Beneficial and harmful insects


ECONOMIC IMPORTANCE OF INSECTS

The estimated annual value of the ecological services provided by insects in the United States alone is at least $57 billion, an amount that justifies greater investment in the conservation of these services.  Without the activities of insects, human life on earth would eventually be extinguished. Over one lakh currently living species of insects have been identified, but the true number is surely much larger, about a million.

BENEFICIAL INSECTS

Pollinators of crops (Bees, wasps, butterflies, moths, hoverflies, beetles)

Many plants depend on insects to transfer pollen as they forage. Plants attract insects in various ways, by offering pollen or nectar meals and by guiding them to the flower using scent and visual cues. This has resulted in strong relationships between plants and insects. Value of crop production from pollination by native insects is estimated to be about $3 billion in US alone.

When we talk about pollinators the ones that come to mind are honey bees and butterflies, but there are also many other insects that perform this job for flowering plants, as well. There are flies, wasps, beetles and even some other insects that most people know nothing about, such as Hemiptera and thrips. There are many important pollinating insect species in the orders: Hymenoptera (bees, wasps, and ants), Lepidoptera (butterflies and moths), Diptera (flies) and Coleoptera (beetles).

As adults these insects feed on pollen and nectar from flowers. They forage from plant to plant and may initiate pollination by transferring pollen from an anther to a stigma. Female bees and pollen wasps provision their nests with pollen and nectar that they actively collect onto their bodies. Their larvae then feed on the collected pollen and nectar. Yucca moth larvae do not feed on pollen or nectar but on the seeds of yucca plants. The adults pollinate the yucca plant by actively collecting pollen onto their palps and then placing the collected pollen on a receptive stigma to ensure proper seed set for their offspring.

Economic value of insect pollination worldwide is estimated at U.S. $217 billion

(Science Daily, Sept. 15, 2008). German scientist found that the worldwide economic value of the pollination service provided by insect pollinators, bees mainly was dollar153 billion in 2005 for the main crops that feed the world. This figure amounted to 9.5% of the total value of the world agricultural food production. The study also determined that pollinator disappearance would translate into a consumer loss of food estimated between dollar 190 to 310 billion.

Predators of pests (Dragonflies, beetles, bugs, lacewings, wasps)

The arthropod predators of insects and mites include beetles, true bugs, lacewings, flies, midges, spiders, wasps, and predatory mites. Insect predators can be found throughout plants, including the parts below ground, as well as in nearby shrubs and trees. Some predators are specialized in their choice of prey, others are generalists. Some are extremely useful natural enemies of insect pests. Unfortunately, some prey on other beneficial insects as well as pests.

Major characteristics of arthropod predators:

  • Adults and immature stages are often generalists rather than specialists.
  • They generally are larger than their prey.
  • They kill or consume many preys.
  • Males, females, immature stages and adults may be predatory.
  • They attack immature and adult prey.

Important insect predators include lady beetles, ground beetles, rove beetles, flower bugs and other predatory true bugs, lacewings and hover flies. Spiders and some families of mites are also predators of insects and mite pests. Natural enemies play an important role in limiting potential pest populations. We have seen what happens when pesticides devastate the natural enemies of potential pests. Surveys of agricultural systems give an indication of the potential number and diversity of predators in a crop.

For example, over 600 species of predators in 45 families of insects and 23 families of spiders and mites have been recorded in cotton. Eighteen species of predatory insects (not including spiders and mites) have been found in potatoes in the northeastern United States.

Parasites of pests (Hymenoptera and Diptera)

Parasitoids are insects with an immature stage that develops on or in an insect host, and ultimately kills the host. Adults are typically free-living, and may be predators. They may also feed on other resources, such as honeydew, plant nectar or pollen. Because parasitoids must be adapted to the life cycle, physiology and defenses of their hosts, many are limited to one or a few closely related host species. Crop losses averted by beneficial insects from predators or parasites of agricultural pests are estimated to be $4.5 billion. The most valuable insect parasites belong to the following groups:

  • Tachinid Flies (Diptera)
  • Ichneumonid Wasps (Hymenoptera)
  • Braconid Wasps (Hymenoptera)
  • Chalcid Wasps (Hymenoptera)

These parasites live in or on one host insect pest which is killed after the parasite completes its development. Parasite (also called parasitoid) adults are free-living; the immature stage lives on or inside a host and kills the host before the host completes its development. Parasites lay one or more eggs on the outside of the host body or they insert the eggs inside their host. The immature parasite feeds on the host and requires only a single individual prey to complete its development.

Free-living adults may feed on nectar from flowering plants or obtain nutrients by piercing the body of host insects and withdrawing fluids (host-feeding). Parasites are often considered more effective natural enemies than predators because many have a narrower host range, require only one host to complete development, have an excellent ability to locate and kill their host and can respond rapidly to increases in host populations.

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