Wild relatives of pigeonpea, such as Cajanus scarabaeoides, C. sericeus, and C. acutifolius show high levels of resistance to the major pest Helicoverpa armigera. Resistance is conferred by nonpreference for oviposition, antibiosis, and tolerance. Antibiosis is expressed as reduced larval weights, prolonged development, and lower fecundity. Trichome structures and chemical compounds in the pods, such as flavonoids and stilbenes, differ between resistant and susceptible varieties and contribute to resistance mechanisms. Wild relatives with different resistance mechanisms can be used to breed cultivated pigeonpea varieties with improved resistance to H.

1. Patancheru 502 324, Andhra Pradesh, India
www.icrisat.org
Sciencewithahumanface
Dec 2009
Mechanisms of Resistance to Pod Borer, Helicoverpa
armigera in Wild Relatives of Pigeonpea
For more information, contact: HC Sharma, Principal Scientist (Entomology), email: h.sharma@cgiar.org
1
International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru 502 324, Andhra Pradesh India.
2
Natural Resources Institute, Chatham, Maritime, Kent, and Jordell Laboratory, Kew Botanical Gardens, London, UK.
Introduction
Helicoverpa armigera is the single most important pest of field crops such as cotton,
legumes, cereals, vegetables and fruit crops, and causes losses of over US$2 billion
annually, in addition to over US$500 million worth of insecticides used to contain this pest
(Plate 1). Control of H. armigera is heavily dependent on insecticides, which has led to
the development of resistance to several insecticides (Sharma 2005). Therefore, there is
an urgent need for alternative control methods, of which host plant resistance is the most
economic. Levels of resistance to H. armigera in cultivated pigeonpea are low to moderate,
hence the need to identify wild relatives with high levels of resistance.
Table 1. Survival and development of neonate larvae of Helicoverpa armigera on wild relatives of pigeonpea.
Accession Species Larval Wt. (mg) (15
Days)
Larval mortality (%)
(20 Days)
Larval period (Days)
ICPW 1 C. acutifolius 154.5 62 25.4
ICPW 159 C. sericeus 112.1 58 22.7
ICPW 125 C. scarabaeoides 46.4 62 33.7
ICPW 152 C. scarabaeoides 112.1 48 36.1
ICPL 332 - R C. cajan 294.5 46 24.3
ICPL 87 - S C. cajan 325.2 36 21.7
SE ± 6.0 5.63 1.63
Table 2. Major compounds1
(in ppm) present in the methanol extracts of the pod surfaces of different
pigeonpea genotypes.
Retention time
(minutes)
Compound (absorbance
maxima) ICPL 87 ICP 7203-1 ICPL 332 ICP 187-1 ICPL 84060
13.8 Isoquercetrin 873 253 412 206 379
17.1 Quercetin 83
18.1 Quercetin-3-methylether 2221 2284 1092 774 893
20.02 (221, 254, 356) √
23.93 (209, 256, 297) √ √ √ √
26.6 “D” √ √ √ √ √
27.4 Stilbene-2-carboxylic acid 351 1852 923 745 738
1
Determined by their peak area at 254 nm. √ = Compound present.
Fig. 2. Compounds identified in methanol extract of pod surface of pigeonpea.
Sources and mechanisms of resistance to Helicoverpa
A diverse array of accessions of wild relatives of pigeonpea belonging to 12 species have
been evaluated for resistance to H. armigera over several seasons. Accessions belonging
to Cajanus scarabaeoides, C. sericeus, C. acutifolius and Rhychosa aurea have shown high
levels of resistance to H. armigera. Oviposition nonpreference, antibiosis, and tolerance
are the major components of resistance to H. armigera. Under field conditions, H. armigera
females did not lay any eggs on the wild relatives C. platycarpus, C. scaraboides and C.
sericeus, while 12 eggs were laid on ICPL 332 (resistant check) and 69 eggs on ICPL 87
(susceptible check). The proportion of larvae (in relation to number of eggs) was much
lower on ICPL 332 than on ICPL 87, suggesting that antibiosis is an important component of
resistance to H. armigera. Antibiosis to H. armigera in resistant wild relatives of pigeonpea is
expressed in terms of reduced larval weights (Table 1), prolonged developmental period, and
reduced fecundity.
Plate 1. Adults and eggs of Helicoverpa armigera (A), and its damage in pigeonpea (B).
A BB
Physico-chemical factors associated with resistance to Helicoverpa
Pods of Cajanus cajan produce chemicals in the glandular trichomes, which act as
phagostimulants to the H. armigera larvae, while the pods of C. scarabaeoides have a
dense covering of short, non-landular trichomes, which act as a physical barrier to feeding
by the young larvae (Plate 2). Quercetrin, iso-quercetin, quercetin-3-methyl ether (feeding
stimulants) (Fig. 1), genistein (antifeedant, present only in C. scarabaeoides), and two other
flavonoids have been identified in pod surface extracts (Green et al. 2002). Guaiene and
beta-caryophyllene, which are present in cultivated pigeonpea but absent in
C. scarbaeoides, act as attractants to H. armigera females.
There are considerable differences in the relative amounts of different flavonoids between
H. armigera-resistant (ICPL 332, ICP 7203-1, ICP 187-1 and ICPL 84060) and the
susceptible cultivar, ICPL 87 (Table 2). The amounts of isoqercetrin, quercetin, and quercetin
3-methyl ether are greater in the susceptible cultivar ICPL 87 than in the resistant ones. The
reverse is true for stilbene-2-carboxylic acid.
Quercetin-3-methyl ether R = CH3
Isoquercetrin R = Glc
Quercetin R = H
A B
Plate 2. Glandular trichomes in Cajanus cajan (A) and nonglandular trichomes in Cajanus
scarabaeoides (B).
Conclusions
Wild relatives of pigeonpea such as Cajanus scarabaeoides, C. sericeus and C. acutifolius
are highly resistant to H. armigera. Ovisposition nonpreference, antibiosis, and tolerance are
the major components of resistance. Wild relatives with different mechanisms can be used to
diversify the genetic base of cultivated pigeonpea and increase the levels of resistance to
H. armigera.
References
Green PWC, Stevenson PC, Simmonds MSJ and Sharma HC. 2002. Can the larvae of
pod borer, Helicoverpa armigera (Lepidoptera: Noctuidae) select between wild and cultivated
pigeonpea, Cajanus sp. (Fabaceae). Bulletin of Entomological Research 92:45-51.
Sharma HC (ed.). 2005. Heliothis/Helicoverpa Management: Emerging Trends and
Strategies for Future Research. New Delhi, India: Oxford & IBH, and USA: Science
Publishers. 469 pp.
3-hydroxy-
4-prenyl-5-
methoxystilbene-
2-carboxylic acid
HC Sharma1
and Phil Stevenson2