As part of the registration process for Bacillus thuringiensis(Bt) crops in the USA, all registrants must submit an insect resistance management (IRM) plan to the Environmental Protection Agency (EPA). The goal of the IRM plans is to delay the development of resistance to the toxins by pro- ducing susceptible insects and planting crops in such a way that the resistant insects are more likely to mate with susceptible insects than other resistant insects (it is generally assumed that resistant insects ini- tially are rare). Development of an appropriate IRM plan for transgenic maize hybrids that control corn rootworms must include, among other things, an understanding of important biological parameters, because the interactions of many factors could affect the duration of the product (Onstad et al., 2001). For instance, movement of larvae from susceptible to transgenic plants, and vice versa, has been hypothesized to adversely affect IRM in several ways (Mallet and Porter, 1992; Davis and Onstad, 2000). In the case of corn rootworms, initial development would need to be relatively close because larval movement is limited (Hibbard et al., 2003). Since a mixed seed (transgenic and non-transgenic seed sold as mixed seed to the grower) has not been registered for sale by the EPA in the USA, this means that the susceptible host would need to be within the maize production field. It has been known for some time that a number of grasses are hosts for larval development of D. v. virgifera (Branson and Ortman, 1967a,b, 1970), but the majority of the species evaluated were tested only in Petri dishes without soil.
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Clark and Hibbard (2004) evaluated larval survivorship and growth parameters of D.v.virgiferalarvae on the roots of 29 plant species, mostly maize field grassy weeds. Larval recovery and growth (measured as increases in head capsule width and accumulation of dry weight) data were recorded at five sample dates (6, 10, 14, 20 and 24 days after infes- tation) after initial infestation of the 29 species. Larvae survived at least 6 days after infestation on 27 species and 24 days on 23 plant species (Table 3.1). Larval recovery and growth was affected by both species and time after infestation. Growth and development of larvae on plant species were significantly slower on most species than growth and development on maize. However, 18 of the species had larvae develop to the second instar while larvae on 14 species had development to the third instar. Adults were recovered from five plant species in addition to maize. Although adults were produced from a smaller percentage of species by Clark and Hibbard (2004) than in a similar study focused on dominant prairie grasses of the western Great Plains (Oyediran et al., 2004), larvae survived for at least 24 days on 23 of the 29 species evaluated.
Because later rootworm instars are more tolerant to transgenic endo- toxins (EPA Scientific Advisory Panel Meeting, 2002), initial develop- ment on a susceptible plant (a grassy weed or maize plant) followed by subsequent migration to a nearby transgenic plant could accelerate the rate of adaptation if heterozygotes with the resistance gene survived expo- sure to the endotoxin at higher rates. Likewise, if larvae briefly fed on a transgenic root and then migrated to a nearby susceptible root, this too could accelerate the rate of resistance development if heterozygotes with the resistance gene were preferentially selected. However, if a low-dose product produced susceptible beetles, movement of larger larvae onto transgenic roots from less suitable alternative hosts could actually increase product durability by producing additional susceptible insects from within the transgenic field. Storer (2003) modelled adaptation of corn rootworms to rootworm-resistant Bt maize. The model predicts that, if as few as 0.5% of the adults come from spatially well-distributed non- maize hosts, the onset of resistance would be significantly delayed in a system with a poorly distributed 5% fixed location refuge, although this delay is not significant under more conservative refuge deployment sce- narios, such as the 20% refuge being required for the product that is cur- rently registered (Nick Storer, personal communication). Both Clark and Hibbard (2004) and Oyediran et al. (2004) have documented significant adult production from hosts other than maize. However, what percentage, if any, of the adults commonly found in maize agroecosystems developed as larvae on hosts other than maize is unknown. Initial development on alternative hosts followed by movement to transgenic roots (perhaps after herbicide sprays) will probably have a greater impact on resistance man- agement than adult production from alternative hosts. However, at this time, we do not know whether grassy weeds present in transgenic fields will speed or slow the development of resistance. The response may be specific to the transgenic product and the dose of toxin associated with it.
52 J. Moeser and B.E. Hibbard
Nutritional Ecology of Larvae and Adults53 Table 3.1.Percentage of western corn rootworm larval recovery from Tullgren funnels (from Clark and Hibbard, 2004). Means between
plant species within a sampling date (columns) followed by the same lowercase letter are not significantly different at P= 0.05. Means between sampling dates (rows) followed by the same uppercase letter are not significantly different.
Time after initial infestation
Host species Day 6 Day 10 Day 14 Day 20 Day 24 Cumulative
Zea mays 42.2 ± 5.1 a A 46.7 ± 7.7 a A 56.7 ± 10.4 a A 42.2 ± 13.9 a–c A 5.0 ± 3.2 e–g B 39.0 a
Pascopyrum smithii 21.7 ± 9.6 c–e BC 43.3 ± 6.9 ab A 45.0 ± 11.7 ab A 36.7 ± 10.1 b–d AB 18.3 ± 10.0 b–d C 33.0 b
Chloris gayana 38.3 ± 14.5 ab A 26.7 ± 4.7 d–f B 31.7 ± 9.6 cd AB 46.7 ± 15.2 ab A 20.0 ± 4.7 ab B 32.7 bc
Phalaris arundinacea 30.0 ± 11.4 cd A 40.0 ± 7.2 a–c A 40.0 ± 17.9 bc A 30.0 ± 10.4 de A 5.0 ± 3.2 ef B 29.0 bcd Agrostis gigantean 23.3 ± 11.4 c–f B 43.3 ± 5.8 ab A 23.3 ± 12.9 ef B 36.7 ± 13.5 c–e AB 16.7 ± 5.8 ab B 28.7 bcd Eragrostis trichodes 36.7 ± 6.9 ab A 10.0 ± 6.4 i–k B 35.0 ± 3.2 bc A 53.3 ± 11.2 a A 6.7 ± 6.7 fg B 28.3 cde Setaria viridis 31.7 ± 15.5 b–d A 16.7 ± 12.6 h–j B 16.7 ± 1.9 e–g AB 31.7 ± 8.8 c–e A 26.7 ± 9.8 a A 24.7 def Setaria vericillatta 20.0 ± 8.6 d–f BC 28.3 ± 7.4 c–f AB 33.3 ± 8.2 bc A 21.7 ± 6.3 e–g A–C 16.7 ± 8.8 b–d C 24.0 ef Digitaria sanquinalis 18.3 ± 9.6 d–f BC 30.0 ± 10.0 c–f AB 35.0 ± 8.8 bc A 21.7 ± 12.0 g–i BC 8.3 ± 4.2 d–f C 22.7 f Panicum capillare 6.7 ± 4.7 h–j C 23.3 ± 12.9 f–h AB 31.7 ± 1.7 cd A 26.7 ± 2.7 d–f AB 16.7 ± 5.8 b–d BC 21.0 fg Echinochloa crus–galli 16.7 ± 16.7 g–i B 38.3 ± 8.3 a–d A 15.0 ± 8.8 f–j B 20.0 ± 14.1 h–j B 15.0 ± 5.0 bc AB 21.0 fg Eriochloa villosa 16.7 ± 6.9 e–g BC 33.3 ± 11.6 b–e A 21.7 ± 7.4 de AB 21.7 ± 11.3 f–h A–C 10.0 ± 5.8 c–e C 20.7 fg Urochloa texana 16.7 ± 10.0 f–h BC 26.7 ± 9.4 ef A 16.7 ± 5.8 e–h AB 16.7 ± 11.1 h–j BC 6.7 ± 6.7 e–g C 16.7 gh
Setaria faberi 28.3 ± 4.2 bc A 23.3 ± 8.4 fg AB 15.0 ± 7.4 e–i B 10.0 ± 4.3 ij B 1.7 ± 1.7 gh C 15.7 h
Eleusine indica 11.67 ± 7.9 g–i A 10.0 ± 4.3 ij A 11.7 ± 5.0 g–j A 18.3 ± 8.3 g–i A 20.0 ± 11.9 bc A 14.3 h
Setaria pumila 30.0 ± 8.4 bc A 20.0 ± 11.9 g–i AB 8.3 ± 6.3 jk B 3.3 ± 3.3 k–m B 5.0 ± 5.0 f–h B 13.3 hij
Eragrostis curvula 5.0 ± 3.2 ij B 6.7 ± 4.7 j–l B 13.3 ± 9.4 ij AB 15.6 ± 5.1 g–i A 8.3 ± 5.0 ef AB 10.6 jk
Panicum miliaceum 2.2 ± 1.9 jk A 20.0 ± 11.9 g–i A 10.0 ± 4.3 h–j A 10.0 ± 7.9 jk A 8.3 ± 8.3 e–g A 9.5 jkl
Bromus tectorum 21.7 ± 7.4 c–e A 11.7 ± 9.6 i–k AB 0.0 l B 13.3 ± 7.2 h–j A 0.0 h B 9.3 jkl
Panicum italicum 5.0 ± 5.0 i–k BC 23.3 ± 12.3 f–h A 10.0 ± 5.8 ij AB 0.0 m C 5.0 ± 5.0 f–h BC 8.7 kl
Triticum aestivum 3.3 ± 1.9 ij B 3.3 ± 1.9 k–m B 20.0 ± 9.3 ef A 3.3 ± 3.3 k–m B 0.0 h B 6.0 lm
Cenchrus tribuloides 6.7 ± 6.7 ij A 0.0 m A 0.0 l A 5.0 ± 5.0 k–m A 1.7 ± 1.7 gh A 2.7 mn
Sorghum halepense 0.0 k A 1.7 ± 1.7 l A 3.3 ± 3.3 kl A 0.0 m A 6.7 ± 3.9 e–g A 2.3 mn
Dactylis glomerata 6.7 ± 3.9 h–j A 0.0 m A 0.0 l A 1.7 ± 1.7 lm A 1.7 ± 1.7 gh A 2.0 mn
Panicum virgatum 1.7 ± 1.7 jk A 0.0 m A 0.0 l A 6.7 ± 6.7 kl A 0.0 h A 1.7 mn
Sorghum drummondii 0.0 k A 0.0 m A 0.0 l A 1.7 ± 1.7 lm A 1.7 ± 1.7 gh A 0.7 n
Amaranthus retroflexus 1.7 ± 1.7 jk A 0.0 m A 0.0 l A 0.0 m A 0.0 h A 0.3 n
Avena sativa 0.0 k A 0.0 m A 0.0 l A 0.0 m A 0.0 h A 0.0 n
Sorghum bicolor 0.0 k A 0.0 m A 0.0 l A 0.0 m A 0.0 h A 0.0 n
Non-infested Z. mays 0.0 k A 0.0 m A 0.0 l A 0.0 m A 0.0 h A 0.0 n
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