Morphological integration and modularity in Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) hind wings

The morphological integration of the hind wings of the western corn rootworm Diabrotica virgifera virgifera LeConte was investigated to get a better insight of the undergone by this invasive species. Geometric morphometric methods were used to test two modularity hypotheses associated with the wing development and function (hypothesis H1: anterior/posterior or H2: distal/proximal wing parts). Both hypotheses were rejected and the results showed the integrated behavior of the hind wings of D. v. virgifera. The hypothesized modules do not represent separate units of variation, so in a similar fashion as exhibited by the model species Drosophila melanogaster, the hind wings of D. v. virgifera act as a single functional unit. The moderate covariation strength found between anterior and posterior and distal and proximal parts of the hind wing of D. v. virgifera confirms its integrated behavior. We conclude that the wing shape shows internal integration, which could enable flexibility and thus enhance flight maneuverability. This study contributes to the understanding of morphological integration and modularity on a non-model organism. Additionally, these findings lay the groundwork for future flight performance and biogeographical studies on how wing shape and size vary across the endemic and expanded/invaded range in the USA and Europe infested with D. v. virgifera.     

Adult movements of newly introduced alien Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) from non-host habitats

Mark-release-recapture experiments were undertaken in order to investigate the movement of adult Diabrotica virgifera virgifera LeConte from accidental introduction points towards suitable habitats, such as its host plant, Zea mays L. In Hungary in 2003 and 2004, nine mark-release-recapture experiments were carried out in a grass steppe area and lucerne field, in which two small maize fields (10x10 m) had been planted 300 m distant from the central release point. After each release of 5500 to 6000 marked D. v. virgifera, beetle recaptures were recorded three times using non-baited yellow sticky traps placed on 30, 105, 205 and 305 m radii around the release point. In seven out of 15 recapture periods (47%), beetle populations showed no directional movements, and their movements towards any particular habitat cannot be predicted. During five recapture periods (33%), beetle populations showed a uni-directional movement, and in three cases (20%) a bi-directional movement was observed. In 10 out of 15 recapture periods (67%), the released populations moved in a direction that was comparable with the mean wind direction during these periods; thus, beetle movements were slightly correlated with wind direction. On average over sites and years, beetles were not preferentially moving towards the two small maize fields (located 300 m from the release point) compared to other directions. However, beetles moved significantly more frequently in the direction of naturally-occurring maize fields within a radius of 1500 m than towards other habitats. Beetles stayed more frequently within flowering lucerne fields out to a radius of 300 and 600 m than in non-flowering lucerne or other habitats. On average, 2.8% (SD 3.2) of all recaptured beetles arrived in one of the two small maize fields located 300 m from their release point indicating that there is a high risk of a founder population establishing. Habitat management cannot be suggested as a means of preventing the beetle's initial dispersal because movement was usually non-directional, and alternative food plants were used prior to reaching maize.     

Reproductive Biology of Celatoria compressa (Diptera: Tachinidae), a Parasitoid of Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae)

The tachinid Celatoria compressa, a parasitoid of adult Diabrotica species in North America has been studied as a candidate classical biological control agent for the western corn rootworm, Diabrotica virgifera virgifera, in Europe. Prior to its potential importation, a thorough understanding of the parasitoid's reproductive biology is essential, and is an important component in the evaluation of a species as a biological control agent. In this study it has been clarified that C. compressa belongs to a group of a few tachinid species characterised by having eggs that contain fully developed larvae which are laid directly into the host. After mating, the egg load of females increased steadily from day 1 to a maximum egg load on day 4. Thereafter eggs containing fully developed first instar larvae reached a maximum of 31 in 69 eggs. At the first day of larviposition, females laid on average only five eggs into multiple hosts, which is in contrast to the availability of 18 eggs containing fully developed first instars in the uterus per female at that time. During a mean female's larviposition period of 23 days, a total of 33 first instars were larviposited into the hosts, which is only half of the female's egg load. Lifetime fecundity of C. compressa was significantly correlated with longevity. However no relationship was found between body size and either lifetime fecundity or longevity. In this study, an inverse host density-dependent pattern of percent parasitism was shown for C. compressa under 24-h fixed-time laboratory conditions, reflecting a Holling type II response. The number of host parasitized per C. compressa female reached an upper limit of 10 hosts with an increasing host density, which can be explained by the long host handling time of C. compressa.     

Western corn rootworm (Diabrotica virgifera virgifera LeConte) (Coleoptera: Chrysomelidae) oviposition can be released by decapitation

Nervous system control of western corn rootworm (Diabrotica virgifera virgifera LeConte, WCR) (Coleoptera: Chrysomelidae) oviposition was studied using decapitation. Gravid females (n = 364) were decapitated with scissors and floated in water-filled Petri dishes. Oviposition by individuals was observed at 30 min intervals for 4 hr after decapitation; cumulative oviposition was tallied at 48 hr post-treatment. Overall, 82.7% of females laid eggs within 48 hr. Oviposition commenced quickly; 78.4% of females laid eggs during the observation period, 81.8% of these began egg laying within 30 min of decapitation. Egg-laying females deposited a total of 66.8 ± 2.1 eggs (mean ± SEM); this was 85.1% of the total mature egg load. Dissection revealed that 31.9% of n = 301 laying females and 14.3% of 63 non-laying females had a rupture of the common oviduct, manifest as an egg-filled hernia containing a mean of 8.17 ± 1.3 eggs (range: 1–83). Among n = 237 females that laid eggs during observations, females with hernias laid significantly fewer eggs (35.8 ± 4.2) than intact females (48.0 ± 2.7) during the 4-hr interval. There was no difference in the mean proportion of hatch for eggs collected from the same n = 10 females before (0.88 ± 0.03) or after decapitation (0.84 ± 0.04). Rapid oviposition following decapitation suggests that WCR egg laying is under constant descending neural inhibition; the motor programme controlling egg laying must reside posterior to the head. Decapitation can be used to quickly collect mature, fertilized WCR eggs.     

Purification and kinetic analysis of acetylcholinesterase from western corn rootworm,Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae)

Acetylcholinesterase (AchE, EC 3.1.1.7) was purified from western corn rootworm (WCR, Diabrotica virgifera virgifera) beetles by affinity chromatography. The purification factor reached over 20,000-fold with a specific activity of 169.5 μmol/min/mg and a yield of 23%. The V_max values for hydrolyzing acetylthiocholine (ATC), acetyl-(β-methyl)thiocholine (AβMTC), propionylthiocholine (PTC), and S-butyrylthiocholine (BTC) were 184.8, 140.5, 150.2, and 18.8 μmol/min/mg, respectively, and K_m values were 19.7, 18.5, 14.1, and 11.0 μM, respectively. The first three substrates showed significant inhibition to the AchE at higher concentrations, whereas BTC showed inhibition at the concentrations of 0.25–2 mM but activation at >4 mM. AchE activity was almost completely inhibited by 1 μM eserine and BW284C15, respectively, but only 12% of AchE activity were inhibited by ethopropazine at the same concentration. These results suggested that the purified AchE from WCR was a typical insect AchE. Insecticides or their oxidative metabolites, chlorpyrifos-methyl oxon, carbofuran, carbaryl, malaoxon, and paraoxon, used in in vitro kinetic study exhibited high inhibition to AchE purified from WCR. However, chlorpyrifos-methyl oxon and carbofuran showed at least 36- and 4-fold, respectively, higher inhibitory potency than the remaining insecticides examined. Results from our in vitro inhibition of AchE agreed quite well with the previously published in vivo bioassay data. Arch. Insect Biochem. Physiol. 39:118–125, 1998. © 1998 Wiley-Liss, Inc.     

Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) larval feeding behavior on transgenic maize (MON 863) and its isoline

Diabrotica species (Coleoptera: Chrysomelidae) larval behavior studies have posed a challenge to researchers because of the subterranean life cycle of this pest. To fully understand how the western corn rootworm, Diabrotica virgifera virgifera LeConte, injures the maize, Zea mays L., root system, its behavior must be studied. For example, larvae that can detect an area of the root that has a lower amount of toxin, whether from an insecticide or a transgenic maize plant, have an increased chance of survival. This study assessed D. v. virgifera larval feeding behavior on rootworm-susceptible maize and maize containing a biotechnology-derived trait (MON 863) with resistance to D. v. virgifera first instar feeding. Maize plants were grown in a medium that allowed for direct observation and measurements during feeding of larval stadia. Neonates were placed on maize seedlings, and data were taken at 3, 6, 9, and 12 d postinfestation on resistant and susceptible maize. On rootworm-susceptible maize, neonate larvae aggregated at the root tips and began actively feeding, and then they moved to older root tissue. Conversely, some larvae that ingested Cry 3Bb1 from the resistant maize exhibited no movement. Other larvae on the resistant maize moved continuously, sampling root hairs or root tissue but not actively feeding. The continuously moving larvae had visibly empty guts, suggesting possible nonpreference for the resistant root. This study contributes to our understanding of D. v. virgifera larval behavior and provides insight into questions surrounding the potential evolution of behavioral and biochemical resistance to Cry3Bb1.     

Basic biology and small-scale rearing of Celatoria compressa(Diptera: Tachinidae), a parasitoid of Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae)

The tachinid Celatoria compressa Wulp has been evaluated as a candidate biological control agent for the western corn rootworm, Diabrotica virgifera virgiferaLeConte, in Europe, where it is an invasive alien pest of maize. Special emphasis has been placed on understanding aspects of the parasitoid basic biology and on developing a rearing technique for a small-scale production of C. compressapuparia. The age of C. compressa adults was found to be the most crucial factor in achieving mating. Only newly emerged, 1-h-old females, mated successfully with 2- to 5-day-old males, achieving a success rate of 74%. After mating, a prelarviposition period of 4 days occurred. The 5-day-old C. compressa females inserted their eggs containing fully-developed first instars directly into adults of D. v. virgifera. Total larval and pupal developmental time, including a pre-larviposition period of 4 days, was 29 days under quarantine laboratory conditions (25 degrees C daytime, 15 degrees C at night, L:D 14:10, 50% +/- 10% r.h). Females of C. compressa were capable of producing on average 30 puparia throughout a female's mean larviposition period of 15 days. A large number of host attacks by C. compressa were unsuccessful, resulting in a mean larviposition success rate of 24% per female. Parasitoid females appear to have difficulties inserting the egg through the intersegmental sutures or membranes around leg openings of the host adults. Although the small-scale rearing technique of C. compressa presented is both time and labour intensive, C. compressa has been reared successfully for at least 20 successive generations without shifting the 1 male : 1 female sex ratio using a non-diapause strain of D. v. virgifera.     

Evolutionary directional asymmetry and shape variation in Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae): an example using hind wings

The western corn rootworm Diabrotica virgifera virgifera LeConte is a pest of maize in the USA and Europe and especially a problem in particular regions of Croatia. In the present study, patterns of variation in hind wing shape were examined. The first objective was to examine the influence of soil type on 10 populations of D. v. virgifera sampled from three regions in Croatia that differed according to edaphic factors and climate. The second objective was to investigate the potential evolutionary presence of directional asymmetry on hind wings. Geometric morphometrics was used to examine these objectives by quantifying the morphological variation within and among individuals and populations. Overall, D. v. virgifera hind wing shape changed according to major soil type classifications in Croatia. The three hind wing morphotypes found varied because of basal radial vein differences, related to landmarks 1, 3, 7, and 14. The findings of the present study show that hind wing shape in D. v. virgifera can be used to differentiate populations based on edaphic factors and may have application as a monitoring tool in the integrated management of D. v. virgifera. In an evolutionary context, the presence of directional asymmetry in the hind wings of D. v. virgifera adds to the ever growing data on the evolution of insect wings. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 110–118.     

Seasonal and diel activity patterns of generalist predators associated with Diabrotica virgifera immatures (Coleoptera: Chrysomelidae)

The diel and seasonal activity of epigeal predators associated with pre-imaginal Diabrotica virgifera was described. Due to its duration, the egg stage was exposed to more predators than the larval stage. Most predators were easily categorized into day- and night-active guilds. Seasonal and diel niche partitioning may contribute to the maintenance of this diverse and abundant predator community.     

Screening of entomopathogenic nematodes for virulence against the invasive western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) in Europe

Entomopathogenic nematode species available in Europe were screened for their efficacy against both the root-feeding larvae and silk-feeding adults of the western corn rootworm, Diabrotica virgifera virgifera LeConte. Laboratory screening tests were aimed at the selection of candidate biological control agents for the management of this invasive alien pest in Europe. Steinernema glaseri, S. arenarium, S. abassi, S. bicornutum, S. feltiae, S. kraussei, S. carpocapsae and Heterorhabditis bacteriophora were studied to determine their virulence against third instar larvae and adults of D. v. virgifera in small-volume arenas (using nematode concentrations of 0.5, 0.8, 7.9 and 15.9 infective juveniles cm-2). All nematode species were able to invade and propagate in D. v. virgifera larvae, but adults were rarely infected. At concentrations of 7.9 and 15.9 cm-2, S. glaseri, S. arenarium, S. abassi and H. bacteriophora caused the highest larval mortality of up to 77%. Steinernema bicornutum, S. abassi, S. carpocapsae and H. bacteriophora appeared to have a high propagation level, producing 5970+/-779, 5595+/-811, 5341+/-1177 and 4039+/-1025 infective juveniles per larva, respectively. Steinernema glaseri, S. arenarium, S. feltiae, S. kraussei and H. bacteriophora were further screened at a concentration of 16.7 nematodes cm-2 against third instar larvae in medium-volume arenas (sand-filled trays with maize plants). Heterorhabditis bacteriophora, S. arenarium and S. feltiae caused the highest larval mortality with 77+/-16.6%, 67+/-3.5%, and 57+/-17.1%, respectively. In a next step, criteria for rating the entomopathogenic nematode species were applied based on results obtained for virulence and propagation, and for current production costs and availability in Europe. These criteria were then rated to determine the potential of the nematodes for further field testing. Results showed the highest potential in H. bacteriophora, followed by S. arenarium and S. feltiae, for further testing as candidate biological control agents.     

Sexual dimorphism in Diabrotica speciosa and Diabrotica viridula (Coleoptera: Chrysomelidae)

Diabrotica speciosa (Germar) and Diabrotica viridula F. (Coleoptera: Chrysomelidae) are the two most abundant species of the genus in South America, and belong to the fucata and virgifera groups, respectively. Here, we characterize the dimorphism of the setae present on the basitarsi of males and females of these species. Dimorphism was confirmed in both species, and it was related to the presence of adhesive setae exclusively in males, which possess these structures on the basal tarsomeres of the pro- and mesothoracic legs.     

Mating behavior of Diabrotica speciosa (Coleoptera: Chrysomelidae)

Diabrotica speciosa (Germar) is an economically important pest of Neotropical cultures and represents a quarantine risk for Neartic and Paleartic Regions. Despite its agricultural importance, few studies have been done on mating behavior and chemical communication, which has delayed the development of behavioral techniques for population management, such as the use of pheromone traps. In this study, we determined 1) the age at first mating; 2) diel rhythm of matings; 3) number of matings over 7 d; 4) the sequence of D. speciosa activities during premating, mating, and postmating; 5) the duration of each activity; and 6) response to male and female conspecific volatiles in Y-tube olfactometer. The first mating occurred between the third and seventh day after adult emergence and the majority of pairs mated on the fourth day after emergence. Pairs of D. speciosa showed a daily rhythm of mating with greater sexual activity between the end of the photophase and the first half of the scotophase. During the 7 d of observation, most pairs mated only once, although 30% mated two, three, or four times. In a Y-tube olfactometer, males were attracted by virgin females as well as by the volatile compounds emitted by females. Neither males nor their volatiles were attractive to either sex. Our observation provide information about mating behavior of D. speciosa, which will be useful in future research in chemical communication, such as identification of the pheromone and development of management techniques for this species using pheromone traps.     

Field-Cage Studies of Beauveria bassiana (Hyphomycetes: Moniliaceae) for the Suppression of Adult Western Corn Rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae)

The efficacy of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin, was tested as a control agent for adult western corn rootworm, Diabrotica virgifera virgifera LeConte, in walk-in field cages. Suspensions of B. bassiana conidia were applied to corn plants in cages into which laboratory-reared beetles had been released. Beetles were collected at 3 and 5 days post-application and evaluated in the laboratory for mortality. Mortality was 10, 29 and 50%, at rate equivalents of 7 X 10 12 , 2 X 10 13 (two applications), and 5 X 10 13 conidia/ ha, respectively. There was no significant difference in mortality of beetles collected at 3 days compared with 5 days post-application. Mortality due to B. bassiana was 24% when beetles were released into field cages 24 h post-application (5 X 10 13 conidia/ha) compared with 50% when beetles were present during the application. Beetle mortality declined significantly with increasing time from application in feeding assays carried out with leaf samples removed from plants at 0, 12, 24 and 72 h post-application. Mortality of beetles collected from treated plants within cages and maintained in the laboratory was found to overestimate the population decline by 10% when compared with beetle estimates from treated plants within field cages.     

The effects of a winter cover crop on Diabrotica virgifera (Coleoptera: Chrysomelidae) populations and beneficial arthropod communities in no-till maize

The effects of an autumn-planted, spring-killed, grass cover crop (Elymus trachycaulus [Link] Gould ex Shinners) on populations of Diabrotica virgifera virgifera LeConte and its predator community were evaluated in South Dakota maize fields over two seasons. Abundance and size of D. virgifera larvae and adults and sex ratio of adults were measured in maize produced under two treatments (i.e., a winter cover crop or bare soil), as were maize root damage and the abundance and diversity of the predator communities collected on the soil surface and in the soil column. First and second instars and adults of D. virgifera were similarly abundant in the two treatments, but third instars were significantly fewer in maize planted after a winter cover crop. Larvae developed at different rates in the two treatments, and second instars were significantly smaller (head capsule width and body length) in the maize planted after a cover crop. First and third instars and adults were of similar size in the two treatments, and adult sex ratios were also similar. Although initially similar, predator populations increased steadily in the cover-cropped maize, which led to a significantly greater predator population by the time D. virgifera pupated. There was significantly less root damage in the cover-cropped maize. Predator communities were similarly diverse in both treatments. Predator abundance per plot was significantly and negatively correlated with the abundance of third instars per plot. Clearly, winter cover crops reduce D. virgifera performance and their damage to the crop, and we suspect that this reduction is caused by both environmental effects of the treatment on D. virgifera size and development, and of increased predation on the third instars of the pest. Additional data on the impact of cover crops on actual predation levels, grain yield and quality, and farmer profitability, and correlations among pest performance, crop characteristics, and predator populations and behaviors are key components of this system that remain to be addressed.     

Constructing life-tables for the invasive maize pest Diabrotica virgifera virgifera (Col.; Chrysomelidae) in Europe

Abstract: The western corn rootworm (Diabrotica virgifera virgifera LeConte, Col.; Chrysomelidae) is an alien invasive species in Europe. It is a univoltine species with eggs that overwinter in the soil and larvae that hatch in spring. Three larval instars feed on maize roots, which can cause plant lodging and yield loss of economic importance. Adults emerge between mid‐June and early August and can reduce yields through intensive silk feeding. In order to provide a thorough understanding of the population dynamics of this invasive pest species in the invaded European region, complete age specific life‐tables were constructed in two maize fields in southern Hungary assessing the significance of natural mortality factors acting on D. v. virgifera populations. This information provides a rational basis for devising sustainable integrated pest management programmes, in particular, by enabling the identification of vulnerable pest age intervals for the timely application of various management tools. The life‐table for D. v. virgifera in Europe resulted in a total mortality of about 99% from the egg stage in the autumn to the emergence of adult females in the following year (K_Total = 2.48), which is comparable with North America. The highest reduction of D. v. virgifera numbers resulted from the mortality in first instar larvae (94% marginal death rate) and from the unrealized fecundity (80%). However, only the variation in mortality between years can change the generational mortality and thus influence population growth. High variation in the marginal death rate between fields and years was found in the second and third instar larval stages, and in the overwintering egg stage. These mortality factors therefore have the potential to cause changes in the total generational mortality. Furthermore, the life‐table suggested that a high fecundity could compensate for a high generational mortality and would lead to population increase.     

Response of larvae of invasive maize pest Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) to carbon/nitrogen ratio and phytosterol content of European maize varieties

We studied the performance of larvae of Diabrotica virgifera virgifera LeConte (Chrysomelidae, Galerucinae) on 17 different maize, Zea mays L., varieties from six European countries. Food conversion efficiency studies were performed using a newly established method. The growth of D. v. virgifera (western corn rootworm) larvae and the amount of ingested food was measured and the food conversion efficiency was calculated. In addition, we analyzed the carbon/nitrogen ratio and the phytosterol content of the different varieties. Significant differences between the maize varieties with regard to larval weight gain, amount of ingested food, and food conversion efficiency were encountered. The efficiency of D. v. virgifera in converting root biomass into insect biomass was positively related to the amount of nitrogen in the plant tissue. Furthermore the root phytosterol content influenced the larval weight gain and the amount of ingested food. It was possible to group the varieties into suitable and unsuitable cultivars with regard to D. v. virgifera larval performance on the basis of the phytosterol content. Our results provide the first evidence of the high variability among European maize varieties with respect to D. v. virgifera nutrition. The use of less suitable maize varieties is discussed with respect to integrated pest management strategies.     

Susceptibility of Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) to the Entomopathogenic Fungus Metarhizium anisopliae when Feeding on Bacillus thuringiensis Cry3Bb1-Expressing Maize

Genetically engineered maize producing the insecticidal protein Cry3Bb1 from Bacillus thuringiensis (Bt maize) is protected against corn rootworms (Diabrotica spp.), which are serious maize pests in North America and Europe. The aim of the present study was to investigate the interaction of Bt maize (event MON88017) and the entomopathogenic fungus Metarhizium anisopliae for controlling the western corn rootworm, Diabrotica virgifera virgifera. Exposure to Cry3Bb1 expressed in Bt maize seedlings resulted in decreased weight gain in D. v. virgifera larvae but did not influence susceptibility to M. anisopliae. Adult beetles were not affected by Cry3Bb1 in their food, but mortality when feeding on maize leaves was higher than when feeding on silk. Adults were more susceptible to the fungus than larvae. The results indicate that the effects of Bt maize and M. anisopliae on D. v. virgifera are additive and that Bt maize does not interfere with the biological control provided by entomopathogenic fungi.One of the most devastating maize pests in North America is the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae). Since the early 1980s, the beetle has been introduced several times to Eastern and Central Europe, where it is spreading rapidly (29). After overwintering as eggs in the soil, larvae of this univoltine species hatch early in the maize growing season, seek maize roots to feed on, and develop in three larval stages before pupation (6). Infested plants suffer from reduced nutrient and water supply, and the decreased plant stability often results in lodging. Further yield reductions can be caused by adults feeding on silk and kernels (19, 23).Options for controlling the western corn rootworm include crop rotation and the application of insecticides (23). However, Diabrotica spp. have developed resistance against both measures (30). Since 2003, Diabrotica-resistant transgenic maize expressing the cry3Bb1 gene from the bacterium Bacillus thuringiensis (Bt maize) has been grown commercially in the United States (12, 15, 49). The deployment of Bt maize has huge potential for reducing insecticide applications and yield losses (8, 39). Continuous exposure to the B. thuringiensis protein, however, puts high selection pressure on the pest population and may lead to the development of resistance, as has frequently occurred with chemical insecticides (10, 23, 41, 44). To delay the development of resistance to Bt crops in the field, a certain percentage of conventional maize is usually grown as a “refuge” adjacent to the Bt crop. The aim is that resistant individuals surviving on Bt maize mate with a large number of susceptible beetles emerging from the refuge. For this strategy to be effective, resistance has to be recessive and the toxin concentration in plants has to be high enough to kill resistance-heterozygous insects (9, 44). The concentration of Cry3Bb1 expressed in Bt maize, however, is not considered a high dose for D. v. virgifera, and resistance was reported to build up within three generations of selection on Bt maize in the greenhouse (28).In agricultural fields, most herbivorous arthropods are attacked by a number of naturally occurring antagonists. This contribution to pest control is considered important because it keeps populations of many herbivores below the economic level of damage. A number of different antagonists (microbial pathogens, nematodes, arthropod predators, and parasitoids) from Central and South America, which have the highest diversity of Diabrotica species and are presumably the regions of origin (45), are known to attack corn rootworms (20). In Europe and North America, generalist predators are also likely to feed nonspecifically on D. v. virgifera, but no effective indigenous parasitoids or other specialized antagonists have been found (23, 46, 48). In addition, infections by entomopathogenic nematodes and the fungal species Beauveria bassiana (Bals.) Vuill. and Metarhizium anisopliae (Metsch.) Sorokin were reported in field-collected D. v. virgifera larvae and adults, but frequencies were very low (38, 46).Entomopathogenic fungi have a long history as biological control agents. They are used to control a variety of pests, and a number of commercial products are registered worldwide (5, 25). Recent laboratory and field studies indicate a potential for entomopathogenic fungi to control D. v. virgifera larvae and adults (18, 33, 37).In integrated pest management, sustainable cropping systems are established by combining environmentally friendly control strategies to reduce the application of harmful, broad-spectrum chemical insecticides. Because Cry3Bb1-expressing Bt maize does not control D. v. virgifera larvae completely (1, 35) and potentially fast evolution of resistance has been demonstrated in the greenhouse (28), biological control by entomopathogenic fungi in combination with Bt maize might help to delay resistance development by decreasing the number of surviving beetles.When different pest control methods, like plants with resistance to herbivores, entomopathogens, and biopesticides, are combined, interactions may lead to synergistic, antagonistic, or additive effects on the target herbivore. For example, when biopesticides containing B. thuringiensis derivates were applied together with entomopathogenic fungi, the effects on herbivores were additive (7) or even synergistic (2, 50). Similarly, host plants with resistance to herbivores due to antibiosis or antixenosis in combination with B. thuringiensis derivates, entomopathogic fungi, or viruses resulted in synergistic or additive effects for herbivore control (11). The interactions of B. thuringiensis proteins produced in genetically engineered plants and entomopathogenic fungi, however, have been examined so far only for lepidopteran pests. Lawo et al. (22) demonstrated that B. thuringiensis-susceptible caterpillars were more sensitive to the fungus when feeding on Bt chickpea than when feeding on nontransformed control plants, while infection rates were similar for a B. thuringiensis-resistant strain. In addition, Johnson et al. (16, 17) reported that B. thuringiensis-susceptible caterpillars showed higher infection rates than B. thuringiensis-resistant ones when feeding on Bt tobacco. The interactions of any commercialized Bt plant with entomopathogenic fungi for controlling beetle pests and potential consequences for biological control have not been studied to date.Similar to the examples listed above, corn rootworm-resistant Bt maize and entomopathogenic fungi may interact in various ways in controlling Diabrotica spp. Larvae that are sublethally damaged by the B. thuringiensis protein might have a weaker immune system that allows easier infection by entomopathogenic fungi, which would be most desirable for pest control (synergistic effects). Alternatively, one can hypothesize that only corn rootworm larvae with a high level of fitness can survive on Bt maize, making infection by the entomopathogen more difficult, which would limit the efficacy for pest control (antagonistic effects). Finally, the two pest control methods may be compatible in such a way that they act independently with additive effects on the pest.We present laboratory data on the interaction of Bt maize, D. v. virgifera, and the entomopathogen M. anisopliae. The specific aims of the study were (i) to assess exposure of D. v. virgifera larvae to root-expressed Cry3Bb1 protein, (ii) to compare the susceptibilities of Bt maize-fed larvae and control maize-fed larvae to M. anisopliae, and (iii) to compare the susceptibilities of adults fed with Bt or control maize silk or leaves to M. anisopliae.     

Western corn rootworm (Diabrotica virgifera virgifera LeConte) population dynamics

1 The western corn rootworm Diabrotica virgifera virgifera LeConte is a major insect pest of field maize, Zea mays L. Larvae can cause substantial injury by feeding on maize roots. Larval feeding may destroy individual roots or root nodes, and reduce plant growth, stability, and yield. Costs associated with managing corn rootworms in continuous maize are annually one of the largest expenditures for insect management in the United States Corn Belt.
2 Even though D. virgifera virgifera has been studied intensively for over 50 years, there is renewed interest in the biology, ecology, and genetics of this species because of its ability to rapidly adapt to management tactics, and its aggressive invasive nature.
3 This article provides a comprehensive review of D. virgifera virgifera population dynamics, specifically: diapause, larval and adult development, seasonality, spatial and temporal dynamics at local and landscape scales, invasiveness in North America and Europe, and non-trophic interactions with other arthropods.
4 Gaps in current knowledge are identified and discussed especially within the context of challenges that scientists in North America and Europe are currently facing regarding pest dynamics and the need to develop appropriate management strategies for each geographic area.