From the point of view of biological control, three groups of arthropods are important, i.e., spiders, mites and insects.
1. Spiders:
The spiders universally live a predatory life and are constantly on the lookout for insects as their food. They catch them directly or with the help of various types of snares made out of webs. Ascyltus pterygodes (Koch) is known to have controlled the outbreak of coconut moths in Fiji.
The important species found actively feeding on sugarcane pyrilla in India are Clubiona atwali Singh and Clubiona drassodes Cambridge, belonging to the family Clubionidae. The populations of these spiders showed a positive correlation with the population of their prey.
The important genera of spiders are Atypus, Scytodes, Pholeus, Lycosa, Argiope and Marpissa. Since the spiders spin threads and are carried away by air over long distances, they are more or less universal in distribution and have acquired specialized habits. The climate inside the houses is controlled for human comfort and the same spider genera are found internationally and they are adapted to feed on household pests.
For example, the spiders, Scytodes and Pholeus, which make irregular cobwebs inside houses, can feed on a large variety of insects attracted to light or which feed near the ceiling. Thus, they trap houseflies, blow flies, moths, mosquitoes, etc. and feed on them.
Among the tree dwelling spiders the most spectacular is the triangular web forming, Hyptiotes, which generally lives on insects. Its snare is most complicated but uniform. The two ends of snare are stuck with two twigs of tree and third end is held by itself. When an insect strikes the snare, it loosens the end held by it thereby engulfing and trapping the insect into the snare.
2. Mites:
Vegetable mites are notorious pests of many crops. Some of them also infest man and domestic animals and cause scabies. Acarapis woodi (Rennie) lives in the thoracic tracheae of the honey bee and causes the acarine disease. Many other species have also acquired a parasitic life on insect pests.
Allothrombium sp. lives as an ectoparasite on many small insects and Entrombidium sp. on the eggs of locust and grasshoppers. Bdellodes lapidaria (Kramer) was introduced from Europe into Australia and it exercised a good check on Sminthurus viridis (Linnaeus) (Collembola), a pest of clovers.
3. Insects:
Insects form the single largest and the most important group of predators and parasitoids.It is believed that 25-33 per cent of the insects are useful to farmers, as they control or suppress populations of known or potential pests. They belong to 15 orders and more than 240 families.
The total number of species recorded in India alone would run into thousands. Praying mantis (Mantidae), even though few in numbers, devour a large number of insects of all sorts. Among Neuroptera, the green lacewing, Chiysoperla spp., and the brown lacewing flies feed voraciously on aphids and other soft-bodied insects at the rate of 160 individuals per day.
Their larvae are very active and have large grooved mandibles. The antlions, Myrmeleon spp., also belong to this order and their larvae catch ants and other insects in cone-shaped pitfalls made in sand. Among the predaceous bugs of the family Reduviidae, Reduvius cincticrus Reuter and Acanthaspis rama Distant.
They suck body juices from other bugs. Pirate bugs (Anthocoridae) prefer thrips larvae, but also feed on eggs and adult thrips, spider mites, insect eggs and small caterpillars. Among beetles, the tiger beetle, Cicindela sexpunctata Fabricius is very common in the North and Western India.
The ladybird beetles form a very important group of predators of aphids in the larval stage as well as in the adult stage. The more well-known species are Meno-chilus sexmaculatus (Fabricius), Brumoides suturalis (Fabricius) and Coccinella septem-punctata Linnaeus , the last one being very common on cabbage aphid in the spring. It diapauses in summer and also has an irrepressible tendency to migrate to the higher altitudes and congregate near the snowline.
On the southern slopes of the Dhaula Dhar range (about 4260 metres above sea level) as many as 200,000 beetles were counted per square metre. The beetles can be collected from there and released in the plains; otherwise they get buried under snow and perish. The larvae of the syrphid fly, Episyrphus balteatus (DeGeer) and Episyrphus viridaureus Wiedemann, are also very important predators of aphids.
The vast majority of the parasitoids belong to Hymenoptera and the two other important orders, Diptera and Strepsiptera. Practically, all the hymenopterous parasitoids belong to the two super-families, Ichneumonoidea and Chalcidoidea, The former group is known through the common examples, Apanteles glomeratus (Linnaeus), A. flavipes (Cameron) and many others belonging to the family Braconidae.
They parasitize a large number of lepidopteran larvae which are pests of crops. Another group of the family Braconidae consists of Stenobracon deesae (Cameron.), Bracon greeni Lefroy and B. hebetor Say, which parasitize larvae, and B. chinensis Szepligeti which parasitize eggs of a number of Lepidoptera. Melcha ornatipennis Cameron also parasitizes lepidopteran larvae and it is placed in the family Ichneumonidae. All these parasitoids attack a number of species of pests.
In the second group is placed the Trichogramma spp. (Trichogrammatidae) which are more or less universal parasitoids of eggs of Lepidoptera. In the family Aphelinidae are the well- known parasitoids of aphids, e.g. Aphidius sp., Aphelinus mali (Haldeman), Aphelinus gossypii Timb.
The other important species are the larval parasitoid of the sugarcane top borer, Elasmus zehntneri Ferriere (Elasmidae), egg parasitoid of the sugarcane pyrilla, Tetrastichus pyrillae J.C. Crawford (Eulophidae) and Aenasius bombowalei Hayat, a nymphal parasitoid of mealybug, Phenacoccus solenopsis Tinsley.