Effects of a potato cysteine proteinase inhibitor on midgut proteolytic enzyme activity and growth of the southern corn rootworm, Diabrotica undecimpunctata howardi (Coleoptera: Chrysomelidae)

The major proteinase activity in extracts of larval midguts from the southern corn rootworm (SCR), Diabrotica undecimpunctata howardi, was identified as a cysteine proteinase that prefers substrates containing an arginine residue in the P1 position. Gelatin-zymogram analysis of the midgut proteinases indicated that the artificial diet-fed SCR, corn root-fed SCR, and root-fed western corn rootworms (Diabrotica virgifera virgifera) possess a single major proteinase with an apparent molecular mass of 25kDa and several minor proteinases. Similar proteinase activity pH profiles were exhibited by root-fed and diet-fed rootworms with the optimal activity being slightly acidic. Rootworm larvae reared on corn roots exhibited significantly less caseinolytic activity than those reared on the artificial diet. Midgut proteolytic activity from SCR was most sensitive to inhibition by inhibitors of cysteine proteinases. Furthermore, rootworm proteinase activity was particularly sensitive to inhibition by a commercial protein preparation from potato tubers (PIN-II). One of the proteins, potato cysteine proteinase inhibitor-10', PCPI-10', obtained from PIN-II by ion-exchange chromatography, was the major source of inhibitory activity against rootworm proteinase activity. PCPI-10' and E-64 were of comparable potency as inhibitors of southern corn rootworm proteinase activity (IC(50) =31 and 35nM, respectively) and substantially more effective than chicken egg white cystatin (IC(50) =121nM). Incorporation of PCPI-10' into the diet of SCR larvae in feeding trials resulted in a significant increase in mortality and growth inhibition. We suggest that expression of inhibitors such as PCPI-10' by transgenic corn plants in the field is a potentially attractive method of host plant resistance to these Diabrotica species.     

The inhibitory effects of the cysteine protease inhibitor, oryzacystatin, on digestive proteases and on larval survival and development of the southern corn rootworm (Diabrotica undecimpunctata howardi)

At least eight proteolytic activities have been identified in the midgut contents of larval Southern corn rootworm (Diabrotica undecimpunctata howardi). Around 70% of protease activity could be arrested by the cysteine protease inhibitors E-64 and chicken egg-white cystatin, while the aspartic acid protease inhibitor pepstatin caused 30% inhibition. The cysteine protease activity was found to be highly sensitive to inhibition by both chicken egg-white cystatin and the rice cystatin, oryzacystatin I. Oryzacystatin I, expressed as a fully functional fusion protein in E. coli, was found to strongly inhibit larval gut protease activity. This recombinant oryzacystatin, incorporated into artificial diet at concentrations of 10 mM and above, caused significant decreases in larval survival and weight gain. E-64 was also shown to cause a significant antimetabolic in vivo effect. These results demonstrate the great potential for cysteine protease inhibitors, such as oryzacystatin, as tools for exploitation in the control of the Southern corn rootworm.     

Interaction between western corn rootworm (Coleoptera: Chrysomelidae) larvae and root-infecting Fusarium verticillioides

A greenhouse experiment was conducted to evaluate the effect of soil-dwelling larvae of the western corn rootworm, Diabrotica virgifera virgifera LeConte, on infection of maize roots by the mycotoxin-producing plant-pathogenic fungus, Fusarium verticillioides (Saccardo) Nirenberg (synonym=Fusarium moniliforme Sheldon). The time and order of application of F. verticillioides and western corn rootworm were varied in three different treatments to investigate the influence of timing on root colonization of F. verticillioides and western corn rootworm larval development. Root feeding by western corn rootworm larvae increased root colonization by F. verticillioides (as determined by real-time polymerase chain reaction) up to 50-fold when a high inoculum (10(7) spores/plant) of F. verticillioides was applied before western corn rootworm eggs were added. This effect was stronger the earlier F. verticillioides was applied relative to the time of western corn rootworm egg application but was only significant for the high F. verticillioides inoculum density treatment; F. verticillioides colonization was not increased when a low F. verticillioides inoculum density (10(6) spores/plant) was applied. F. verticillioides slightly suppressed larval development in that the ratio of second- to third-instar larvae was higher in treatments with F. verticillioides than without F. verticillioides. F. verticillioides reduced western corn rootworm head capsule width when applied before or simultaneously with western corn rootworm. The results of this study are discussed focusing on conditions that favor root colonization by F. verticillioides and its influence on western corn rootworm larval development.     

Discovery and characterization of Sip1A: a novel secreted protein from Bacillus thuringiensis with activity against coleopteran larvae

Bioassay screening of Bacillus thuringiensis culture supernatants identified strain EG2158 as having larvicidal activity against Colorado potato beetle (Leptinotarsa decemlineata) larvae. Ion-exchange fractionation of the EG2158 culture supernatant resulted in the identification of a protein designated Sip1A (secreted insecticidal protein) of approximately 38 kDa having activity against Colorado potato beetle (CPB). An oligonucleotide probe based on the N-terminal sequence of the purified Sip1A protein was used to isolate the sip1A gene. The sequence of the Sip1A protein, as deduced from the sequence of the cloned sip1A gene, contained 367 residues (41,492 Da). Recombinant B. thuringiensis and Escherichia coli harboring cloned sip1A produced Sip1A protein which had insecticidal activity against larvae of CPB, southern corn rootworm (Diabrotica undecimpunctata howardi), and western corn rootworm (Diabrotica virgifera virgifera).     

Larval mortality and development for rotation‐resistant and rotation‐susceptible populations of western corn rootworm on Bt corn

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is one of the most economically important insect pests threatening the production of corn, Zea mays (L.), in the United States. Throughout its history, this insect has displayed considerable adaptability by overcoming a variety of pest management tactics, including the cultural practice of annual crop rotation. Since first reported in Illinois in the late 1980s, populations of the rotation‐resistant western corn rootworm have spread over a wide area of the eastern Corn Belt. Currently, little information is available concerning the interaction of rotation resistance with the use of genetically modified corn expressing insecticidal toxins from Bacillus thuringiensis Berliner (Bt), a popular tactic for preventing larval injury and its associated yield loss. The goal of this greenhouse experiment was to determine whether rotation‐resistant and rotation‐susceptible western corn rootworm larvae differ with respect to survival or development when exposed to single‐ or dual‐toxin (pyramided) Bt corn. Individual corn plants were infested with 225 near‐hatch eggs at the V5 (five leaf collar) growth stage. Larvae developed undisturbed on the root systems for 17 days, after which they were recovered using Berlese–Tullgren funnels. Surviving larvae were counted to estimate mortality, and head capsule widths were measured to assess development. Rotation‐resistant and rotation‐susceptible larvae had statistically similar mean levels of mortality and head capsule widths when exposed to both single‐toxin (Cry3Bb1 or Cry34/35Ab1) and pyramided (Cry3Bb1+ Cry34/35Ab1) Bt corn, suggesting that these two populations do not differ with respect to survival or development when exposed to Bt corn. Additionally, the statistically similar mean levels of mortality for larvae exposed to single‐toxin and pyramided Bt corn suggest that pyramided Bt hybrids containing the Cry3Bb1 and Cry34/35Ab1 toxins do not result in additive mortality for western corn rootworm larvae. Implications for management of this economically important pest are discussed.     

Effect of Cry3Bb1-expressing transgenic corn on plant-to-plant movement by western corn rootworm larvae (Coleoptera: Chrysomelidae)

Dispersal of larvae of the western corn rootworm, Diabrotica virgifera virgifera LeConte, in specific combinations of transgenic corn expressing the Cry3Bb1 protein and nontransgenic, isoline corn was evaluated in a 2-yr field study. In total, 1,500 viable western corn rootworm eggs were infested in each subplot. Each year, plant damage and larval recovery were evaluated among four pedigree combinations (straight transgenic; straight nontransgenic corn; nontransgenic corn with a transgenic central, infested plant; and transgenic corn with a nontransgenic central, infested plant) on six sample dates between egg hatch and pupation. For each subplot, the infested plant, three successive plants down the row (P1, P2, and P3), the closest plant in the adjacent row of the plot, and a control plant were sampled. The number of western corn rootworm larvae recovered from transgenic rootworm-resistant plants adjacent to infested nontransgenic plants was low and not statistically significant in either 2001 or 2002. In 2001, significantly fewer larvae were recovered from transgenic rootworm-resistant plants than from nontransgenic plants when both were adjacent to infested, nontransgenic plants. In 2002, significantly more neonate western corn rootworm larvae were recovered from nontransgenic plants adjacent to infested, transgenic rootworm-resistant plants than nontransgenic plants adjacent to infested, nontransgenic plants on the second sample date. Together, these data imply that both neonate and later instar western corn rootworm larvae prefer nontransgenic roots to transgenic rootworm-resistant roots when a choice is possible. However, when damage to the infested, nontransgenic plant was high, western corn rootworm larvae apparently moved to neighboring transgenic rootworm-resistant plants and caused statistically significant, although only marginally economic, damage on the last sample date in 2001. Implications of these data toward resistance management plan are discussed.     

Evaluation of soil‐applied insecticides with Bt maize for managing corn rootworm larval injury

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is considered one of the most significant insect pests of maize in North America. Larvae of other secondary subterranean pests such as grape colaspis, Colaspis brunnea (F.), and Japanese beetle, Popillia japonica Newman, can also injure maize seedlings and cause yield loss. In the past decade, maize hybrids containing Bt proteins have been used to manage the western corn rootworm; additionally, seeds are commonly treated with a neonicotinoid and fungicide combination to control secondary pests. Recently, soil‐applied insecticides have been used in conjunction with rootworm Bt hybrids (and seed‐applied insecticides) in areas with perceived risk for increased rootworm larval or secondary pest damage. We conducted a series of trials from 2009 to 2011 that examined multiple rootworm Bt hybrids and their near‐isolines, along with two soil‐applied insecticides, to determine whether the Bt plus insecticide combination resulted in an increased level of efficacy or yield. We also sampled for Japanese beetle and grape colaspis larvae to determine their potential for reducing yield. Densities of secondary pests in our trials were low and likely had no effect on maize yield. The addition of a soil‐applied insecticide to rootworm Bt hybrids improved efficacy only once across 17 location‐years, when overall corn rootworm injury was highest; an improvement in yield was never observed. Our results suggest that the use of a soil‐applied insecticide with a rootworm Bt hybrid should only be considered in scenarios with potentially significant rootworm larval populations. However, potential negative consequences related to trait durability when soil insecticides are used with rootworm Bt maize should be considered.     

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.     

Effect of four cropping systems on variant western corn rootworm (Coleoptera: Chrysomelidae) adult and egg densities and subsequent larval injury in rotated maize

The cultural practice of rotating corn, Zea mays L., with soybean, Glycine max (L.) Merrill, to manage larval injury by the western corn rootworm, Diabrotica virgifera virgifera LeConte, was used extensively throughout east central Illinois and northern Indiana until the mid-1990s. The effectiveness of this management tactic diminished due to a shift in the ovipositional behavior of the western corn rootworm. The variant western corn rootworm has since spread as far as northwestern Illinois, southern Wisconsin, southern Michigan, and eastern Ohio. The objective of this study was to evaluate the influence of four cropping systems on adult and egg densities of the western corn rootworm and to quantify the level of root injury in rotated corn after each system. The four cropping systems used included: 1) corn; 2) soybean; 3) double-cropped winter wheat, Triticum aestivum L., followed by soybean; and 4) winter wheat. Research trials were conducted near Monmouth (northwestern), DeKalb (northern), and Urbana (east central), IL, during 2003 and 2004. Results indicated variant western corn rootworm adults can be found in all four treatments at each location and consequently no crop was immune to oviposition or root injury by corn rootworm larvae in rotated corn the following season. Adults were found primarily in corn and soybean, whereas egg densities were greatest in corn plots in all three locations in both years of the study. Root injury by larvae was most abundant in corn following corn at all three sites. Of the four systems evaluated, the use of wheat demonstrated the most potential for preventing yield reducing levels of root injury in rotated corn.     

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.     

Mortality of western corn rootworm larvae on MIR604 transgenic maize roots: field survivorship has no significant impact on survivorship of F1 progeny on MIR604

Mortality of western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae due to MIR604 transgenic corn, Zea mays L., expressing the modified Cry3A (mCry3A) protein relative to survivorship on corn with the same genetic background without the gene (isoline corn) was evaluated at three Missouri sites in both 2005 and 2006. We made these comparisons by using wild-type western corn rootworm at three different egg densities (6,000, 3,000, and 1,500 eggs per m) so that the role of density-dependent mortality would be known. The mortality due to the mCry3A protein was 94.88% when averaged across all environments and both years. Fifty percent emergence of beetles was delayed approximately 5.5 d. Beetles were kept alive and their progeny evaluated on MIR604 and isoline corn in the greenhouse to determine whether survivorship on MIR604 in the field for one generation increased survivorship on MIR604 in the greenhouse in the subsequent generation. There was no significant difference in survivorship on MIR604 in greenhouse assays between larvae whose parents survived isoline and larvae whose parents survived MIR604 in the field the previous generation, indicating that many susceptible beetles survived MIR604 in the field the previous season along with any potentially resistant beetles. The data are discussed in terms of rootworm insect resistance management.     

Parasitism of Western Corn Rootworm Larvae and Pupae by Steinernema carpocapsae

Virulence and development of the insect-parasitic nematode, Steinernema carpocapsae (Weiser) (Mexican strain), were evaluated for the immature stages of the western corn rootworm, Diabrotica virgifera virgifera LeConte. Third instar rootworm larvae were five times more susceptible to nematode infection than second instar larvae and 75 times more susceptible than first instar larvae and pupae, based on laboratory bioassays. Rootworm eggs were not susceptible. Nematode development was observed in all susceptible rootworm stages, but a complete life cycle was observed only in second and third instar larvae and pupae. Nematode size was affected by rootworm stage; the smallest infective-stage nematodes were recovered from second instar rootworm larvae. Results of this study suggest that S. carpocapsae should be applied when second and third instar rootworm larvae are predominant in the field.     

Binary insecticidal crystal protein from Bacillus thuringiensis,strain PS149B1: effects of individual protein components and mixtures in laboratory bioassays

A family of novel binary insecticidal crystal proteins, with activity against western corn rootworm, Diabrotica virgifera virgifera LeConte, was identified from Bacillus thuringiensis Berliner. A binary insecticidal crystal protein (bICP) from B. thuringiensis strain PS149B1 is composed of a 14-kDa protein (Cry34Abl) and a 44-kDaprotein (Cry35Ab1). These proteins have been co-expressed in transgenic maize plants, Zea mays L., and effectively control western corn rootworm larvae under field conditions. Laboratory experiments were conducted to better understand the contribution of each component protein to the in vivo activity of the bICP. The 14-kDa protein is active alone against southern corn rootworm, Diabrotica undecimpunctata howardi Barber, and was synergized by the 44-kDa protein. In mixtures, the concentration of the 14-kDa protein had a greater impact on efficacy than the 44-kDa component. Although both proteins are clearly required for maximal insecticidal activity, laboratory results did not support the formation of a stable, fixed-ratio complex of the two component proteins.     

Western corn rootworm larval movement in SmartStax seed blend scenarios

Insect resistance management (IRM) can extend the lifetime of management options, but depends on extensive knowledge of the biology of the pest species involved for an optimal plan. Recently, the Environmental Protection Agency (EPA) registered seed blends refuge for two of the transgenic Bacillus thuringiensis (Bt) corn products targeting the western corn rootworm, Diabrotica virgifera virgifera LeConte. Larval movement between Bt and isoline plants can be detrimental to resistance management for high dose Bt products because the larger larvae can be more tolerant of the Bt toxins. We assessed movement of western corn rootworm larvae among four spatial arrangements of SmartStax corn (expressing both the Cry34/35Ab1 and Cry3Bb1 proteins) and isoline plants by infesting specific plants with wild type western corn rootworm eggs. Significantly fewer western corn rootworm larvae, on average, were recovered from infested SmartStax plants than infested isoline plants, and the SmartStax plants were significantly less damaged than corresponding isoline plants. However, when two infested isoline plants surrounded a SmartStax plant, a significant number of larvae moved onto the SmartStax plant late in the season. These larvae caused significant damage both years and produced significantly more beetles than any other plant configuration in the study (including isoline plants) in the first year of the study. This plant configuration would occur rarely in a 5% seed blend refuge and may produce beetles of a susceptible genotype because much of their initial larval development was on isoline plants. Results are discussed in terms of their potential effects on resistance management.     

Supercooling and cold-hardiness in eggs of western and northern corn rootworms

Oviposition by northern corn rootworms, Diabrotica barberi Smith and Lawrence, and western corn rootworms, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), key pests of corn in the Great Plains of the USA, occurs in the soil during late summer. Overwintering eggs are exposed to variable soil moisture and temperatures below ?5 °C. The winter mortality of eggs in the soil is a primary factor that determines the potential for larval injury to corn the following spring. Our studies aimed to determine the comparative supercooling capacities of northern and western corn rootworm eggs and to assess egg mortality following brief exposure to extreme low temperature, ranging from ?12.0 to ?21.5 °C, under three moisture regimes. Eggs of northern corn rootworm were supercooled to a temperature as low as ?27 °C, and survived supercooling to a greater extent than did western corn rootworm eggs. Moisture treatment prior to supercooling had little effect on northern corn rootworm eggs. Western corn rootworm eggs were more resistant than northern corn rootworm eggs to the effects of desiccation followed by supercooling. The survival of northern corn rootworm eggs was better than western corn rootworms under dry conditions, followed by exposure to temperatures of ?12.0 and ?17.5 °C, but was very low at ?21.5 °C, regardless of the moisture regime. The results suggest that moisture and temperature may interact in the soil environment to determine the overwintering survival of corn rootworms. It is evident from these studies that both rootworm species experience mortality at temperatures well above the supercooling points of the eggs, but that differences exist in the effects of substrate moisture treatments on the cold‐hardiness of eggs from the two species.     

Field studies of the temporal response patterns of male Diabrotica virgifera virgifera LeConte and D. barberi Smith and Lawrence to 8R-methyl-2R-decyl propanoate

ABSTRACT. Variations in the diel response of male northern and western corn rootworms attracted to a synthetic pheromone source were examined in southwestern Ontario. Activity was measured by the number of males caught at hourly intervals in sticky traps baited with 8 R -methyl-2 R -decyl propanoate. Northern corn rootworm males were attracted to the pheromone between 22.30 and 0330 hours and were unresponsive at all other times at temperatures above 15°C. Western corn rootworm males had a bimodal response with peaks in attraction from 08.30 to 12.30 hours and again from 15.30 to 18.30 hours. While the period of peak attraction for northern corn rootworm males was well defined, the pattern for western corn rootworm males was erratic and many beetles were caught throughout the day. Evening temperatures below 15°C suppressed the activity of both northern and western corn rootworms. While the nightly period of activity for the northern corn rootworms was shifted by low temperatures to the following morning, that of the western corn rootworm was not altered substantially. As a result, peaks in the response of northern and western corn rootworm males occurred simultaneously under these conditions, suggesting strongly that temporal separation of activity periods was not an important factor in maintaining sexual isolation for these species in Ontario.     

Evaluating multiple approaches for managing western corn rootworm larvae with seed blends

Corn rootworm, Diabrotica spp., larvae represent a significant and widespread economic threat to corn, Zea mays (L.), production in the United States, where control costs and yield losses associated with these insect pests exceed $1 billion annually. Preventing root injury and associated yield loss caused by corn rootworm larvae may be accomplished by the independent use of planting time soil insecticides or transgenic Bt hybrids. However, recent reports of both confirmed and suspected Bt resistance in corn rootworm populations throughout the Corn Belt have led to significant interest in the use of these two management tactics simultaneously. Although this approach has been investigated to some extent previously, information is lacking on how the use of a soil insecticide in tandem with a Bt seed blend—Bt and refuge (non‐Bt) seed mixed into a single product—may affect root protection and yield. We describe an experiment including six trial sites conducted over a three‐year period where various seed blends and soil insecticide/seed blend combinations were evaluated. The predominant species contributing to root injury across all sites was the western corn rootworm (Diabrotica virgifera virgifera LeConte). A weighted technique is presented for evaluating root injury for seed blends that offers a reliable estimate of product performance. The addition of a soil insecticide to the seed blend treatments never resulted in significantly improved root protection and failed to provide a consistent yield benefit. Our results suggest that a soil insecticide/seed blend combination approach is not warranted. Additionally, a subanalysis performed on individual refuge and nearby Bt root systems for seed blend treatments provides insight into the spatial characteristics of root injury in seed blend scenarios.     

Insecticidal proteins from Bacillus thuringiensis protect corn from corn rootworms.

Field tests of corn co-expressing two new delta-endotoxins from Bacillus thuringiensis (Bt) have demonstrated protection from root damage by western corn rootworm (Diabrotica virgifera virgifera LeConte). The level of protection exceeds that provided by chemical insecticides. In the bacterium, these proteins form crystals during the sporulation phase of the growth cycle, are encoded by a single operon, and have molecular masses of 14 kDa and 44 kDa. Corn rootworm larvae fed on corn roots expressing the proteins showed histopathological symptoms in the midgut epithelium.     

Field screening maize germplasm for resistance and tolerance to western corn rootworms (Col.: Chrysomelidae)†

Abstract: In the hopes of lessening the current reliance on soil insecticides, developing a viable alternative for transgenic maize hybrids, and providing sustainable options for Europe, researchers recently have been developing novel maize lines that exhibit resistance and/or tolerance to corn rootworm larvae. Here we report the results of a 2‐year field experiment in a northern growing region assessing the resistance and tolerance of 10 experimental synthetic maize populations selected for varying levels of damage from western corn rootworm larvae, Diabrotica virgifera virgifera LeConte (Col.: Chrysomelidae) and four maize hybrids. Maize non‐preference, antibiosis and tolerance to rootworms was evaluated using previously established methods, including: the Iowa 1–6 root damage rating scale, root fresh weight, compensatory root growth ratings and adult rootworm emergence. Among the experimental synthetic maize populations, BS29‐11‐01 was the most susceptible, and had a mean root damage rating that was greater than the highly susceptible maize hybrid B37 × H84. This line also had the lowest mean root fresh weight and one of the lowest mean compensatory root growth ratings. In contrast, CRW8‐3 appeared to be tolerant to western corn rootworms, and had the lowest mean root damage rating, which was comparable with that of the non‐transgenic hybrid DeKalb^® 46‐26.     

Estimation of efficacy functions for products used to manage corn rootworm larval injury

Maize, Zea mays L., is an economically important crop grown throughout the world. Corn rootworm, Diabrotica spp. (Coleoptera: Chrysomelidae), larvae constitute a significant economic threat to maize production in the United States, where yield losses and management costs associated with corn rootworm species exceed $1 billion annually. Furthermore, the introduction of the western corn rootworm, D. virgifera virgifera LeConte, into maize‐producing regions of Europe has made managing corn rootworm larval injury an international concern. Larvae injure maize plants by feeding on root tissue and are the primary target of management activities. Products commonly used to protect root systems from injury include chemical insecticides (seed or soil applied) and genetically modified maize hybrids expressing toxins derived from Bacillus thuringiensis Berliner (Bt). The confirmation of field‐evolved resistance to various Bt toxins in populations of the western corn rootworm presents a significant management challenge. We performed a meta‐analysis to provide a broad understanding of the relative efficacy of the primary products currently being used to manage corn rootworm larval injury, including insecticidal seed treatments, soil insecticides and Bt hybrids (with and without the addition of soil insecticide). Our analysis is unique in the breadth of locations and years included – we analysed 135 individual trials conducted from 2003 through 2014 at multiple sites in both Illinois and Nebraska. Panel data were produced by pairing the mean node‐injury rating for each treatment of a given trial with the mean node‐injury rating for untreated maize. Linear regression models were developed to estimate the relationship between the potential for corn rootworm larval injury and product performance. For a given level of injury potential, the parameters estimated reveal differences in the degree of root protection offered by the various product categories analysed. Implications for developing long‐term, integrated, and sustainable practices for managing this important pest of maize are discussed.