Diversity and performance of egg parasitoids occurring on fall armyworm (spodoptera frugiperda j.E smith) on maize in Kenya.

Abstract

Fall armyworm, Spodoptera frugiperda (J.E Smith) (Lepidoptera: Noctuidae) is an insect of economic importance, indigenous to tropical and sub-tropical regions of America. This pest mainly attacks maize crop and it also has a wide host range. Characteristic damage symptoms on maize crop include ragged feeding, windowing and short holes on maize leaves and presence of moist saw dust-like frass found on top surface of maize leaves and whorl of the crop. Trichogramma and Telenomus species are egg parasitoids which are commonly used worldwide to keep lepidopteran pest under check. This study aimed at carrying out field survey across selected agro-ecological zones in Kenya, collect egg parasitoids of FAW and determine the exact native parasitoid species using morphological and molecular techniques and determine their effectiveness under laboratory condition. A field survey of egg parasitoids of FAW was conducted in eighty farms in five counties across different agro ecological zones namely, Kilifi, Kwale and Taita Taveta (coastal lowlands), Makueni (midland) and Kirinyaga (highland). About fifty maize plants (from young maize crop to before flowering) per field were randomly searched for FAW egg masses adopting a “W” pattern of sampling. Fall armyworm egg masses collected were put in glass vials and taken to the International Centre for Insect Physiology and Ecology (icipe). Egg masses were incubated under room temperature, 25 ± 1°C, 12:12 h (L: D) photoperiod and 60–70% relative humidity (RH) in the laboratory to observe for emergence of parasitoids or FAW larvae and percent parasitism determined. The egg parasitoids recovered from the field were identified using both morphological and molecular techniques to determine the native parasitoid species of FAW present in Kenya. Morphological characterization was done based on antennal type, while molecular characterization involved the use of LCO/HCO primer targeting mitochondrial cytochrome oxidase 1 xi (COI) barcode gene region. Colonies of FAW, factitious host, Corcyra cephalonica (Stainton) and the egg parasitoids were established in the laboratory. The egg parasitoid, Telenomus remus Nixon (Platygastridae: Hymenoptera) was raised using FAW eggs, while Trichogramma chilonis Ishii (Trichogrammatidae: Hymenoptera) was raised using factitious eggs, C. cephalonica and FAW eggs. Subsequently, performance of egg parasitoid species on FAW eggs was assessed under controlled conditions in the laboratory. Fall armyworm egg densities (20, 40 and 60) were used as treatments and exposed to an individual mated female parasitoid (T. remus or T. chilonis) for 24 hours. Data on percent parasitism, FAW emergence, egg mortality and sex ratio were recorded. Sex ratio was determined after emergence; whereby adult parasitoids were killed by freezing them at 0°C for 5 minutes. They were sexed by observing individual adults using a dissecting microscope (Leica Laz EZ), at magnification of 10x, and this was based on the antennal characteristics. Field data collected was analyzed using GLM with binomial distribution while data collected from the assessment of egg parasitoids under laboratory conditions was analyzed using one-way Analysis of Variance (ANOVA) and the mean separation was done using Tukey multiple comparison test. The two native parasitoids species to Kenya, namely: T. remus and T. chilonis were key egg parasitoid species recovered from the FAW eggs. Diversity of egg parasitoids and percent parasitism was higher in midland and highland than lowland regions. Field percent parasitism significantly varied with altitudes for both parasitoid species where it ranged from 4.4 to 13.2% for T. chilonis and 0 to 21.4% for T. remus, with the latter being predominant. Percent parasitism, FAW emergence, egg mortality and sex ratio varied significantly per female parasitoid species and with FAW egg densities offered. More female parasitoids emerged from the FAW eggs which is beneficial in relation to biological control. However, the male parasitoids were few but could still mate with many females. Under controlled conditions, 57 to 96% and 53 to 90% of FAW eggs were either killed or parasitized by T. remus and T. chilonis respectively. Egg parasitoid species kill the host eggs by attacking it more than once (super parasitism). Sex ratio (% female progeny) varied significantly among the parasitoid species with T. remus recording the highest sex ratio (83.3%). The parasitoids T. remus and T. chilonis could be adopted as biocontrol agents in the management of FAW through conservation or augmentative release.