Insect populations that use different local
host plants frequently show divergent egg laying or feeding preferences on those hosts
(Bernays
and Chapman 1994). Because plants represent
habitats as well as food, host–plant variation
could also affect behaviors not directly related to finding or consuming plants (Carroll
et al. 1998). Several authors have recently
addressed the microevolutionary change is deemed predictable if replicate populations
in a new environment converge towards populations already adapted to that
environment (Messina
2004). Our results demonstrate that
both a juvenile trait and an adult trait (host discrimination) in C.
maculatus population can be rapidly and predictably modified by a ancestral
host shifting. Furthermore, results revealed that C. maculatus behaviors were
largely affected by the host choice tests, hosts and generation number. In no host
choice tests, female C. maculatus were highly attracted by the seeds of chickpea
and cowpea. However, they showed a strong preference for seeds of cowpea only
in free host choice test. This fitted with Rees (2004) who indicated that the bruchids have
varied preferences for different hosts.

Previous works indicated that bruchid species are known to exhibit
specificity in the choice of legumes they attack (Rees 2004). C. maculatus is primarily a pest of cowpea but has many alternative
hosts among leguminous seeds (Haouel-Hamdi
et al. 2017a). Being a field-to-store pest suggests that
dispersing individuals are guided by specific cues to their preferred hosts (Ajayi et
al. 2015).

According to Messina (2004) the oviposition preference is rather
labile but that a new host is readily accepted after 40 generations. In addition, even after many years
of laboratory culturing, there is still considerable plasticity in traits
related to host use in C. maculatus (Guedes et
al. 2003). However, a host shift exposes larvae to a
completely new environment causing changes in larval feeding behavior (Guedes et
al. 2003), larval competitive behavior (Tuda and
Iwasa 1998; Messina 2004), and adult life-history traits (Messina
2004). Thus, host shifts lead to adaptations to
new food sources both in the laboratory and in the wild conditions (Tuda et al.
2006). Female oviposition behavior is known to
show heritable genetic variation and egg-laying preferences in C. maculatus seems to be quite evolutionary
labile in general (Messina
2004).

On the other hand, Sankara et al. (2010) showed that females of C. maculatus
were able to recognize odors from their egg-laying substrates and to find their
way to the sources of these odors. When these females have the choice between
clean air and air containing seed odors, they are significantly more attracted
to the smell of seeds. This would be explained by the volatile substances
emitted by the seeds. In addition, Ignacimuthu et al. (2000) confirmed that these chemicals would have
an attractive effect on C. maculatus females.

In this study, we found rapid evolution of female oviposition behavior
with an increase in novel host acceptance, in agreement with Wasserman and
Futuyma’s (1981) as well as Messina and Karren’s (2003) findings. Most importantly, we observed
acceleration in means growth rate and an increase in weight and body size.
Thus, evolution of demographic traits to the novel host occurred in concert
during adaptation, together with elevating population fitness on cowpea and
lentil.

Previous
study of Agosta (2008) indicated that in the free host choice test both the survival of offspring
and the host seed size showed a positive relationship with the numbers of eggs
laid on the seed. However, in the no host choice test, only the survival of
offspring and the numbers of eggs laid showed a positive relationship, while
there was a significant correlation between seed size and the number of eggs
laid on the seed.

The developmental plasticity is illustrated in this
study by several biological indicators, which gave evidence of an important
potential for a relatively rapid adaptation of C. maculatus on the three
tested hosts. Other indicators of C. maculatus adaptability to cowpea
seeds came from the extent of larval survival inside the hosts and the
intrinsic rate of natural increase. Although both parameters were higher on chickpea
until the 4 generation, the increase in the intrinsic rate of increase was
faster and higher when developing on chickpea compared with cowpea. Furthermore,
Sankara et al. (2016) showed that larval mortality was important
inside Bambara seeds, whatever the number of eggs laid, indicating that
acceptability is not always correlated with suitability (Giga and
Smith 1987; Shazali 1989). The chemistry of seeds may be the most
important factors determining interactions between the development of bruchids
and their host plants (Tuda et al.
2006). Besides, Sankara et al. (2016) showed that females of C. maculatus
seem to remember the volatile signals from cowpea, the original host, even
after three years of adaptation or development on the Arachis
hypogaea and Cajanus cajan. For
instance, if a population has a means to detoxify a particular secondary
compound in a novel host, it may be able to exploit closely related hosts that
contain similar compounds (Agrawal 2000). According to Nicole (2002), an insect is specific to a plant if it
can first recognize it. This author has shown that C. maculatus was able
to recognize its original host cowpea and to deposit its eggs in all tested
situations (free choice, semi choice and no choice). In addition, this author
has reported that the transfer of C. maculatus to other legumes and its
maintenance on these hosts for a relatively long time influences its penchant
for this host, but does not alter its ovipositional preference for the host
plant.

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