The univoltine adults of the WCR are present in maize fields from July until the first frost and feed on maize foliage, pollen, silks and develop- ing kernels. Oviposition has traditionally taken place almost exclusively in maize fields from late July through to mid-September (Shaw et al., 1978; Levine and Oloumi-Sadeghi, 1991). The diapausing eggs spend the autumn and winter in the soil until late May and early June, when they hatch and the larvae begin to feed on maize roots (Levine and Oloumi- Sadeghi, 1991). Larval feeding reduces the amount of water and nutrients available to developing plants, disrupting root system function, which, in turn can reduce grain yield. Larval feeding also may facilitate infection by root- and stalk-rot fungi, resulting in further damage. Extensive root injury makes plants more susceptible to lodging and direct yield losses may result from the difficulty in harvesting this maize. Finally, high den- sities of adults (usually > 5 WCR/plant) may interfere with maize polli- nation due to silk feeding (Levine and Oloumi-Sadeghi, 1991).
The WCR was first collected in 1865 and described in the report of LeConte (1868), who observed it on a wild gourd (probably Cucurbita foe- tidissima) near Fort Wallace, Kansas, USA. Smith (1966) suggested that the native hosts of WCR populations in the Colorado–New Mexico–Arizona region of the USA were probably Tripsacum (any of approximately 15 species of perennial grass closely related to maize, Zea mays). Despite a close host association with plants in the Graminaceae, WCR adults will feed compulsively on species in the Cucurbitaceaecon- taining cucurbitacins B and E (Metcalf, 1979). This compulsion is taken as evidence of diabroticite coevolution with Cucurbitaceaeprior to a host shift onto graminaceous plants, after which the cucurbitacin compulsion was retained (Metcalf, 1983).
Gillette (1912) was first to observe WCR as a pest of cultivated maize (sweetcorn) in 1909 and 1910 near Fort Collins and Loveland, Colorado, respectively. In subsequent years, the distribution of WCR expanded slowly eastward across the western maize-growing region through the 1920s–1940s (reviewed by Metcalf, 1983). The WCR was first noticed and caused some damage in south-west Nebraska in 1929 and 1930 and caused heavy damage in the early 1940s (Tate and Bare, 1946). Large-scale appli- cation of soil insecticides against the WCR began in Nebraska in 1949; treat- ment with aldrin, chlordane and heptachlor followed into the mid-1950s.
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By 1954, WCR were present along the Missouri River from South Dakota to Missouri. The first WCR control failures were reported in 1959; by 1961 WCR in central Nebraska were 100-fold more resistant to cyclodiene insec- ticides than susceptible populations in the eastern portion of the state.
Concurrent with the rise of widespread cyclodiene resistance was an accel- eration in the rate of WCR population expansion (Metcalf, 1982, 1983).
Between 1909 and 1948, WCR moved 756 km from Colorado to the Missouri River, averaging just 19 km/year. In Kansas during 1953, the WCR range expanded 56 km (Burckhardt and Bryson, 1955). From 1961 to 1964 cyclodiene-resistant WCR moved 579 km from Nebraska to Eau Claire, Wisconsin (c. 193 km/year), and by 1968 had advanced 805 km to north-western Indiana (c. 113 km/year). By 1979, WCR were found throughout most of the Corn Belt; even at the leading edge of the expand- ing front some still retained 1000- to 2500-fold resistance to cyclodienes (reviewed by Metcalf, 1983). In the 1980s, the eastward WCR expansion finally reached US East Coast maize-producing areas. Metcalf (1983) hypothesized that the increased movement rate was related to the increased fitness among cyclodiene-resistant beetles and to a behavioural change associated with the genes for resistance. A very mobile WCR may be a legacy of the chemical era.
Though insecticides are a favourite WCR management tool, cultural controls have been recommended for nearly as long as the WCR has been a recognized pest. In a paper focusing on the NCR, Gillette (1912) noted that the obligate host relationship between corn rootworm beetles and maize suggested an obvious method of pest management, crop rotation, a cultural control that has been recommended for control of both the NCR and the WCR ever since. A maize and soybean rotation (the most common rotational pattern in the Midwest USA, accounting for much of the 80% of US rotated maize (Power and Follett, 1987)) disrupts the rootworm life cycle because corn rootworm larvae cannot survive on soybean roots. Crop rotation proved so effective that farmers using it could safely discount the possibility of significant corn rootworm larval injury to the roots of first- year maize. Despite a history of insecticide resistance and encouragement to use crop rotation, the availability of new insecticides and inexpensive nitrogen fertilizers worked against widespread adoption of cultural control in favour of more profitable continuous maize production.
The WCR first entered north-west Illinois in 1965 and moved across the northern counties to reach Indiana by 1970. In Illinois, the use of soil- applied insecticide in maize increased through the late 1960s when
> 60% of maize hectares were treated for corn rootworms (primarily for the NCR; problems with the NCR’s prolonged diapause predate the arrival of the WCR (Levine et al., 1992)). Insecticide usage remained at ≥50% of hectares until the late 1970s when, thanks to educational efforts that emphasized rotation as an alternative to chemicals, insecticide use began to decline steadily into the 1990s, by which time approximately only 13%
of maize was treated (Pike and Gray, 1992). The shift from broad-scale prophylactic chemical application to cultural control constituted a great
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victory for integrated pest management. Ironically, even this level of insecticide use was probably excessive. Steffey et al. (1992) conducted surveys of root injury to maize after soybean (first-year maize) around Illinois from 1986 to 1989. Only 1.7% of rotated maize fields had injury levels that exceeded the theoretical economic injury level of 4.0 on the Iowa State University root-injury rating scale (equal to one full node of roots removed (Hills and Peters, 1971)). Steffey et al. (1992) concluded that Illinois maize producers rarely needed to apply soil insecticides to prevent rootworm injury in maize rotated with soybean. At the time, most of the economic injury reported by Steffey et al. (1992) was attributed to NCR’s prolonged diapause. East-central and north-eastern Illinois were the areas identified as being most at risk; these districts encompass the Ford County epicentre of rotation resistance and the areas where the WCR problem is currently most serious.
Unfortunately, it now appears that the enthusiastic adoption of crop rotation over a broad area (98% adoption in parts of east-central Illinois) combined with the great effectiveness of the technique created conditions that favoured an existing, but rare, WCR variant with reduced egg-laying fidelity to maize fields (Onstad et al., 2001). The presence of some WCR expressing what we would now call ‘variant behaviour’ may be evident in late-1970s to late-1980s records of root injury for first-year maize (Shaw et al., 1978; Steffey et al., 1992). This unexpected, but not alarming, injury was frequently attributed to the presence of volunteer maize or grassy weeds in rotated soybean fields that were known to be attractive to ovipositing WCR and NCR (Shaw et al., 1978). Even if this injury had been viewed as a manifestation of a natural variation in WCR host fidelity, it seems unlikely that scientists at the time could have imagined that a rapid development of resistance was already under way. An appre- ciation for widespread crop rotation as a selective force was lacking. As late as 1993, it was suggested that ‘it is highly unlikely that the WCR could become adapted to crop rotation by oviposition in the alternate crop’ (Krysan, 1993). There are large areas in Illinois (e.g. Monmouth, located c. 220 km north-west of Champaign-Urbana, in western Illinois) where WCR can be very abundant in maize, but only low numbers of WCR adults are ever found outside of maize fields and there is no evi- dence of rotation resistance. The distance between these areas and the rotation-resistance epicentre (see below) has protected them from the problem; however, even these areas will eventually experience rotation resistance as the problem continues to spread north and west in Illinois (Onstad et al., 1999, 2000; Fig. 6.1).
Where crop rotation is applied nearly universally, the practice imposes a strong (though unintended) selection that favours individuals expressing reduced ovipositional fidelity to maize fields. Where maize is rotated, females with reduced fidelity to maize fields realize a reproduc- tive advantage over females with perfect ovipositional fidelity to maize fields since they lay some of their eggs in non-host fields (e.g. soybean) which are rotated to maize with high probability. Eggs deposited in maize
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fields rotated to soybean are lost. We hypothesize this selection generated a WCR population in which a high proportion of females have some propensity to exit maize fields and oviposit in soybean fields (or other locations in addition to maize fields). Extensive adult monitoring and
Movement, Dispersal and Behaviour of Adults in Maize 125
Fig. 6.1.WCR abundance (mean number of WCR beetles per 100 sweeps per county with a 38 cm diameter sweep net) in soybean fields for Illinois (USA). Each mean represents four to eight samples. Collections were made during the last week of July and first week of August 1997–2003. Sampled counties have a heavy border outline, unsampled counties have thin outlines with a stippled interior.
1997 1998 1999
2000 2001 2002
2003 WCR Abundance in Illinois
Soybean Fields: 1997–2003 (WCR/100 sweeps)
11–50
> 50 Not sampled 0
≤10 chap06.qxd 24/11/04 1:42 PM Page 125
analysis of soybean field soil samples confirm that WCR resistance to crop rotation occurs because females lay eggs in crops rotated with maize (eggs are still also laid in maize fields) (Levine et al., 2002; Pierce, 2003).
The first hints that the crop rotation success story was failing came in 1987 when unexpected WCR root injury to rotated seed maize was reported near Piper City in Ford County, Illinois. Local use of pyrethroid insecticide in seed production maize fields was initially proposed as a mechanism that could have repelled gravid WCR from treated maize fields and into surrounding untreated seed soybean fields where females oviposited (Levine and Oloumi-Sadeghi, 1996). The pyrethroid hypothe- sis was found to be false as the primary cause of the problem in 1993 when circumvention of crop rotation occurred in commercial maize fields where pyrethroids were not used. However, the possibility that pyrethroids provided the ‘push’ needed to concentrate a critical mass of WCR with variant behaviour outside of maize fields remains an intrigu- ing hypothesis for how the more widespread problem of behavioural resistance in WCR was initiated. By 1993 and 1994, WCR injury to rotated maize was reported in several east-central Illinois counties and, in 1995, failure of crop rotation was devastating in nine Illinois counties and 15 adjacent Indiana counties (Levine et al., 2002). As a consequence of rota- tion resistance, growers in east-central Illinois and western Indiana have experienced economic losses due to WCR larval injury to first-year maize since 1995. For these growers and those in parts of Michigan and Ohio, where this problem has now spread, the only reliable option to manage this pest is a planting-time application of soil insecticide (Levine et al., 2002). Modelling studies of the spread of the problem also strongly suggest that it originated very close to Piper City (Onstad et al., 1999;
Levine et al., 2002).