Female WCR are mated by protandrous males, who often intercept teneral virgins shortly after they emerge from the soil and begin releasing sex pheromone (Ball, 1957; Hill, 1975; Branson et al., 1977; Sherwood and
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Levine, 1993; Hammack, 1995). Mating pairs are commonly observed in copulonear the base of maize plants. Once mated, WCR females feed on the maize tissues available in the field where they emerged. After a period of several days, between 15% (Coats et al., 1986) and 24% (Naranjo, 1990) of the newly mated females engage in sustained, migratory flights of > 30 min duration (as measured with tethered flight mills) that would facilitate long-distance dispersal from their natal field. In the field, many young WCR females may be observed ascending during the hours just before dusk from mid-July to early August. Those that engage in migratory flight are believed to fly for a period at an elevation significantly higher than the plant canopy before they descend into another maize field, where they resume feeding and mature their first clutch of eggs. In laboratory tests, only WCR between 2 and 10 days old engaged in sustained flights (Coats et al., 1987). Long-distance, sustained flight is also associated with mod- erate juvenile hormone (JH) titres (Coats et al., 1987). Application of a JH mimic or inhibitor had significant effects on WCR sustained (and trivial) flight tendencies and capabilities.
Concurrent collection of adult WCR from within the canopies of maize and soybean fields, and those flying at 1 and 10 m above soybean canopy level reveal that populations vary significantly between elevation and collection site. The highest-flying (10 m) population is composed pri- marily of young, newly mated females (containing a recognizable sper- matophore, and sperm in their spermathecae, but lacking any mature eggs) with residual maize tissue in their gut contents (Table 6.1). Adults collected from within the maize canopy are similar to those collected from high elevation; however, there is a higher proportion of unmated females in the maize canopy (7%) than at high elevation (< 2%). The sim- ilarities suggest WCR at 10 m recently originated from maize fields.
Insects collected within the soybean canopy have soybean tissues in their gut contents, are all mated and carry oocytes that are significantly more mature than those of females from in or above maize fields. The presence of spermatophores of any size was significantly less common among these insects than in those collected elsewhere. Females flying within 1 m of the soybean canopy are very similar to those within the soybean field canopy. The proportion of female WCR among insects collected in the soybean canopy and those flying just above the canopy is significantly less than that among the high-flying population, though it is still strongly biased toward females. Overall, the array of characteristics suggests that the WCR in and around soybean fields are more mature than those col- lected at high elevation. We hypothesize that females moving in the soybean canopy have dispersed from their natal field and begun periodic movement between maize and soybean fields.
A several day’s to a week delay in the movement of WCR adults from their natal maize fields into soybean (or other non-host fields) is typical of WCR abundance patterns previously characteristic of continuous vs.
rotated maize fields (Godfrey and Turpin, 1983). Prior to rotation resist- ance, few WCR were present in rotated crops and few WCR emerged in
128 J.L. Spencer et al.
first-year maize. WCR abundance in first-year maize remained low until females mated and dispersed from continuous maize fields. Because females also predominate among the dispersing adults, sex ratios outside continuous maize were significantly biased toward females. Today, due to rotation resistance, there is now a lagging relationship between WCR abundance in first-year maize and soybean (as well as other crops in rota- tion with maize). This pattern is characteristic of rotation-resistant WCR populations (O’Neal et al., 1999; Levine et al., 2002; Rondon and Gray, 2003; Fig. 6.2).
The effect of rotation resistance on WCR abundance and movement between maize and soybean fields is illustrated with a comparison of sea- sonal WCR abundance patterns between areas with and without rotation resistance. Beetle abundance patterns are dramatically different between Monmouth (Warren County), Illinois, and Urbana (Champaign County), Illinois (Fig. 6.3). Abundance patterns in these locations (separated by 200 km) typify rotation-susceptible and rotation-resistant populations. At Monmouth WCR are uncommon visitors to soybean fields (as measured
Movement, Dispersal and Behaviour of Adults in Maize 129
Table 6.1.The characteristics of dissected female WCR captured in maize and soybean fields in 2000 at Urbana, Illinois, USA. Values in the same row bearing the same letter are not statistically different at α= 0.05. Female weight was compared using Fisher’s protected LSD test following significant analysis of variance (ANOVA) at α= 0.05. Per cent data were compared as proportions using the methods of Zar (1996) for comparing multiple
proportions.
WCR collection location and method
Maize fields Soybean fields
Live Aerial net Aerial net Sweep net collection at 10 ma at 1 m in canopyb
Measured quantity n= 502 n= 502 n= 113 n= 120
Female weight (g/103) 12.3 ab 11.7 b 99.1 a 12.9 a
% mated 93.0 b 99.1 a 44.5 ab 0.0 a
% with maize tissue in gutc 79.5 a 44.5 b 12.0 c 8.3 c
% with soybean tissue in gut 1.3 d 12.0 c 84.0 b 85.8 a
% with spermatophoresd 59.6 b 84.0 a 43.7 c 49.2 bc
% with large spermatophores 47.4 a 43.7 a 0.6 c 25.8 b
% with mature eggse 1.3 ab 0.6 b 2.7 ab 7.5 a
a10 m aerial collections were made from the top of scaffolding towers.
bCollection made with a 37 cm diam. net.
cPlant tissues were visually identified in gut contents at 100×magnification.
dIn newly mated females, the spermatophore fills the bursa copulatrix and the milky lobe of the spermatophore is as large as or larger than the pink lobe. The milky lobe shrinks and is gone within 3 days, evidence of the spermatophore is gone by 7 days post-mating (Lew and Ball, 1980).
eMature eggs had full sculpturing on the chorion.
LSD, least significant difference.
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with vial traps and sweep samples) while they are very common in Urbana soybean fields. In recent years, rotation-resistant WCR popula- tions have made significant advances into northern and north-west Illinois. At Shabbona (De Kalb County), Illinois, WCR abundance in soybean fields has changed from the Monmouth-like pattern to one like Urbana since 1998 as rotation-resistant populations have become firmly established in north-eastern Illinois (Levine et al., 2002).
Although WCR are capable of long, sustained flight, the bulk of WCR movement is probably quite local (trival adult flight duration aver-
130 J.L. Spencer et al.
Fig. 6.2.2001 WCR abundance in Illinois soybean fields and WCR abundance data for maize and soybean fields located at Monmouth (Warren County; rotation-susceptible population) and Urbana (Champaign County; rotation-resistant population), Illinois (USA).
Bar charts present seasonal average capture rates for equally spaced positions along a linear transect of traps intersecting the interface between a maize and soybean field. Line charts present weekly WCR abundance for maize (n= 5) or soybean (n = 5) traps along a linear transect over the 2001 growing season. Bar charts depict mean daily capture rates of WCR adults in insecticide + cucurbitacin-baited vial traps positioned at ear height in maize, or the same traps at the top of the soybean plant canopy.
Warren County
Rotation-susceptible population 1.3 WCR/100 sweeps
Champaign County Rotation-resistant population 234 WCR/100 sweeps
C5 C4 C3 C2 C1 S1 S2 S3 S4 S5 Transect position
187 194 201 208 215 222 229 236 243 250 Julian date
187 194 201 208 215 222 229 236 243 250 Julian date
C5 C4 C3 C2 C1 S1 S2 S3 S4 S5 Transect position
Maize Soybean
35 30 25 20 15 10 5 0
20 18 16 14 12 10 8 6 4 2 0
120 100 80 60 40 20 0
50 40 30 20 10 0
Mean no. WCR/trap/day (±SEM)Mean no. WCR/trap/day (±SEM) Maize Soybean Maize Soybean
ages 3.1 min/flight and covers an average of just 68 min/flight (Coats et al., 1986)). While sustained flight-mill flights were much longer (averaging 71.8 min/flight) and could result in calculated daily displace- ments of up to 36 km/day, Illinois weather conditions probably limit the intervals when sustained flight would be possible. Based on 181.7 h of meteorological observations concurrent with flying WCR collec- tions at our Urbana location (18 July to 5 September 1997), Onstad et al. (1999) estimated that WCR adults would have only 1.45 h suitable for sustained flight and a typical beetle could travel only c. 4.4 km/year during that period without the aid of wind. The historical record of WCR population spread and the recent expansion of rotation-resistant WCR populations greatly exceed 4.4 km/year and indicate that there must be another contributing mechanism that facilitates long-distance dispersal.
Before rotation resistance was evident, Grant and Seevers (1989) doc- umented long-distance adult WCR transport associated with passage of summertime convective storms from 1984 to 1986. They report 16 occa- sions when large numbers of WCR beetles washed up on the southern shores of Lake Michigan after the passage of weather fronts in July and August. Onstad et al. (1999) used assumptions related to the WCR sus- tained-flight capacity and other weather-related indicators to show that storm- and wind-supported movement could explain much of the expan-
Movement, Dispersal and Behaviour of Adults in Maize 131
Fig. 6.3. Pattern of mean weekly WCR abundance (±SEM) in adjacent maize and soybean fields in an area where rotation-resistant WCR beetles are present. WCR per trap were measured with insecticide + cucurbitacin-baited vial traps positioned at ear height in maize and at the top of the plant canopy in soybean. Data are from Urbana, Illinois (USA), in 1999.
Maize Soybean
174 181 188 195 202 209 216 223 230 237 244 251 258 Julian date of weekly sampling period midpoint
60 50 40 30 20 10 0 1999 WCR/vial trap/day (Mean ±SEM)
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sion of rotation resistance between 1987 and 1997. With the aid of wind, it was calculated that an adult WCR could move 10–30 km/year, depend- ing on storms and prevailing winds (Onstad et al., 1999). Onstad et al.
(2003) revisited this problem and, using additional weather and land- scape data, illustrated how various models for the spread of rotation resistance were more or less sensitive to other factors (e.g. local landscape diversity) that were not considered in the original model. Allowing for combinations of dispersal-promoting conditions, a more accurate fit between observed root injury and various measures of WCR abundance was possible. Annual statewide sampling of WCR adults in soybean fields and root-injury evaluations for maize fields in 30 or more select Illinois counties have provided data that have proved useful for evaluating the yearly spread of the rotation-resistance problem area (Shroeder and Ratcliffe, 2003).
The Onstad et al. (1999, 2003) models suggest that an important factor in the evolution and spread of rotation resistance is the local proportion of land that is not in rotation with maize and does not provide host plants for WCR larvae (i.e. ‘extra vegetation’). These areas provide a counteract- ing selection against oviposition outside of maize and become a sink for eggs. Onstad et al. (2001) suggest that, where less than c. 80% of the plant landscape is rotated, the presence of ≥20% of the land cover as ‘extra veg- etation’ (i.e. non-maize and non-rotated soybean vegetation) does not provide adequate selective advantage to maintain rotation resistance when populations move into these areas. The east-central Illinois epicen- tre of the rotation-resistance problem has a very low percentage of extra vegetation in the landscape; six of the nine counties have ≤ 10% extra vegetation and the remainder have ≤20% extra vegetation (Onstad et al., 2003). In western and southern Illinois, most of the counties have ≥20%, with many having ≥30–50% extra vegetation (Onstad et al., 2003). In a 3- year sticky trap survey of Michigan counties, O’Neal et al. (2003) report that very few soybean fields supported WCR populations indicative of a rotation-resistance threat. In contrast to Illinois, no Michigan counties have percentages of extra vegetation at or below the 20% hypothesized by Onstad et al. (2001) as necessary to sustain rotation resistance. O’Neal et al. (2003) report high adult WCR abundance in some soybean fields, but suggest that normal WCR responses to crop maturity (Darnell et al., 2000) and farming practices (e.g. early season harvest of silage maize) can ade- quately explain beetle movement out of maize fields. They also suggest that low levels of adult WCR emergence in Michigan fields of first-year maize after soybean is consistent with the innate tendencies of WCR beetles to oviposit outside maize at some low level rather than expression of rotation resistance. Slower than predicted expansion of rotation- resistant WCR populations into Michigan may be a consequence of the greater vegetational diversity in this state, as hypothesized by Onstad et al. (2003).
132 J.L. Spencer et al.