Interaction of crops with insects

I. Insects Eat Plants

• Many insect species are herbivores or plant eaters.
• Caterpillars are immature moths or butterflies that often feed on plant material.
• Insects can eat leaves, sap, seeds, and roots.
• Many insects, such as caterpillars and Japanese beetles, eat entire leaves.
• Aphids, cicadas, and other bugs can suck the juices or sap out of the plant, much like mosquitos suck the blood out of people.
• Various beetles, such as weevils, can bore into and lay their eggs inside of seeds, where the grub eats the seed from the inside.
• Other insects, such as Mole crickets, June bug grubs, or corn borer larvae, can tunnel underground and eat plant roots, which kills the plant because the plant cannot effectively absorb nutrients from the ground, or the loss of roots can even cause the plant to tip over.

II. Plants Defend Themselves against Harmful Insects in Many Ways

a) Mechanical defenses:

• Trichomes : fine hairs keep insects from being able to get to the plant’s stem, and in some cases, these hairs can even impale or stab the insect. Some trichomes are equipped with sticky sap on the tip of hair which eventually kills the insects.

b) Chemical Defenses:

• Many plants produce chemical compounds that are poisonous to insects, so insects avoid eating those plants Example: The milkweed plant is toxic to many types of insects because of the cardenolides or cardiac glycosides produced in the plant. This chemical is quite unpalatable to most insects and mammals.
• Plants can also produce special odors that attract predatory insects. Example: When damaged by moth caterpillars, corn plants produce odors that attract tiny parasitoid wasps. The parasitoid wasp goes to the damaged corn plant, finds a caterpillar, and lays her eggs inside the caterpillar. After hatching, the wasp larva becomes a parasite and eats the caterpillar from the inside out.

III. Insects Have Developed Ways to Overcome Plant Defenses – Co-evolution

a) Sequestration: Insect can store and use chemicals from plants it eats.

Example: Caterpillars of swallowtail butterflies feed on toxic mulberry plants. The caterpillars can then use these chemicals to defend themselves from predators. When frightened, a swallowtail caterpillar will eject a red, forked organ or osmeterium from its head, which releases a nasty odor. The caterpillar is able to produce this odor only when it feeds on mulberry plants because the plant produces a chemical that the caterpillar needs.

b) Filtration/ Resistance: Insect can eat toxic plants by filtering out and excreting the toxins.

Example: The Cabbage White Butterfly can break down toxic plant chemicals, and therefore, feed on plants that would kill most other insects.

Management strategies for insect pests

1. Identify the pest
2. Obtain the information about the life cycle, habitats, plant host range, natural controlling
3. Decide the need for control measures
4. Select control measures on the basis of effectiveness, fumigant properties, toxicity, formulation