Repellent Effects of Onion, Garlic and Marigold Intercropped with Potato (Solanum tuberosum L.) Against Potato Tuber Moth (Phthorimaea aperculella Zeller)

Abstract

The potato tuber moth, Phthorimaea operculella (Zeller)(Lepidoptera; Gelechiidae) is a cosmopolitan, oligophagous insect pest of solanaceous crops. It attacks potatoes, tomatoes, tobacco, eggplant, pepper and wild solanaceous plants. A study was conducted to quantify the effects of intercropping of potato (Solanum tuberosum L.) with insect-repellent crops on PTM larval infestation and foliage-feeding insects, tuber infestation intensity, infestation preferences on the canopy and larval parasitism. The study was conducted at Solusi Farm (18.5830S 32.7570E, 1200 m altitude), Solusi University, Zimbabwe. The potato variety Montclare was used in three consecutive field trials. Potatoes were intercropped with onion, garlic and marigold. Sole potato was included among the treatments, with a Nuvacron 40 WSC insecticide (monocrotophos) treatment as the control. Each treatment was replicated three times in a randomized complete block design. Every seven days from plant emergence to pre-harvest defoliation, three plants were randomly selected from each treatment plot. The stems and leaves were inspected for presence of larvae and their developmental stages, aphids and leafhoppers. For PTM larval canopy infestation preferences, the plants were divided into three categories; the leafy canopy, the upper stem and the bottom stem. The PTM larvae and their developmental stages were determined. The degree of tuber infestation by larvae was determined at harvesting. Samples of ten plants were selected from each plot and classified into two categories, the infested tubers and the green tubers. The parasitism of the PTM was also assessed. The larvae in leaves and stems were each kept in plastic containers with cloth lids until adult moths or parasitoids emerged. Mean populations of adult parasitoids were square root transformed (p = √(x +1). Percentage parasitism was calculated. The numbers of larvae per plot were transformed and means subjected to ANOVA using the GenStat software, Release 14.1.There were no significant differences (P¿0.01) in the PTM larval density on foliage between the garlic and marigold intercrops. A significant difference (P¿0.01) was noted between the Nuvacron-treated potatoes and the sole potato. There were no significant differences (P¿0.01) in the density of aphids among the garlic, marigold intercrops and the Nuvacron-treated potatoes. A significant difference (P¿0.01) was noted between onion intercrop and the marigold intercrop in the leafhopper density. There was also no significant difference (P¿0.01) in the large larval density within the leafy canopy, upper stem and bottom stem regions of the potato plant in all the treatments. One mechanism that may account for low PTM larval density and foliage-feeding pests in onion, garlic and marigold intercrops, could be mortality in the eggs, thus preventing larval eclosion. Garlic compounds may toughen the structure of the egg, preventing hatching in a way similar to that in which dehydration can act, with embryos apparently developing normally but hatching inhibited. The green tubers in all plots were more vulnerable to PTM infestation as the larvae could access these easily. The sandy soils in Solusi farm provide easy entry of the larvae into the soil to damage the tubers. During three season trials, in the Nuvacron-treatedpotatoes, it was found that 22.33% to 44.80% of the PTM larvae were parasitized by Apanteles subandinus and Copidosoma koehleri, which indicated that the parasitoids could tolerate relatively severe pesticide spray regimes. Garlic and marigold intercrops are effective insectrepellent crops that suppress the PTM larval density, foliage-feeding insects and reduce tuber infestation intensity and can be used to replace Nuvacron insecticide. The repellent properties of onion, garlic and marigold have no direct influence on the plant infestation preferences of PTM larvae. A. subandinus and C. koehleri PTM parasitoids are promoted by intercropping.