Introduction

Insects and diseases in the field and in storage deplete bean (Phaseolus vulgaris L.) production, reducing not only yield but also grain quality of this legume. In the 1980s, the Centro Internacional de Agricultura Tropical (^{CIAT, 1988}) pointed out that the Mexican bean weevil (Zabrotes subfasciatus Boh.) and the common bean weevil (Acanthoscelides obtectus Say; Ordet Coleóptera, family Bruchidae) are the most important pests of beans. Specifically the latter insect is catalogued as the primary pest because of the damage it causes to stored grains (^{Vera et al., 2011}).

It is widely distributed throughout the high valleys of Mexico, but it has also been detected in tropical regions. Damage severity and population parameters of A. obtectus vary largely depending on the bean cultivar, testa hardness and the naturally occurring antibiotics in the colyledons (^{Ramirez et al., 2003}).

The Bean Breeding Program of the Colegio de Postgraduados and the Department of Plant Science of the Universidad Autónoma Chapingo selected five bean cultivars resistant to rust (Uromyces phaseoli var. typka or U. appendiculatus). Their general characteristics are the following:

The objectives of this study were to estimate and compare the population parameters of A. obtectus in these bean cultivars to detect the cultivars most resistant to weevil infestation.

Materials and Methods

The study was conducted in the insect ecology laboratory of the graduate program in Plant Health of the Colegio de Postgrraduados, Campus Montecillo, in 2014. One rearing chamber was used with controlled environment (28±2 °С, relative humidity 70±3% and continuous illumination).

The bruchid population was incremented to obtain sufficient insect specimens, following Vera et al. (1997). In addition, the grains of each genotype were washed, dried and kept in paper bags at 4 °С for three months to prevent damage by insects, mites or pathogens. Initial moisture of the grains was 7 and 8 % measured with an automatic moisture meter (Stein lite™).

Population parameters were estimated with the technique of life and fertility tables, mainly with the Logrank test (p≤0.05) (^{Méndez et al, 1984}). In this way, weevil survival (n_x), larva mortality, mean life expectancy (e_x), adult emergence, net reproductive rate (R₀) (p≤0.05), intrinsic rates of population growth (rm) (p≤0.01) and average generation time (Т) curves were obtained (^{Krebs, 1985}).

The bruchid's life tables were constructed with eggs obtained from the mentioned offspring. To stimulate oviposition, 300 adults were placed in a glass Petri dish (9 cm in diameter) with CV Peruano beans, and 24 h later the eggs were separated to have the certainty that they had been laid on the same day and that the proportion of females to males was approximately 1:1. For each bean cultivar, an initial cohort was established with 100 eggs taken at random. The eggs were placed in glass Petri dishes (9 cm in diameter), and 100 beans of each cultivar were placed in each dish. In this way, intraspecific competition for food was avoided because of the grain:egg ratio of 1:1. The bean grains were agglomerated in the Petri dishes so that, once the larvae hatched, they would have support from adjacent grains to penetrate other grains. The Petri dishes were handled carefully to prevent inhibiting the larvae from penetrating (^{Quentin et al, 1991}).

The number of newly hatched larvae was recorded daily, as well as number of first stage larvae that did not penetrate the testa of the grains. For a count of larvae that died inside the grains and determine the stage in which they died, 30 to 31 d after initiating the cohorts, when adults emerged from the grains, the non-perforated (apparently healthy) grains were cooked and dissected to find the dead larvae. Their development stage was estimated by their size and shape (^{El-Achkar et al., 1991}).

Survival curves were constructed with data from the life tables and compared with the Logrank test. The death frequencies of each population were used to assess the discrepancies between these frequencies (o) and the expectation (E) of their occurrence when the null hypothesis is true. These discrepancies are measured with χ ^{2 cal}, which is compared with χ ² ₀ . _{05 (2 d.f.)} (^{Méndez et al, 1984}). With these data, the mean life expectancy of the insects in each cultivar was estimated.

Fertility tables were constructed with data of the weevils that reached adulthood (males and females). The adult weevils were placed in Petri dishes (9 cm radius) that contained bean grains of the same genotype from which they had originated and the number of eggs laid was recorded daily. With these data the net reproductive rate (R_o), intrinsic rate of population growth (r_m) and mean generation time (Τ) were estimated, following the methodology described by ^{Sánchez et al. (1997)} and Vera et al. (2002). The five rates of population growth (r_m) were compared with the ^{Vera and Sotres (1991)} Overlapping Interval test (p≤0.09).

Results and Discussion

The A. obtectus life tables in each bean genotype are presented as survival curves (Figure 1). The Logrank test did not detect significant differences (p>0.05) between bruchid cohorts in the cultivars Blanco Tabasco, Flor de Mayo and Pinto Durango. There were, however, differences between these three cultivars and the Rayado Rojo and Línea Elite No. 4 cultivars. This was due to the fact that insect mortality was higher inside the grains of the first three cultivars (Table 1) than in Rayado Rojo and Línea Elite No. 4.

Figure 1 Survival curve of Acanthoscelides obtectus (Say) reared in five bean (Phaseolus vulgaris L.) cultivars.  

Table 1 Mortality of Acanthoscelides obtectus larvae reared in five bean (Phaseolus vulgaris L.) cultivars. 

In Blanco Tabasco, Pinto Durango and Flor de Mayo, the number of emerged adults (30 to 31 d old) was lower than in Rayado Rojo and Línea Elite No. 4 (Table 2).

Table 2 Emerged Acanthoscelides obtectus adults reared in five bean (Phaseolus vulgaris L.) cultivars. 

Mean insect life expectancy (e_x), in general, was also longer in the last two genotypes (Table 3)·

Table 3 Mean life expectancy (ex) of Acanthoscelides obtectus reared in five bean (Phaseolus vulgaris L.) cultivars. 

^†Values recorded every 10 days.

The above was reflected in the weevil's reproductive rate, R₀ and r_m (Table 4); thus, the rates of Blanco Tabasco, Flor de Mayo and Pinto Durango were the lowest. The Overlapping Intervals test did not detect significant differences (p>0.09) among Pinto Durango, Flor de Mayo and Blanco Tabasco, nor between Rayado Rojo and Línea Elite No. 4.

Table 4 Reproductive rates (R₀ and r_m) and generation time (T) of Acanthoscelides obtectus reared in bean (Phaseolus vulgaris l.) cultivars. 

^a,bStatistically different (p≤0.09). r_m (individual d^-1).

A higher reproductive rate does not mean longer generation times (T) (^{Vera et al., 2012}). Generation is understood as the period between birth of parents and birth of offspring.

Blanco Tabasco was the most damaged cultivar; the reproductive rate rm (which is expressed as individual d^-1) was lower in a longer Т The opposite occurred with Rayado Rojo and Línea Elite No. 4, whose generation times were close to those of Blanco Tabasco. Consequently, it would be recommendable to cross initially Blanco Tabasco, Flor de Mayo and Pinto Durango with cultivars known for resistance to the insect (with r_m≤0.02), such as Colorado landrace beans and black beans from Chiapas and Tabasco (^{Bolaños and Vera, 1997}). With this, resistant beans can be successfully produced in a short time.

Regarding adult emergence and reproductive rate, it should be pointed out that the populations with high numbers of emerged adults may not have higher reproductive rates (^{Ramirez et al., 2003}). This is observed more often in landrace beans (^{Vera et al., 1998}), whose reproductive capacity (r_m) is equal to or lower than 0.02.

No relationship between growing region, resistance to rust and growth habit of the beans was identified with the Overlapping Intervals test. This means that in storage A. obtectus can parasite (with higher or lower intensity) any of the studied cultivars.

Conclusions

No statistically significant differences were detected in the survival curves of A. obtectus Say in Blanco Tabasco, Flor de Mayo, Pinto Durango, Rayado Rojo and Línea Elite No. 4, but there were differences between the first three and the second two cultivars.

Susceptibility of Rayado Rojo and Línea Elite No. 4 to the coleopteran indicates that these cultivars could confer resistance to sensitive cultivars.