Effect of plant growth stimulants on the response of Phthorimaea absoluta (Lepidoptera: Gelechiidae) larvae and moths to tomato plants

Abstract

Phthorimaea absoluta (Lepidoptera: Gelechiidae), the tomato leaf miner, is the most serious pest of tomato in the world and a major threat to cultivation of this crop. Moths of this pest lay their eggs mostly on plants leaves and stems, and the endophytic larvae feed on the parenchyma of leaves. Insecticide applications are the most common strategy used for control of P. absoluta larvae. However, pesticides have detrimental effects on biological control agents and disrupt the use of IPM programs. Phthorimaea absoluta populations have also evolved resistance to chemical insecticides wherever tomatoes are cultivated. Other management strategies for P. absoluta include mating disruption, changes in agronomic practices and the general management of plant health. Plant growth stimulants are commonly used in tomato production to benefit plant health and are reported to result in increased availability of nutrients and increases in various plant growth parameters. One of the benefits of plant growth stimulants is increased plant °Brix, which is considered a measure of plant health and the indicator of total soluble solids in plants. Various plant growth- and biostimulants are used in tomato production in South Africa. These products include Seabrix (RealIPM, Thika, Kenya) and RealTrichoderma (RealIPM) formulated from Trichoderma asperellum and registered for the control of pathogenic fungi. Another such product is NewSil (RealIPM), a product that is applied as foliar fertilizer treatment. The adverse effects of these products on pests are often ascribed to increased plant °Brix. The potential of using watering regimes and plant growth stimulants to manipulate °Brix of crops as a pest management strategy have been investigated in various crop species such as potato and other vegetables, raspberry and grapes. The aim of this study was to determine whether plant °Brix could be manipulated by application of plant growth stimulants, and different soil moisture conditions, and if such treatments result in differences in °Brix which may influence the oviposition and larval preferences of P. absoluta. Experiments were conducted with potted tomato plants maintained in greenhouses and P. absoluta moths and larvae, sourced from an insect rearing colony I maintained throughout the course of this study. Results of the three experiments during which °Brix readings were taken showed that neither soil water conditions nor growth stimulant treatments had pronounced effects on leaf °Brix. °Brix varied significantly over weeks and growth stimulants had only minor effects on °Brix. The differences in °Brix resulting from growth stimulant treatments in this study was, however, masked by the high variation recorded in °Brix at different sampling times. No significant correlation existed between the numbers of eggs laid on plants in no-choice (r = -0.41; p = 0.086) and two-choice (r = 0.039; p = 0.711) tests and larvae only showed preference for leaves of plants in two of the 15 combinations that they were provided with. Results from the study illustrate the difficulty to manipulate °Brix by means of growth stimulants, and showed no relationship between leaf °Brix of tomato plants and the preference of P. absoluta moths, or larval response to plants that were treated with stimulants for prolonged periods. The leaf °Brix of tomato plants in this study was much higher than those reported for tomato in the literature. A lack of knowledge of the relationship between leaf and fruit °Brix was identified during this study. While moths of this pest lays their eggs on plant leaves and larvae feed predominantly on leaf parenchyma tissue,°Brix of tomato plants that were reported to influence the life history parameters of P. absoluta were based on fruit °Brix. The results therefore, provide no support for either a positive or negative relationship between leaf °Brix of tomato plants and the preference of P. absoluta larvae or moths for plants with higher or lower °Brix. °Brix levels alone cannot be used to indicate host plant suitability for this pest.