Which insect is often found on the underside of leaves and can be controlled with insecticidal soap?

Whiteflies are small, white, winged insects that often congregate on the underside of leaves, making them a common pest in many gardens and crops. These insects feed on the plant's sap, which can weaken the plants and lead to yellowing leaves and overall decline in health.

Insecticidal soap is particularly effective against whiteflies because it suffocates them upon contact and disrupts their cell membranes. The soap's action is non-toxic to plants when applied correctly, targeting the soft-bodied insects like whiteflies without harming beneficial insects that may be present.

The other pests listed do not have the same characteristics or vulnerabilities to insecticidal soap. Thrips are more slender and can cause damage in different ways; leafhoppers tend to jump away easily and may require different control methods; scale insects have a hard protective covering that makes them less susceptible to soaps. Therefore, whiteflies are the specific insect that can be effectively managed using insecticidal soap due to their biology and the nature of the product.