Behaviour and ecology of the western corn rootworm (Diabrotica virgifera virgifera LeConte)

1 The western corn rootworm (WCR) is a historic pest with a legacy of resistance and behavioural plasticity. Its behaviour and nutritional ecology are important to rootworm management. The success of the most effective and environmentally benign rootworm management method, annual crop rotation, was based on an understanding of rootworm behaviour and host–plant relationships. Enthusiastic adoption of crop rotation, provided excellent rootworm management, but also selected for behavioural resistance to this cultural control.
2 Though well-studied, significant gaps in WCR biology remain. Understanding the topics reviewed here (mating behaviour, nutritional ecology, larval and adult movement, oviposition, alternate host use, and chemical ecology) is a starting point for adapting integrated pest management and insect resistance management (IRM) to an expanding WCR threat. A presentation of significant questions and areas in need of further study follow each topic.
3 The expansion of WCR populations into Europe exposes this pest to new environmental and regulatory conditions that may influence its behaviour and ecology. Reviewing the state of current knowledge provides a starting point of reference for researchers and pest management decision-makers in North America and Europe.
4 The trend toward increasing adoption of transgenic maize will place an increasing premium on understanding WCR behaviour. IRM plans designed to promote sustainable deployment of transgenic hybrids are grounded on assumptions about WCR movement, mating and ovipositional behaviour. Preserving the utility of new and old management options will continue to depend on a thorough understanding of WCR biology, even as the ecological circumstances and geography of WCR problems become more complex.     

How to measure the food utilization of subterranean insects: a case study with the western corn rootworm (Diabrotica virgifera virgifera)

To date, nutritional studies on subterranean insects have focused on qualitative aspects due to experimental limitations. We have developed a method of studying insect–plant interactions quantitatively in subterranean environments. The initial and final weights of larvae of Diabrotica virgifera virgifera LeConte (Chrysomelidae, Galeruncinae) and those of maize roots, which served as food items were determined. The difference between initial and final weight of larvae and roots allowed for the calculation of the food conversion efficiency. This can be used to portray differences in food quality and its impact on larval performance and development.     

Toxic and behavioural effects of free fatty acids on western corn rootworm (Coleoptera: Chrysomelidae) larvae

Feeding behaviour, feeding intensity and staying behaviour of neonate western corn rootworm (Diabrotica virgifera virgifera LeConte) larvae were evaluated in response to synthetic feeding stimulant blends. All of the treatments contained a 3‐sugar blend (glucose : fructose : sucrose, 30 : 4 : 4 mg/ml) and one of twelve free fatty acids. Each free fatty acid was tested in this blend at three different concentrations. The addition of the 12 : 0, 16 : 0, 16 : 1, 18 : 0, 18 : 1, 18 : 2 and 18 : 3 free fatty acids to the sugar blend significantly (P < 0.05) increased the percentage of larvae feeding, but did not increase food consumption per larva. Most of the free fatty acids elicited staying behaviour. At the lowest dose (0.1 mg/ml), all of the free fatty acids except the 18 : 0 and the 20 : 0 elicited staying by significantly more larvae than the sugar blend, and at the highest dose (1.0 mg/ml), eight free fatty acids (8 : 0, 10 : 0, 12 : 0, 14 : 0, 16 : 1, 18 : 1, 18 : 2 and 18 : 3) caused more larvae to stay compared to the sugar blend. Larvae were visibly impaired after exposure to some of the free fatty acids. At the highest dose, the 8 : 0, 10 : 0, 12 : 0, 14 : 0, 16 : 1, 18 : 1 and 18 : 2 free fatty acids were toxic to the larvae. At least 60% of larvae were impaired after exposure to the 12 : 0, 16 : 1 and 18 : 2 free fatty acids and the 8 : 0 and 10 : 0 free fatty acids caused 100% impairment or death. Synthetic blends were compared with liquid from crushed maize roots and with a methanol extract of maize roots. Feeding intensity and staying behaviour on the root liquid and the root extract were significantly greater than on any of the synthetic blends, suggesting the presence of additional compounds in maize roots that serve as feeding cues for western corn rootworm larvae.     

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.     

Quarantine host range of Bikasha collaris,a potential biological control agent of Chinese tallowtree (Triadica sebifera) in North America

Chinese tallowtree, Triadica sebifera (L.) Small (Euphorbiaceae), is one of the worst invasive weeds of the southeastern USA impacting coastal wetlands, forests, and natural areas. Traditional mechanical and chemical controls have been unable to limit the spread, and this invasive species continues to expand its range. A proposed biological control candidate, the flea beetle Bikasha collaris (Baly) (Coleoptera: Chrysomelidae), shows high specificity for the target weed Chinese tallowtree. Results from a series of no‐choice and choice feeding tests of B. collaris adults and larvae indicated that this flea beetle was highly specific to Chinese tallowtree. The larvae of B. collaris feed by tunneling in the roots, whereas the adults feed on the leaves of Chinese tallowtree. A total of 77 plant taxa, primarily from members of the tallow plant family Euphorbiaceae, were tested in numerous test designs. Larval no‐choice tests indicated that larvae completed development only on two of the non‐target taxa. Of 80 B. collaris larvae fed roots of Hippomane mancinella L. and 50 larvae fed roots of Ricinus communis L., two and three larvae completed development, respectively. The emerging adults of these five larvae died within 3 days without reproducing. Larval choice tests also indicated little use of these non‐target taxa. Adult no‐choice tests indicated little leaf damage by B. collaris on the non‐targets except for Ditrysinia fruticosa (Bartram) Govaerts & Frodin and Gymnanthes lucida Sw. When given a choice, however, B. collaris adults consumed much less of the non‐targets D. fruticosa (7.4%) and G. lucida (6.1%) compared with the control leaves. Finally, no‐choice oviposition tests indicated that no eggs were produced when adults were fed all non‐target taxa, except those fed G. lucida. These B. collaris adults fed G. lucida leaves produced an average of 4.6 eggs compared with 115.0 eggs per female when fed Chinese tallowtree. The eggs produced from adults fed G. lucida were either inviable or the emerging larvae died within 1 day. These results indicate that the flea beetle B. collaris was unable to complete its life cycle on any of the non‐target taxa tested. If approved for field release, B. collaris will be the first biological control agent deployed against Chinese tallowtree in the USA. This flea beetle may play an important role in suppressing Chinese tallowtree and contribute to the integrated control of this invasive weed.     

Minnesota field population of western corn rootworm (Coleoptera: Chrysomelidae) shows incomplete resistance to Cry34Ab1/Cry35Ab1 and Cry3Bb1

In the United States of America, the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is commonly managed with transgenic corn (Zea mays L.) expressing insecticidal proteins from the bacteria Bacillus thuringiensis Berliner (Bt). Colonies of this pest have been selected in the laboratory on each commercially available transformation event and several resistant field populations have also been identified; some field populations are also resistant. In this study, progeny of a western corn rootworm population collected from a Minnesota corn field planted to SmartStax^® corn were evaluated for resistance to corn hybrids expressing Cry3Bb1 (event MON88017) or Cry34/35Ab1 (event DAS‐59122‐7) and to the individual constituent proteins in diet‐overlay bioassays. Results from these assays suggest that this population is resistant to Cry3Bb1 and is incompletely resistant to Cry34/35Ab1. In diet toxicity assays, larvae of the Minnesota (MN) population had resistance ratios of 4.71 and >13.22 for Cry34/35Ab1 and Cry3Bb1 proteins, respectively, compared with the control colonies. In all on‐plant assays, the relative survival of the MN population on the DAS‐59122‐7 and MON88017 hybrids was significantly greater than the control colonies. Larvae of the MN population had inhibited development when reared on DAS‐59122‐7 compared with larvae reared on the non‐Bt hybrid, indicating resistance was incomplete. Overall, these results document resistance to Cry3Bb1 and an incomplete resistance to Cry34/35Ab1 in a population of WCR from a SmartStax^® performance problem field.     

Behavioral responses of New Zealand fur seals (Arctophoca australis forsteri) to darting and the effectiveness of midazolam and tiletamine‐zolazepam for remote chemical immobilization

We evaluated the behavioral response of 125 free‐ranging New Zealand fur seals (75 females and 50 males) to darting and the effectiveness and safety of midazolam and tiletamine‐zolazepam for remote chemical immobilization. Behavioral reactions to darting were minor and brief. Overall, severe reactions to darting such as long flight responses (7%) and escape to the sea (7%) were uncommon. Midazolam administered by dart failed to produce a satisfactory level of immobilization. Tiletamine‐zolazepam was administered to 120 animals (35 females and 85 males), 104 of which were successfully immobilized and 16 escaped to the water following darting or attempted net capture. At least 10 of the 16 animals are known to have survived. Tiletamine‐zolazepam caused moderate depression of swimming and diving behavior in the animals that escaped to the water. Data from dive loggers confirmed that seals that escaped remained near the sea surface for extended periods. Tiletamine‐zolazepam administered by dart at a mean dosage of 1.87 ± 0.18 mg/kg for females and 1.49 ± 0.23 mg/kg for males provided effective and safe immobilization, reducing capture stress for both animals and personnel. Although there is still a risk of drugged animals escaping to the water and possibly drowning, this risk is much lower than previously reported for other pinnipeds.     

Contrasting responses to Pleistocene climate changes: a case study of two sister species Allium cyathophorum and A. spicata (Amaryllidaceae) distributed in the eastern and western Qinghai–Tibet Plateau

It has been hypothesized that species occurring in the eastern and the western Qinghai–Tibet Plateau (QTP) responded differently to climate changes during the Pleistocene. Here, we test this hypothesis by phylogeographic analysis of two sister species, Allium cyathophorum and A. spicata. We sequenced two chloroplast DNA (cpDNA) fragments (accD‐psaI and the rpl16 intron) of 150 individuals, and the nuclear (ITS) region of 114 individuals, from 19 populations throughout the distributional ranges of these species. The divergence between the two species was dated at 779 ‐ 714 thousand years before the present and was likely initiated by the most major glaciation in the QTP. Analysis of chlorotype diversity showed that A. spicata, the species occurring in the western QTP, contains much lower genetic diversity (0.25) than A. cyathophorum (0.93), which is distributed in the eastern QTP. Moreover, multiple independent tests suggested that the A. spicata population had expanded recently, while no such expansion was detected in A. cyathophorum, indicating a contrasting pattern of responses to Pleistocene climate changes. These findings highlight the importance of geographical topography in determining how species responded to the climate changes that took place in the QTP during the Pleistocene.