Resistance mechanisms against the pseudostem weevil odoiporus longicollis olivier (coleoptera : curculionidae) in banana
Loading...
Files
Date
1999
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Department of Agricultural Entomology, College of Horticulture, Vellanikkara
Abstract
The present investigation on resistance mechanisms against pseudostem
weevil Odoiporus longicollis Olivier (Coleoptera:Curculionidae) in banana was
undertaken in the Department of Entomology, College of Horticulture, Vellanikkara
during 1997-99. In insect-plant interactions, it is always the survival of the fittest.
There is also a constant enhancement in the physiological preparedness both by the
insect and the host plant to gain the upper hand. It is felt that even though the insect
conqu6¥$ the resistance of the host plant in majority of cases, there may be a
threshold of insect attack, which would induce plant defenses to deter the attack. It
was also possible that some of these plant defenses may be triggered off by enzymes
produced in the plant system consequent to the attack. These incitory enzymes or
chemicals may be transferred to the offspring, which would make them resistant to
attack, by the weevil. Hence, major objectives of the study were to evaluate sucker
and in vitro regenerated progenies of infested Nendran (AA B) for resistance against
0. longicollis, to evolve methods for screening resistance under artificial conditions,
laboratory rearing of 0. longicollis and identification of morphological, biochemical
and anatomical bases of resistance using Nendran as susceptible and Njalipoovan as
the resistant source.
Preliminary field screening of Musa (AAB) Nendran for resistance
against pseudostem weevil was done based on the number of ovipunctures/feeding
holes on the pseudostem, surface area of infestation and the size of feeding holes.
All of these contributed to the intensity of the attack. The multiplication product of
these factors led to the development of a new 0-4 rating scale, for scoring the
damage by 0. longicollis in the field.
No-choice tests conducted on sucker and in vitro regenerated progenies
of such graded mother plants revealed a better performance by plants having lower
history of infestation. This suggested the existence of induced resistance involving
triggering off of enzymatic reactions. Besides these lower grades, the plantlets of G4
exhibited better tolerance than control by exerting greater potential to stand amidst
heavy winds till complete maturity of the bunch. The in vitro regenerated plants
performed much better than the sucker derived treatment plants. This offers
potential for mass multiplication of such in borne resistance for commercial release.
The results obtained so far opens up the possibility of induced resistance that may be
genetically inherited to the plantlets from its mother as an evolutionary character.
Such triggering mechanism operating in vivo offers greater potential to select
resistant clones.
The methodology for screening banana for resistance to 0. longicollis by
artificial infestation in field situations was standardized. Four adult weevils in the
sex ratio 1: 1 (2!f' and 2 r:J) was found to be optimum to create moderate levels of
infestation during the most favourable season (May-August).
Mass rearing of 0. longicollis in laboratory conditions using natural diet
was quite successful for maintaining a large-scale population throughout the year. A
continuous supply of uniformly aged test insects for exposing the experimental
plants at specific pest load was made possible by this.
Studies involving the duration of development of the weevil revealed
acute antibiosis in Njalipoovan manifested by reduced egg laying prolonged
duration of life cycle, reduced hatching of eggs (39.5%) and reduced adult
emergence from eggs (29.7%) in contrast to 94.9 per cent hatching of eggs and 9l.3
per cent adult emergence in Nendran.
A no-choice test was designed to study the appropriate age of Nendran
from which it becomes susceptible to infestation. Results indicated a distinct
preference for oviposition by the adults on pseudostem of banana from seventh
month onwards. This information helps to reduce the insecticidal load in the banana
ecosystem by advocating pesticides from seventh month only. The existence of
oviposition deterrency when more females were confined to Nendran pseudostem
pieces revealed the deposition of spacing pheromones by conspecific females after
egg laying. It was seen that the acetone and ether extracts of pseudostem pieces
where ten females had laid their eggs, when sprayed on fresh pseudostem, deterred
the oviposition by other females. Isolation and identification of such epidectic
compounds will help in their utilisation in the field to prevent egg laying on banana
pseudostem.
The morphological, anatomical and biochemical bases of resistance were
also identified. The morphological parameters associated with resistance were
smaller width and breadth of the sheath coupled with more number of sheaths in the
resistant variety. These increased the number of cuticular layers and cell walls the
grub had to traverse before entering the central pith. The pith formation is of prime
importance to the curculionid as it becomes the target site of feeding for third and
fourth in stars. There are certain cues emanating from the pseudostem subsequent to
maturity, which aids the adult weevils in host selection and oviposition.
The lesser moisture and higher crude fibre content of Njalipoovan
pseudostem exhibited greater influence on resistance by posing a mechanical barrier
to egg laying and feeding of neonate larva. The information on moisture content is
of practical utility in the management of the pest in endemic areas. It may be
possible to artificially create moisture stress in the plant attaining flower bud
initiation stage by regulating irrigation. The level of irrigation has to be adjusted so
as not to induce a yield reduction but at the same time to induce moisture stress at
least in the outer sheaths to prevent oviposition. The water suckers served as more
conducive hosts for the pest even before sufficient maturity and this highlighted the
need for removal of water suckers from endemic fields.
The anatomical sections of pseudostem sheath clearly revealed a thick
cuticle on epidermal cells with four to five layers of dense collenchymatous
hypodermis imposing a physical barrier against oviposition in the resistant cultivar.
Further, the ground tissues which constituted of compactly arranged
parenchymatous cells with several starch grains offered feeding deterrency. The
closely packed bigger sized lower epidermal cells along with additional
parenchymal layers below the air cavity operated as a mechanical barrier which
caused the failure of the neonate larva to penetrate inner tissues.
Higher content of phenolics generally showed a positive correlation with
resistance. However, the role of ortho dihydric phenolic substrates in resistance is
greatly dependent on the enzymatic activity of polyphenol oxidase to form toxic
metabolites like quinones and tannins. Hence, the oxidation factor of OD phenols
clearly indicated the dominance of the resistant variety, Njalipoovan. The
accelerated activity in Njalipoovan is endowed with greater potentiality to activate
polyphenol aggressively effecting more production of lignin, quinone and tannins, in
the event of egg laying or feeding by the pseudostem weevil.
Description
MSc
Keywords
Citation
171567