Contents - Faperta

162 Biotechnological Approaches for Pest Management and Ecological Sustainability
humile Poir. respond to pea weevil, Bruchus pisorum L., eggs by forming callus (Clement,
Hardie, and Elberson, 2002). Mustard plants with a hypersensitive response to insect
feeding produce a necrotized zone around the base of eggs of the cabbage worm, Artogeia
rapae L., leading to desiccation of the eggs (Shapiro and DeVay, 1987).
Biochemical Markers
Secondary plant substances and poor nutritional quality of the host plant are important
components of resistance to insects. Antibiosis to insects in general is due to secondary
plant substances, for example, gossypol and tannins in cotton (Lukefahr, Houghtaling,
and Graham, 1975; Sharma, 1982; Sharma and Agarwal, 1983a, 1983b), tridecanone and
tomatine in tomato (Farrar and Kennedy, 1988, 1990), maysin and DIMBOA in maize (Klun,
Tipton, and Brindley, 1967; Waiss et al., 1979), oxalic and malic acids in chickpea (Yoshida,
Cowgill, and Wightman, 1995), and isofl avonoids in pigeonpea (Sharma et al., 2001; Green
et al., 2002, 2003) (Table 6.2). The role of secondary plant substances as biochemical markers
in plant resistance to insects is discussed below.
Attractants
Chemical cues involved in long-range identifi cation of the host plant are called attractants,
although the fi nal acceptance or rejection of a host plant depends on contact with the host
plant for feeding and oviposition. Dipropyl disulfi de is involved in host fi nding by the
onion fl y, Delia antigua (Meigen) (Stadler, 1986). Carrot fl y, Psila rosae (F.), is guided to the
host plant by a combination of compounds with varying degrees of volatility (Stadler and
Buser, 1984). Glucosinates and isothiocyanates play an important role in host fi nding by
the insects feeding on cruciferous plants (Renwick and Radke, 1988). Plant odors emanating
from the sorghum panicles at anthesis are involved in host fi nding by the sorghum
midge, S. sorghicola (Sharma, Leuschner, and Vidyasagar, 1990). In olfactometer tests, some
cotton genotypes are less attractive to the larvae of spotted bollworm, E. vittella (Sharma
and Agarwal, 1981).
Repellents
Plant defense chemicals that prevent or reduce the contact between the insect herbivores
and their host plants are called repellents. Steam distillates of rice and soybean varieties
that are resistant to insects are repellent to brown planthopper, Nilaparvata lugens (Stal)
(Saxena and Okech, 1985) and cabbage looper, T. ni (Khan, Ciepla, and Norris, 1987). Thymol
and carvacresol are repellent to rice leaf folder, Cnaphalocrocis medinalis Gn.; α-pinine and
3-carene are repellent to pine beetle, Blastophagus piniperda (L.) (Oksanen, Pertlunen, and
Kangas, 1970), while tomatine and capsicin are repellent to Colorado potato beetle, L.
decemlineata (Schreiber, 1958). Essential oils are repellent to corn earworm, H. zea (Starks
et al., 1965), and tannic acid to alfalfa weevil, Hypera postica (Gyllen.) (Bennett, 1965). Genetic
engineering can be used to increase the production of repellents as a strategy for developing
crop plants with better resistance to insects through changes in the metabolic pathways.
Phagostimulants
The chemicals that stimulate feeding by insects are called phagostimulants. Several
chemicals on the pod surface of pigeonpea that are absent from the pods of wild relatives
infl uence host plant selection for feeding by the larvae of H. armigera (Sharma et al., 2001).