Interactions between the germplasm of okra (Abelmoschus spp.) and Aphids with special reference to Aphis gossypii Glover (Hemiptera: Aphididae) in Cameroon

Aphis gossypii Glover (Hemiptera: Aphididae) is one of the major pests of okra (Abelmoschus spp). On one hand, direct damages due to its feeding habit results in curling and deformation of young leaves. On the other hand, indirect damages are caused because of honeydew secreted on fruits and leaves with, which in turn may promot growth of black sooty mould. The black sooty mouls stain and reduce fruit and leaf quality and reduce photosynthetic activity. In addition, honeydew attracts ants that fend off natural enemies of Hemipterans. The severity of aphid infestation has led to widespread use of chemical pesticides for its control with adverse effect that it also eliminates the natural enemies. Pests including aphids such as A. gossypii are becoming resistant to pesticides. Most vegetable farmers in Cameroon accept that they use chemical pesticides, and are equally willing to accept new varieties that are resistant to pests and diseases, to minimize the use of pesticides. The objective of this work was to identify aphid-resistant okra germplasm for a better management of A. gossypii. Screening trials were conducted under natural field conditions without pesticide application. Aphid infestations per variety were directly scored on one leaf per stratum on three strata of five plants randomly selected. The number of aphids was recorded using the following scale: 0 = no aphids present; 1 = 1 to 10 aphids per leaf; 2 = 11 to 100 aphids per leaf; 3 = 101 to 500 aphids per leaf; and 4 = >500 aphids per leaf. Phenotypic structures and secondary metabolites that could affect the life traits of Aphis gossypii were analysed. In the case of phenotypic structures, trichome density, hardness and chlorophyll content of okra leaves were taken into consideration. Concerning secondary metabolites, leaf contents in total phenols, total tannins, free amino acides, total sugars, reducing sugars, total nitrogen and potassium were considered. The implications of mechanisms of tolerance, antibiosis and antixenosis were evaluated in the analysis of resistance of plants of Abelmoschus spp. Nine okra accessions were therefore identified as resistant or moderately resistant to A. gossypii. The most resistant ones were VI041210, VI057245 and Gombo caféier. The farmers’ check Kirikou and VI060694 were the most tolerant. Resistant accessions produced fewer pods than susceptible and tolerant accessions. In this study, non-preference (antixenosis) was not a category of resistance. The non-discrimination between susceptible and resistant accessions in aphid settling behaviour indicates that phenotypic structures and plant metabolites did not influence attraction and settling behaviour. The trichome density was highest on the leaves of the top stratum, higher at the middle stratum and lower at the bottom; it was lower on VI060794 and the farmers’ check, Kirikou, at all plant strata, and may favour infestation of these susceptible accessions. The current study revealed the role of total nitrogen content in leaves leading to the susceptibility of okra accessions to aphids. VI060794 that was the most susceptible in Taiwan in 2013 and in the second season of the confirmatory screening trial in Cameroon in 2014 had significantly higher leaf Nitrogen content than in other accessions. Constitutively, the role of free amino acids, tannins and total phenols in imparting resistance against A. gossypii in the identified okra accessions during our study is inconclusive. Biochemical studies of accessions of okra at 6 and 10 weeks after sowing showed that total phenols and tannins content changed following aphid infestation. Total tannins increased in the resistant accessions and reduced in Kirikou, the susceptible farmers’ check at all plant growth stages. The total sugars, potassium and reducing sugars played a role in offering resistance in plants with or without aphids. As a susceptible accession, VI060794 had higher nitrogen content significantly at vegetative stage following aphid infestation and at reproductive growth of plant even when plants were not infested. The farmers’ check Kirikou that was one of the most susceptible to aphids had the highest intrinsic rate of natural increase of aphid population, which was significantly different from that of VI057245, one of the most resistant accessions during confirmatory and multilocation trials. When plants were previously infested with aphids at vegetative and reproductive stages, the developmental time was significantly longer on VI041210 than on all accessions except VI060688 at vegetative growth. No mortality of aphids was observed on VI033805, VI033824 and on the farmer’s check Kirikou. Results from the multilocation trials indicated that the farmers’ varieties were more susceptible to aphids than most of the selected resistant accessions, across all agro-ecological zones. VI057245 and VI036213 are suitable for resistance to aphids in the western highland; VI060818, VI060794 and VI039614 in the monomodal humid rain forest; VI060794, VI057245, VI051114 and Gombo caféier for the bimodal humid rain forest, VI060818 and VI041210 in the Sudano-Sahelian region. VI060794 was also the most yielding in all ecozones in Cameroon and with some acceptable level of resistance. We recommend the following accessions for the presence of resistant traits: VI041210, VI051114, VI033824, VI057245 and VI036213 for leaf trichomes; VI051114 and VI036213 for fruit size; VI041210, VI060794 and Gombo cafiere for plant vigour. VI041210, VI057245 and Gombo cafiére for higher secondary metabolites and lower plant nutrients contents leading to antibiosis. VI060794 presents superior qualities in terms of yields and management of aphids. It will also be interesting to study the genotypes of the selected accessions to identify genes associated with resistance to A. gossypii.