The Bio-ecology and Genetic Structure of Tuta absoluta (meyrick) in Kenya

Abstract

Tomato (Solanum lycopersicum L.) is an important vegetable in Kenya in terms of its economic value and production. However, successful production is hampered by insect pests, and invasion by tomato leafminer, Tuta absoluta (Meyrick) has proved to be a serious menace. Since its first detection in 2014, Kenyan farmers have mainly relied on the routine application of synthetic pesticides to reduce T. absoluta’s infestation and damage to tomatoes. Chemical control has multiple adverse effects; hence the need for pest management alternatives. Being an alien species in Kenya, there is limited knowledge of key bio-ecological aspects of T. absoluta that could assist in designing effective, sustainable, and species-specific pest control packages. The aim of this study, therefore, was to determine the bio-ecology and genetic structure of T. absoluta populations in Kenya through field surveys and laboratory experiments. DNA barcoding confirmed the presence of T. absoluta and also revealed that field populations were mixed with four Gelechiidae pests in the commonly practiced poly-cultural cropping systems in Kenya. Tuta absoluta was more closely related to Phthorimaea operculella (Zeller) than other studied Gelechiidae pests based on nucleotide composition, evolutionary divergences, and phylogenetic results. Analysis of 11 microsatellite loci of T. absoluta showed that the genetic structure of the invading populations is homogenous. Six Kenyan populations and additional 11 populations from 7 African countries displayed a weak genetic clustering of 2 hypothetical clusters (ΔK = 2) with an admixture of alleles. Also, a high degree of gene flow (Nm = 5) was detected among the populations and there was no clear genotypic differentiation based on their geographical origin. A countrywide field survey showed that T. absoluta is widely spread in Kenya with no altitudinal limits. High levels of pest abundance were recorded in different localities and the highest capture/trap/day was 115.38 ± 15.90 adults. The highest infestation on tomato leaves was 92.22 ± 3.38%, whereas the highest damage level on fruits was 59.60 ± 12.13%. Two predators and nine parasitoid species were identified as the indigenous natural enemies that have adapted to T. absoluta, although the parasitism rates were low (7.26 ± 0.65%). The predatory activity of Nesidiocoris tenuis Reuter on T. absoluta eggs indicated that this predator has the potential to effectively regulate the field populations of the pest. Both adults and nymphs of N. tenuis preyed on a significant number of eggs and exhibited a type II functional response. The highest count of T. absoluta eggs consumed per day by an adult predator was 86.60 ± 2.35, while the fifth instar nymphs recorded the highest predation activity of 47.00 ± 1.96 eggs. Generally, adult females (86.53 ± 0.86%) were better consumers than males (77.46 ± 1.08%). Adults also preyed on T. absoluta larvae but preferred the first instars (3.20 ± 0.37). Tuta absoluta eggs were proven to be a suitable host for the growth of N. tenuis nymphs, which had a survival rate of 73.00 ± 0.08%. Seventeen plant species were recorded as host plants of T. absoluta, out of which three were new records. In addition, T. absoluta preferentially selected tomato for feeding and oviposition. However, it was clear that the pest has the potential to attack and reproduce successfully on alternative Solanaceae hosts including giant nightshade, eggplant, and potato. Biological parameters of T. absoluta also revealed that both nightshade and eggplant have good nutritional quality comparable to tomato. The findings of this study showed that T. absoluta is an important agricultural pest in Kenya and also provide knowledge that could be useful in designing effective, sustainable, and environmentally friendly integrated pest management (IPM) strategies