Eggplant Integrated Pest Management AN ECOLOGICAL GUIDE

_____________________________________________________________________Ecology of insect pests and natural enemies4.4 How can an insect damage a plant?A plant needs its leaves to absorb sunlight to make sugars for energy and growth(this process is called photosynthesis). The sugars are transported through theveins of the plant to other parts like roots and stems.When an insect feeds on the leaves and reduces the leaf area, like somecaterpillars do, less sugar is produced and the plant has less energy for growthand development.When insects are sucking on the leaves of the plant, like aphids do, they aresucking the sugars out of the plant cells or the veins. This leaves less sugaravailable for the plant for its growth and development. In additions, some insectsexcrete sugary wastes (honeydew) on which fungi can grow. Leaves becomeblack with these fungi and as a result, photosynthesis is reduced.Other insects like leafminers feed inside the leaf and destroy part of the veins, resulting in less sugartransport. Less sugar available for plants means less plant growth and reduced plant health, and thatmay eventually lead to an overall lower yield.It is important to note that not all insect feeding reduces yield! See section on compensation below.A special case are insects that can transmit virus or mycoplasma diseases. These insects are usuallysucking insects like aphids, jassids, thrips and whitefly. The jassid Cestium phycitis for example, is animportant vector of little leaf disease caused by a mycoplasma (see section 8.2.3). A mycoplasmainfected plant has mycoplasma parts in most cells and sometimes inside veins. When an insect feeds onan infected plant, it will also suck with the plant juice some mycoplasma particles. These particles eitherstick to the mouth parts of the insect or they are swallowed into the stomach of the insect. When theinsect starts feeding on a fresh plant, the mycoplasma particles are transmitted from the mouth parts orstomach into the new plant. This plant then becomes infected too.4.5 Plant compensationNot all insect feeding reduces yield. Eggplant is able to compensate for feeding because more leavesand roots are produced than are actually needed for fruit formation, and new shoots are regularly formed.Even when the plant is almost completely cut back to about 15 cm above soil level, the plant will producenew shoots and form a new plant (ratooning – see section 3.12). This regrowth is often very strongbecause the roots are already there and can provide the nutrition and water from the soil to feed the newshoots.Low levels of insect feeding and minor disease infections on leaves, stems and roots do not significantlyreduce yields. Loss of leaves can be caused by feeding damage of Epilachna beetles, but also by aperson removing diseased leaves in order to reduce the source of disease spread. It depends on theextent of leaf removal and the growth stage if the production is reduced or delayed. See box below.Summarized field data on plant compensation studies from 4 Training of Trainers held in Bangladesh,eggplant could compensate for 50% loss of leaves up to 85 DAT without affecting yield (pers. comm. Md.Atiur Rahman, May 2001).51Eggplant Ecological Guide