Tobacco budworm Heliothis virescens L. infests more than 19 crops, while bollworm Helicoverpa zea (Boddie) infests at least 30 (Metcalf et al. 1962; Payne & Polles 1974; Martin et al. 1976; Young & Price 1977; Lopez et al. 1984; Rummel et al. 1986; Slosser et al. 1987; Kogan et al. 1989; Blanco et al. 2002; and references within). Both species have been observed feeding on at least 80 (H. virescens) and at least 76 (H. zea) uncultivated plants (Morgan & Chamberlin 1927; Isley 1935; Lincoln et al. 1967; Roach 1975; Stadelbacher 1981; Pair 1994; Sudbrink & Grant 1995; Parker 2002; and references within). Apart from the vast number of reports cited above, there are few reports that compare larvae or moth production per area under natural conditions in cotton and other plant species and even fewer studies were conducted for more than 1 growing season.

The tobacco budworm and the bollworm are important pests of a variety of cultivated plants in North America, especially of upland cotton Gossypium hirsutum L. (Luttrell 1994). Although these insects (heliothines) have been considered as the most important pest complex of cotton in 22 of the last 25 years in the U.S. cotton producing states (Williams 2005), tobacco budworm larvae in cotton plants and moths captured in pheromone traps have declined in certain areas in recent years (Parajulee et al. 2004; Blanco et al. 2005; Adamczyk & Hubbard 2006). Tobacco budworm has developed resistance to certain classes of insecticides (Sparks 1981; Luttrell et al. 1987; Hardee et al. 2001; Terán-Vargas et al. 2005), and since 1996 it has been effectively controlled, and bollworm partially controlled, by commercial varieties of cotton genetically engineered to express the insecticidal Cry proteins from the bacterium Bacillus thuringiensis Berliner (Bt). Because this is a valuable agricultural biotechnology, the US Environmental Protection Agency (EPA) and the Mexican Agricultural, Livestock, Rural Development, Fisheries and Nutrition Department (SAGARPA) requests Bt cotton registrants to conduct annual monitoring of Bt susceptibility in target pests (Matten & Reynolds 2003). The U.S. Department of Agriculture, Agricultural Research Service (USDA-ARS) in Stoneville, Mississippi and the National Agriculture and Forestry Research Institute (INIFAP) in Mexico conduct monitoring programs to assist industry and regulatory agencies with their need for Bt-susceptibility information. Due to the fact that Bt-resistance frequency in tobacco budworm field populations was established as 1.5 × 10-3 prior to commercialization of Bt cotton (Gould et al. 1997), in order to detect possible shifts from the above mentioned Bt-resistance frequency a high number of samples should be obtained and screened. Efficient trapping methods and/or plant hosts that produce high densities of tobacco budworms, ideally in synchrony with those occurring in cotton, would prove beneficial to satisfy those sampling needs.

The goal of this research was to determine population densities and synchrony of tobacco budworm and bollworm among 3 plant species that are relatively easy to establish: garbanzo bean (Cicer arietinum L.), upland cotton, and velvetleaf (Abutilon theophrasti Medikus), reported as capable of hosting large numbers of heliothines. This information could be useful for determining easier and reliable captures of naturally occurring tobacco budworms and bollworms. We also were interested in determining the contribution of each plant species to the overall insect population and moth emergence synchrony. Lastly, these data on larval-to-moth densities could be important for the development or refinement of theoretically-based models assessing mitigation of insecticide resistance in these important pests.

Materials and Methods

This study was conducted over a 3-year period in 2 geographical locations assessing naturally occurring populations of heliothines.

Results

Discussion

A clear advantage for the field collection of tobacco budworm larvae is presented by garbanzo bean plots. Significantly higher densities of this insect were observed on this plant in all years in both locations during 85% of the garbanzo's growing season (120 d). On cotton tobacco budworm was observed only 19% of the time of the 160 d of its growing season, while velvetleaf hosted this insect during 51% of its 150 d growing season. A disadvantage for using cotton or velvetleaf as a reliable source of H. virescens is the fact that this insect was not found on them every year. Tobacco budworms present in garbanzo coincided with larvae present in cotton or velvetleaf during most of the study, except in TAM in 2003. Heliothis virescens moth densities followed closely what was previously described, showing this time overlap in TAM in 2003 between moths emerging from garbanzo and cotton. The presence of H. zea larvae and moths in all three plant hosts was more sporadic compared with H. virescens. There was less bollworm larval overlap on all the plants hosts.

Although comparisons of insect populations between years and geographies are influenced by regional annual cropping systems, the intrinsic movement capacity of the species and habitat suitability (Kennedy & Storer 2000), heliothine densities recorded in this study are similar to those of reports that evaluated naturally-occurring populations in North America. Tobacco budworm larval densities reported here (500-57,000/ha) were estimated as 7-3,053 on cotton by Henry & Adkisson (1965), Lincoln et al. (1967), and Graham & Robertson (1970). Heliothis virescens larvae on velvetleaf in our study (125-81,000) also compare to 54,895-98,003 larvae/ha reported by Stadelbacher et al. (1986). Naturally-occurring tobacco budworm moth emergence has been reported as 1,008 moths from cotton by Roach & Ray (1976), while our collections ranged between 312-2,100 moths/ha. Our estimates of bollworm densities were somewhat lower than those of Henry & Adkisson (1965), Lincoln et al. (1967), Graham & Robertson (1970), Young & Price (1975 and 1977), and Jackson et al. (2003), which ranged from ≥59 up to 370,828 larvae per hectare (Rummel et al. 1986). Our bollworm densities only ranged between 247-17,000 larvae/ha. Stadelbacher et al. (1986) reported a density of 6,297 bollworm larvae/ha on velvetleaf and in this study they ranged between 125-81,000 larvae/ha. Bollworm moth emergence under natural conditions was reported by Roach & Ray (1976) as 459 moths per hectare in cotton while in this study bollworm emerging moths ranged between 300-2,100 bollworm moths/ha.

The relationship between larval densities and moth emergence under field conditions can be estimated from this study. Tobacco budworm peak larval densities yielded 1 moth for every 13, 22 and 55 larvae on garbanzo, cotton, and velvetleaf, respectively. Bollworm peak larval densities produced 1 moth for every 1,290 and 656 larvae on garbanzo or cotton, while no estimates for bollworm moth emergence were established for velvetleaf plots. This information reflects on the relative importance of these plant species as heliothine hosts. The differences between the emergence of moths from the three plant hosts could have been influenced in part by the high number and diversity of predators found in cotton and velvetleaf plots (data not shown) as compared with those observed on garbanzo bean (lower abundance and diversity, data not shown), throughout the entire growing seasons. The variation in moth emergence among years and locations might have been caused by abiotic factors such as precipitation. According to Young & Price (1968), rain in excess of 2.0 cm can cause 100% mortality in H. zea, while Hendricks (1991) concluded that 50% of heliothine pupae in water-saturated soil for ≥50 h die, a likely possibility in the MS study site's alluvial soils. Peak tobacco budworm larval densities in MS in weeks 9 and 13 (yr 2002), 6 and 26 (yr 2003), and 7, 16, and 17 (yr 2004) were closely followed by ≥3.0 cm of rain, indicating the possibility that this factor alone might have had in the survivorship of both insect species under natural conditions (Schneider 2003). The influence that biotic and abiotic factors have on naturally-occurring heliothine populations can be corroborated with those field-collected larvae placed in insect artificial diet in the laboratory for species identification that yielded ≈85% moths (1 moth for every 1.2 larvae placed in insect artificial diet under laboratory conditions). These estimates of moth emergence densities between field conditions and laboratory conditions provide a useful indicator for understanding heliothine densities in other studies.

Garbanzo bean, the third most important legume in the world (Romeis et al. 2004) is a minor crop in the U.S., representing less than 1% of the world production, but it is an important export commodity in Mexico representing 3% of the world production. An area that covers ≅3,300 ha (data of 2002) (Johnson & Jimmerson 2003) of this crop in North America, has a great potential in the production of heliothines.