Bio-ecological studies of Amaranth’s Lepidopteran Defoliators and Development of IPM Technologies for their Management

Abstract

One of the most important and largely consumed African Indigenous Vegetables (AIV) is amaranth. It is known to have a high nutritional value, agronomical assets and economic attributes. Amaranth production is however constrained by numerous biotic factors such as insect pests, occurring as a complex of species among which lepidopteran defoliators are found to be the most destructive. Investigations on their bio-ecological and control options were conducted. Field experiments were conducted from September 2015 to June 2017 in two agro-ecology zones (mid and high altitudes) in Kenya using two species of amaranth (Amaranthus dubius L. and Amaranthus cruentus L.). Results indicated that three lepidopteran species namely Spoladea recurvalis Fabricius (Lepidoptera: Crambidae), Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) and Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) were the most important. In mid-altitude, S. recurvalis was the most significant defoliator followed by the two other Spodoptera species while in high altitude S. recurvalis was of minor importance. Population dynamics studies revealed that S. recurvalis was more abundant from December to February where the mean infestation rate reached 24 larvae per plant and the incidence of 100 % during the outbreak. Spodoptera exigua and S. littoralis were present throughout the year with variable population levels. Leaf damages ranged from 0.85 to 62.45 % according to amaranth species, the site and the season. Tested attractants were neither able to significantly reduce the damage level nor induce significant catch in traps. A total of four indigenous parasitoids were found associated to S. recurvalis including Apanteles hemara (Hymenoptera: Braconidae), Atropha tricolor (Hymenoptera: Ichneumonidae), Phanerotoma sp. (Hymenoptera: Braconidae), and Schoenlandella testacea (Kriechbaumer) (Hymenoptera: Braconidae) while four others that were found associated with the Spodoptera species were Chelonus curvimaculatus Cameron (Hymenoptera: Braconidae), Coccygidium luteum Brulle (Hymenoptera: Braconidae), Charops ater Szepligeti (Hymenoptera: Ichneumonidae) and Cotesia icipe Fernandez-Triana & Fiaboe (Hymenoptera: Braconidae) which was a newly discovered species during the present work, with variable level of field parasitism rates. The results of laboratory experiments on the performance of the newly discovered parasitoid C. icipe showed a high potential for the use of this species. On S. littoralis a single female of C. icipe parasitized 42.99 ± 2.66 % of the 50 exposed larvae in 24 hours whereas a cohort of five females parasitized 85.59 ± 1.46 % of same host density. On S. exigua parasitism rates were 9.72 ± 0.76 % and 59.53 ± 3.1 % for a single and cohort of 5 females released respectively in the same conditions. Life history studies of A. hemara on S. recurvalis revealed that the parasitoid was a good candidate in the management of S. recurvalis. The temperatures range for its development was from 15 and 30°C. However, the total developmental time was affected by temperature. Female A. hemara developmental times were 50.5 ± 0.29, 21.78 ± 0.17, 12.88 ± 0.14 and 9.13 ± 0.06 days at 15, 20, 25 and 30 °C respectively. No pre-oviposition period was observed. Aiming at a successful biological control program for S. recurvalis in the sub-region, the genetic taxonomy studies were conducted using the cytochrome Kenya and Tanzania as well as data available in the Genbank from Pakistan, Florida, India, Japan, Greece, Canada, China and Costa Rica, belong to the same species. This represents an important result that may have implications for a future biological program in the way that unlike the current finding, cryptic species usually lead to the failure of the management options.