Patancheru 502 324, Andhra Pradesh, Indiawww.icrisat.org

Science with a human faceDec 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

1International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru 502 324, Andhra Pradesh India.2Natural Resources Institute, Chatham, Maritime, Kent, and Jordell Laboratory, Kew Botanical Gardens, London, UK.

IntroductionHelicoverpa 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 25.4ICPW 159 C. sericeus 112.1 58 22.7ICPW 125 C. scarabaeoides 46.4 62 33.7ICPW 152 C. scarabaeoides 112.1 48 36.1ICPL 332 - R C. cajan 294.5 46 24.3ICPL 87 - S C. cajan 325.2 36 21.7SE ± 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 37917.1 Quercetin 8318.1 Quercetin-3-methylether 2221 2284 1092 774 89320.02 (221, 254, 356) √23.93 (209, 256, 297) √ √ √ √26.6 “D” √ √ √ √ √27.4 Stilbene-2-carboxylic acid 351 1852 923 745 7381 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 HelicoverpaA 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 HelicoverpaPods 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 = CH3Isoquercetrin R = GlcQuercetin R = H

A B

Plate 2. Glandular trichomes in Cajanus cajan (A) and nonglandular trichomes in Cajanus scarabaeoides (B).

ConclusionsWild 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.

ReferencesGreen 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