Agriculture Reference
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considerable importance as a cosmopolitan pest of stored cowpea
Vigna unguiculata
(L.) and
to a lesser extent of other stored legumes like mung bean (
Vigna radiata
), adzuki beans
(
Vigna angularis
) and pigeon pea (
Cajanus cajan
) (Jackai & Daoust, 1986). More recently,
adaptation to a novel host legume,
Cicer arietinum,
has also been reported (Fricke &
Arnqvist, 2007). The female and male couple and the females begin oviposition within 1
hour.
C. maculatus
females cement their eggs onto the seed surface. Oviposition is completed
in about 8 days, and adults die about 10-12 days after emergence. Eggs hatch after 3-5 days.
Before the larva arrives to the cotyledons, where it will complete the life cycle, it is necessary
that the larvae cross the seed coat, which represents a critical event (Credland, 1987). Larval
development and pupation take place inside the seed, adults emerge from the seed some 25-
30 days after oviposition, and adult emergence leads to a reduction in seed quality, poor field
performance and lower acceptability in the food industry (Caswell, 1968; Southgate, 1979;
Credland, 1987). The larvae of this species feed and develop exclusively on the seed of
legumes and the adults do not require food or water and spend their limited lifespan mating
and laying eggs on beans (Beck & Blumer, 2007).
Seed defense against insect attacks may involve a diverse set of contributors, going from
a tissue to a molecular level of action. Approaches on the seed coat level contribution are not
numerous. One of these studies analyzed the perforation capacity of
C. maculatus
larvae in
seeds of 73 species and showed that the seed coat prevented the penetration of larvae in
69.5% of the tested seeds. However, there was no direct relationship between hardness and /
or thickness of the seed coat with the larval abilities for crossing or not this tissue (Janzen,
1977), since the work showed a high mortality of larvae during the perforation of the very
thin seed coats of
Erythrina berteroana
and
Ormosia venezolana
seeds (Janzen, 1977).
Research on infestation of
Phaseolus lunatus
by
Acanthoscelides obtectus
larvae showed that
these were unable to penetrate the seeds and the reasons for that also pointed to not physical
aspects of the tissue, since
P. lunatus
seed coat is not as hard as others from different species
of
Phaseolus
(Thiéry, 1984). Thiéry
et al
. (1994) showed that the survival of
A. obtectus
depends on the ability of the first instar larvae to pierce the seed coat of
P. vulgaris
and only
57 % of the larvae successfully penetrated the seeds. The seed coat of fourteen
Vicia faba
genotypes acted like a barrier for the two bruchid species,
Callosobruchus chinensis
(L.) and
Callosobruchus maculatus
(F.). Only 45-58% of the neonate larvae were able to perforate the
seed coat and thus reached the cotyledons; the other larvae began to perforate the seed coat
and died while trying to penetrate it (Boughdad et al. 1986; Desroches et al., 1995).
Previous studies have shown the presence of vicilin-like 7S storage globulin, normally
thought to be expressed only in the embryo, in the seed coat of some legume seeds. Oliveira
et al. (1999) isolated, from
Canavalia ensiformis
seed coat, a protein that was seen to be
homologous to canavalin (vicilin-like protein from
Canavalia
seed cotyledons) by N-terminal
amino acid sequence determination. This protein interfered with the normal development of
the cowpea weevil
C. maculatus,
reducing the mass and number of surviving larvae. The
presence of phaseolin (a vicilin-like protein) peptides in the seed coat of the legume
Phaseolus lunatus
L. (lima bean) was demonstrated by N-terminal amino acid sequencing.
Utilizing an artificial seed system assay it was showed that phaseolin was detrimental to
C.
maculatus
with a LD
50
of 3.5%. The level of phaseolin in the seed coat (16.7% of the total
protein) was found to be sufficient to deter this bruchid larval development (Moraes et al.
2000). Silva et al. (2004) have confirmed that the seeds of the common bean (
Phaseolus
vulgaris
L.) do not support development of the same bruchid. Analysis of the
P. vulgaris
seed
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