Insect Pest Diversity and Damage Assessment in Field Grown Okra (Abelmoschus esculentus (L.) Moench) in the Coastal Savannah Agro-ecological Zone of Ghana

Abstract

Aims: The specific objectives of this study were: to identify the diversity of insect species associated with ten okra cultivars, and to assess the abundance of the insect species and the extent of leaf damage during vegetative, flowering and fruiting stages of ten okra cultivars under field conditions.
 Study Design: The experimental treatments were deployed in a Randomized Complete Block Design (RCBD), replicated four times.
 Place and Duration of Study: The research was conducted at Nuclear Agriculture Research Center (NARC) farms and the laboratories of Radiation Entomology and Pest Management Center (REPMC) of Biotechnology and Nuclear Agriculture Research Institute (BNARI), between July 2017 and March 2018. The study area is located at Kwabenya, Accra on latitude 5º40' N, longitude 0º13' W with Ochrosol (Ferric Acrisol) soil type, derived from quartzite Schist.
 Methodology: Plant materials used for the study consisted of five local and five exotic okra cultivars. The local cultivars were Asutem (AS), Togo (TG), Labadi dwarf (LD), Kwab (K1) and Adom (AD). These were obtained from the market (Asamankese and Dome) and okra farmers’ fields. The exotic cultivars were Lucky 19F1 (LF1), F1 Kirene (F1K), F1 Sahari (F1S), Kirikou F1 (KF1) and Clemson Spineless (CS). These cultivars were obtained from a commercial seed shop, Technisem, Accra. Land preparation of the research site involved plowing and harrowing. The prepared land was lined and pegged into 40 plots using a Randomized Complete Block Design with four replications. Each replicate measured 35 m x 7 m and separated by 2 m from each other with 10 subplots within a block. Each subplot measured 3 m x 3 m and spaced from one another by 1 m. The total size of the experimental area was 646 m2. The okra seeds were manually sown to a depth of 2 cm directly at a spacing of 0.50 m x 0.60 m. Four seeds per hill were sown and later thinned to one seedling per hill after emergence. Field management practices such as weed control and watering were done as and when required. Data on insects were collected from five okra plants randomly selected from the middle rows. Okra leaves were carefully examined by observing both the abaxial and adaxial surfaces. Insects found on the surfaces of the leaves were identified, counted manually and recorded as either major or minor based on their incidence pattern. Data was taken daily because the ten cultivars have different vegetative, flowering and fruiting dates. Insects were counted between the hours of 6.00 am and 8.00 am when they are inactive and cannot fly. In order to determine the extent of leaf damage, the following described scoring scale was designed for this work. Leaf damage was determined by counting the total number of perforations created by the insects in all leaves found on the five randomly selected test plants. This was then divided by the total number of leaves on the five selected test plants to obtain the average number of perforations per leaf. Leaves were visually assessed and scored for severity of damage using a damage rating where; 1 very mild damage (1 to 15 perforations); 2 mild damage (16 to 30 perforations); 3 moderately severe damage (31 to 45 perforations); 4 very severe damage (46 to 60 perforations); 5 extremely severe damage (more than 60 perforations).
 Results: A total of thirteen insect pests belonging to six orders (Coleoptera, Homoptera, Lepidoptera, Hymenoptera, Orthoptera and Mantodea), and thirteen families Chrysomelidae, Coccinellidae, Pyrgomorphidae, Meloidae, Noctuidae, Nolidae, Cicadellidae, Aleyrodidae, Aphididae, Pseudococcidae, Mantidae, Formicidae and Acrididae) were found to be abundant in the field. Among these, the highest number of insect species belonged to Homoptera group viz., Green Peach Aphid (Myzus persicae) Okra leafhopper (Amrasca biguttula), Whitefly (Bemisia tabaci), and striped mealybug (Ferrisia virgata) followed by Coleoptera (Flea beetle (Podagrica sp.) and Ladybird beetle (Cheilomenes lunata). On the vegetative stage of the okra, Flea beetle had the highest number on Lucky 19F1 (36.00±9.66 insects/plant). During the flowering stage, plants of L-19F1 had the highest mean number of Flea beetles (32.25±10.30 insects/plant). On the fruiting stage, plants of LD had the highest mean abundance of flea beetles (47.50±13.53 per plant).
 Conclusion: A total of 1,439 insects were recorded at the fruiting stage which was significantly higher than the flowering (855) and vegetative stages (693). Mean Whitefly counts were relatively low at the vegetative, flowering and fruiting stages of the cultivars. However, Flea beetle (Podagrica sp.) and Green Peach aphids (Myzus persicae) mean numbers increased progressively throughout all the stages. In the present study, the severity of leaf damage was significantly higher at the fruiting stage compared with the flowering and vegetative stages. Plants of cultivars LD and AS were the most promising recording the least leaf damage (111.95) and (119.10) respectively.