea mays (L.). Improvements in sampling methodology, including the development of a sequential sampling plan, have reduced the minimum sampling time required to make a management decision to 20 min or less per field per visit. However, many growers and crop consultants still find this time commitment a constraint to repeated scouting. A common currently used sampling method involves systematically covering most of the field following a "W" pattern. The feasibility of replacing the current sampling pattern with a simpler and less time-consuming transect (straight line) pattern was assessed. When sampling methods were compared, computer simulations demonstrated that treatment decisions based on transect sampling would have an acceptably low error rate averaging 10% over a range of realistic corn rootworm densities (0-2 adults per plant). This error rate represented a decrease in accuracy of <1% compared with systematic sampling. Field trials using transect, systematic, and random sampling in each field were used to compare the categorization of adult corn rootworm densities into "above" or "below" threshold with a sequential sampling plan. Efficiency measured in time to reach a decision, number of corn plants evaluated, and time divided by plants observed were compared between sampling methods. The three methods did not differ significantly in the number of plants evaluated or in the categorization of corn rootworm populations. Transect sampling resulted in a significantly shorter time divided by plants observed (38 s), than either systematic (78 s), or random sampling methods (166 s). Based on these results transect sampling reduces sampling time 51% compared with systematic sampling and thus could be used to reduce total sampling times substantially.     

Post-establishment movement of western corn rootworm larvae (Coleoptera: Chrysomelidae) in Central Missouri corn

If registered, transgenic corn, Zea mays L., with corn rootworm resistance will offer a viable alternative to insecticides for managing Diabrotica spp. corn rootworms. Resistance management to maintain susceptibility is in the interest of growers, the Environmental Protection Agency, and industry, but little is known about many aspects of corn rootworm biology required for an effective resistance management program. The extent of larval movement by the western corn rootworm, Diabrotica virgifera virgifera LeConte, that occurs from plant-to-plant or row-to-row after initial establishment was evaluated in 1998 and 1999 in a Central Missouri cornfield. Post-establishment movement by western corn rootworm larvae was clearly documented in two of four treatment combinations in 1999 where larvae moved up to three plants down the row and across a 0.46-m row. Larvae did not significantly cross a 0.91-m row after initial host establishment in 1998 or 1999, whether or not the soil had been compacted by a tractor and planter. In the current experiment, western corn rootworm larvae moved from highly damaged, infested plants to nearby plants with little to no previous root damage. Our data do not provide significant insight into how larvae might disperse after initial establishment when all plants in an area are heavily damaged or when only moderate damage occurs on an infested plant. A similar situation might also occur if a seed mixture of transgenic and isoline plants were used and if transgenic plants with rootworm resistance are not repellent to corn rootworm larvae.     

Mating success and spermatophore composition in Western corn rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm (Diabrotica virgifera virgifera LeConte) resistance management for transgenic (or Bt) corn hinges on understanding the mating behavior and biology of this adaptable insect pest. During mating, the male transfers sperm and additional, previously uncharacterized material, to the female in the form of a spermatophore. We investigated the composition of rootworm spermatophores. Proteins were found to be a major component, and the stable isotope (15)N was used to assess the fate of spermatophore nitrogen in mated female beetles and their eggs. We also performed longevity studies on mated and virgin females under three different diet treatments and investigated the relationships between morphometric characteristics and spermatophore volume of mating pairs of beetles. The stable isotope analysis determined that nitrogen provided to the female in the spermatophore was incorporated into the eggs. We found that virgin female beetles on a corn diet lived significantly longer than mated female beetles on the same diet. There were significant positive relationships between male size parameters (head capsule width, pronotum width, and elytral length) and spermatophore volume, and ampulla and spermatophylax volume.     

Modeling the dynamics of adaptation to transgenic corn by western corn rootworm (Coleoptera: Chrysomelidae)

A simulation model of the population dynamics and genetics of the western corn rootworm, Diabrotica virgifera virgifera LeConte, was created for a landscape of corn, soybean, and other crops. Although the model was created to study a 2-locus problem for beetles having genes for resistance to both crop rotation and transgenic corn, during this first phase of the project, the model was simulated to evaluate only resistance management plans for transgenic corn. Allele expression in the rootworm and toxin dose in the corn plant were the two most important factors affecting resistance development. A dominant resistance allele allowed quick evolution of resistance to transgenic corn, whereas a recessive allele delayed resistance >99 yr. With high dosages of toxin and additive expression, the time required to reach 3% resistance allele frequency ranged from 13 to >99 yr. With additive expression, lower dosages permitted the resistant allele frequency to reach 3% in 2-9 yr with refuges occupying 5-30% of the land. The results were sensitive to delays in emergence by susceptible adults and configuration of the refuge (row strips versus blocks).     

Identification of feeding stimulants in corn roots for western corn rootworm (Coleoptera: Chrysomelidae) larvae

Using a bioassay-driven approach, we have isolated and identified a blend of compounds from the roots of germinating corn, Zea mays L., that serve as feeding stimulants for neonate western corn rootworm larvae, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae). The active blend is a combination of simple sugars (30:4:4 mg/ml glucose:fructose:sucrose in the corn root) plus at least one of the free fatty acids in germinating corn roots (2:5 mg/ml oleic acid:linoleic acid in the corn root). When an extract of germinating corn was partitioned into an ethyl acetate fraction and an aqueous fraction, full feeding occurred only when the two fractions were recombined, indicating that the phagostimulant was comprised of both polar and nonpolar components. Gas chromatography-mass spectrometry analysis of root extracts from germinating corn seedlings revealed a blend of 20 compounds from a variety of chemical classes, including small sugars, diacids, amino acids, inorganic compounds, and free fatty acids. When the major components were tested in feeding bioassays, the sugars and lipids were shown to be essential for feeding by larvae, but the two classes of compounds were only effective when combined. The sugars alone elicited feeding by only 40% of larvae, but the percentage of larvae feeding was increased significantly with the addition of linoleic acid (91.7% larvae feeding) or oleic acid (85.8% larvae feeding). The amino acids alone were not essential elements for feeding by western corn rootworm larvae.     

Effect of extended diapause on evolution of resistance to transgenic Bacillus thuringiensis corn by northern corn rootworm (Coleoptera: Chrysomelidae)

We develop a population genetics model for the northern corn rootworm, Diabrotica barberi Smith & Lawrence, to examine the effect of extended diapause on the evolution of resistance to transgenic Bacillus thuringiensis (Bt) corn, Zea mays L. We model conditions found in the center of the extended diapause problem along the Minnesota-South Dakota-Iowa borders. The proportion of resistance alleles in eggs oviposited after 15 simulated years is used to measure the evolution of resistance. Sensitivity analysis indicates that although population genetics parameters (fecundity, initial egg density, density-dependent larval survival, random mating, insecticide mortality, and gene expression) affect the evolution of resistance, product characteristics (e.g., Bt toxin dose) and farmer management practices (e.g., insecticide use on refuge corn and rotation pattern) generally have a larger impact on the development of resistance. Exceptions to this generalization exist: 1) if the resistance allele is dominant, resistance evolves quickly; 2) the level of random mating is an important determinant of how quickly resistance evolves for a theoretical high dose product; and 3) small differences in insecticide mortality imply large differences in resistance for medium- and low-dose products with high levels of Bt corn adoption and a predominance of 1- and 2-yr corn rotations. When extended diapause spreads into a new area, it typically reduces resistance to Bt corn, assuming Bt corn is used only on continuous corn. In the study region where extended diapause already exists, increasing extended diapause (increasing hatch rates after two or three winters while holding total hatch constant), tends to increase resistance because the resistance increasing effect of the hatch rate after two winters dominates the resistance decreasing effect of the hatch rate after three winters. However, this is not always the case, because combinations of rotation pattern, toxin dose, and soil insecticide use exist for which the net effect of extended diapause decreases resistance. Results are interpreted as a combination of two offsetting effects. First, extended diapause injects older alleles with lower resistance allele frequencies into the breeding population, which slows resistance. Second, extended diapause speeds the population's recovery from perturbations (reduces the undercompensating density dependence of population dynamics), which accelerates resistance.     

In-field labeling of western corn rootworm adults (Coleoptera: Chrysomelidae) with rubidium

Field and laboratory studies were conducted in 2000 and 2001 to determine the feasibility of mass marking western corn rootworm adults, Diabrotica virgifera virgifera LeConte, with RbCl in the field. Results showed that application of rubidium (Rb) in solution to both the soil (1 g Rb/plant) and whorl (1 g Rb/plant) of corn plants was optimal for labeling western corn rootworm adults during larval development. Development of larvae on Rb-enriched corn with this technique did not significantly influence adult dry weight or survival. Rb was also highly mobile in the plant. Application of Rb to both the soil and the whorl resulted in median Rb concentrations in the roots (5,860 ppm) that were 150-fold greater than concentrations in untreated roots (38 ppm) 5 wk after treatment. Additionally, at least 90% of the beetles that emerged during the first 3 wk were labeled above the baseline Rb concentration (5 ppm dry weight) determined from untreated beetles. Because emergence was 72% complete at this time, a significant proportion of the population had been labeled. Results from laboratory experiments showed that labeled beetles remained distinguishable from unlabeled beetles for up to 4 d postemergence. The ability to efficiently label large numbers of beetles under field conditions and for a defined period with virtually no disruption of the population provides an unparalleled opportunity to conduct mark-recapture experiments for quantifying the short-range, intrafield movement of adult corn rootworms.     

Insecticide enhancement with feeding stimulants in corn for western corn rootworm larvae (Coleoptera: Chrysomelidae)

Amounts of the insecticide thiamethoxam required for 50% mortality of western corn rootworm larvae, Diabrotica virgifera virgifera LeConte, were reduced 100-fold when extracts of germinating corn, Zea mays L., were used to entice neonate larvae to feed on it. In behavioral bioassays, neonate rootworm larvae fed vigorously on filter paper disks treated with liquid pressed from corn roots. Moreover, disks treated with an acetone extract of corn (dried and rewetted with water) also elicited strong feeding from larvae. Larvae wandered away from filter paper disks treated with distilled water without feeding. Dilutions of thiamethoxam were tested in the bioassay alone or with corn extract and the efficacy of this insecticide was improved by the addition of the corn extract. For solutions containing 10 ppm thiamethoxam, 95% larval mortality occurred after 30 min of exposure when corn extract was present, but only 38% mortality occurred when the same concentration of insecticide alone (no feeding stimulants) was tested. Larval mortality after 24 h was significantly higher for corn extract-treated disks with 0.01, 0.1, 1, or 10 ppm insecticide than for the same concentrations without corn extract. Thiamethoxam did not deter larval feeding on corn extract, even at the highest concentration of thiamethoxam tested.     

Inheritance of methyl-parathion resistance in Nebraska western corn rootworm populations (Coleoptera: Chrysomelidae)

Field populations of western corn rootworm, Diabrotica virgifera virgifera LeConte, were collected from three different sites (York Co., Phelps Co., and Saunders Co.) in Nebraska during 1996. Adult bioassays of these three populations were conducted with different concentrations of methyl-parathion and at a diagnostic concentration (1.0 microg/ml) to determine resistance levels among these populations. Self and reciprocal crosses were made between the two resistant and one susceptible laboratory-reared populations. Dose-responses and dominance ratios calculated for the four reciprocal crosses indicated that resistance was incompletely dominant in both strains, although in one of the strains there was an indication of sex linkage. However, evaluation of native polyacrylamide gels stained for nonspecific esterases and nonspecific esterase activity of parents and F1 progeny of the crosses suggested that esterase inheritance was completely dominant and autosomal. The results of this study were inconclusive with regard to the precise nature of inheritance, because the bioassays and esterase assays could not discriminate between heterozygotes and homozygotes. However, they do provide insight into the potential for developing simple diagnostic assays to assess resistance frequencies. Based on the inheritance studies described in this investigation, we can begin to generate information on specific genetic factors that dictate the evolutionary divergence of discrete resistant populations and facilitate modeling efforts designed to approximate the movement of genes for resistance among populations.     

Economic analysis of dynamic management strategies utilizing. Transgenic corn for control of western corn rootworm (Coleoptera: Chrysomelidae)

We studied management strategies for western corn rootworm, Diabrotica virgifera virgifera LeConte, using transgenic corn, Zea mays L., from both a biological and an economic perspective. In areas with and without populations adapted to a 2-yr rotation of corn and soybean (rotation-resistant), the standard management strategy was to plant 80% of a cornfield (rotated and continuous) to a transgenic cultivar each year. In each area, we also studied dynamic management strategies where the proportion of transgenic corn increased over time in a region. We also analyzed management strategies for a single field that is the first to adopt transgenic corn within a larger unmanaged region. In all areas, increasing the expression of the toxin in the plant increased economic returns. In areas without rotation-resistance, planting 80% transgenic corn in the continuous cornfield each year generated the greatest returns with a medium toxin dose or greater. In areas with alleles for rotation-resistance at low initial levels, a 2-yr rotation of nontransgenic corn and soybean, Glycine max (L.) Merr., may be the most economical strategy if resistance to crop rotation is recessive. If resistance to crop rotation is additive or dominant, planting transgenic corn in the rotated cornfield was the most effective strategy. In areas where rotation-resistance is already a severe problem, planting transgenic corn in the rotated cornfield each year was always the most economical strategy. In some cases the strategies that increased the proportion of transgenic corn in the region over time increased returns compared with the standard strategies. With these strategies the evolution of resistance to crop rotation occurred more rapidly but resistance to transgenic corn was delayed compared with the standard management strategy. In areas not managed by a regional norm, increasing the proportion of transgenic corn and increasing toxin dose in the managed field generally increased returns. In a sensitivity analysis, among the parameters investigated, only density-dependent survival affected the results.