(By Dr. Girish Chandra)
Cultural control includes such methods of planting, growing and harvesting crops, which will reduce crop damage. Slight modification in the crop rotation, weed control, disposal of crop remnants, resistant varieties, time of planting and harvesting may prove important in combating some insect pests. Control by cultural means can be achieved by the following methods:
1. By resistant varieties: Some varieties of plants show greater resistance to pest damage and should be planted if found desirable from other standpoints. For example, Pawnee-wheat is resistant to Hessian fly; Atlas-sorghum is resistant to chinch bug, rice varieties, namely, IR-36, Pattambi-33, IR-42 are resistant to brown planthoppers. Resistance is hereditary and can be utilized in breeding programmes. There can be several types of characters that make the variety resistant. They may be physical, like hairy leaves, which pose difficulty to potato leafhopper in feeding. Varieties of corn with long tight husk are injured less by corn-earworm. Shape of the onion leaves has an important bearing on the resistance against thrips. Certain chemicals like oxalic acid provide resistance to rice varieties against plant hoppers.
2. By ploughing: Timely ploughing can disturb and kill the insect pests, eradicate weeds upon which they might feed, expose them to natural enemies or to harsh weather, and bury the pest stages so deeply that few adults can emerge.
Plowing effectively controls corn root-aphids because it disturbs and kills them and destroys the weeds upon which they might feed before the crop is planted. Early ploughing exposes many white root-feeding grubs, which are then destroyed by birds.
3. By planting practices: Some pests remain in the seed from harvest to planting time, and can start infestation. Good seed-bed preparation can not only bury many potential pests like Hessian fly but by conserving moisture gives the plant a higher chance of survival and the pests can be easily sprayed down in a smaller area.
Date of planting may have an important bearing upon insect infestation. If planting coincides with the emergence or immigration of the adult pest, the crop is likely to be infested severely. Late planting of cotton is likely to be severely infested by the boll weevil because it will fruit at a time when the weevils would be most numerous.
Depth of planting is important in the case of potato which if planted deeper, checks infestation of potato tuber moth, since the moth is unable to lay eggs through the cracks in soil.
4. By harvesting practices: Control of a number of leguminous pests is possible by timely and thorough mowing of the field. Clean cutting eliminated any leftover plants on which the insects may breed. Uniform mowing on community bases eliminated the existence of tender side-growths on which the insects can breed and live. Mowing indirectly kills the nymphs of Lygus bugs since they are exposed to high temperature, and prompt removal of the hay from the field exposes the pest on bare ground. No stubbles should be left while harvesting.
5. By crop sanitation: Destruction of crop residues is a good preventive measure, since it eliminates further insect breeding by cutting off their food supply. Destruction of corn, rice, sugarcane and wheat stubbles is recommended against shoot borer larvae and pupae. Harvested sugarcane crop produces secondary growth on which a sizable population of borers and hoppers can build up for the next season.
Weed control: Many insects will develop on weeds very different from the crop botanically. Weed control therefore reduces infestation. Leptocorisa and Pyrilla can feed on many grasses in and around the fields. Green leafhoppers of rice can breed on barnyard grass.
Destruction of volunteer crops: Volunteer crops are those which grow from self-sown or spilled seeds, or which sprout from stubbles. They should be destroyed. A summer rain may cause germination of seeds scattered during the harvesting times. Roots of harvested cotton will sprout and produce an earlier crop on which the pink bollworm can complete one generation and when the main crop emerges there is already a sizable population of the pest available.
6. By isolation from the secondary host plants: Insect damage can often be reduced by planting the crop as far away as possible from the other crops, which may serve as secondary hosts. Armyworms may breed on wheat and barley and later may migrate to corn and sorghum. The infestation of potato by potato-aphid, Macrosiphum solanifoli has been found to be associated with rose bushes on which this species passes the winter. Sesamia inferens can migrate from wheat to rice and sugarcane.
7. By closed seasons: A monophagous species can be eradicated by not growing the crop for a year or two. Closed seasons are practiced in tropical countries to decrease the damage of the pink bollworm. This is done by not growing cotton collectively by all farmers for a couple of years.
8. By crop rotation: Crop rotation is effective against pests that feed on relatively few plant species, and are incapable of long distance migration. Usually graminaceous and leguminous crops are grown alternatively.
9. Pasturing: Where enough animals are available, pasturing heavily damaged fields may utilize the remaining plants as fodder and pest stages are destroyed by feeding and trampling by the animal herds.
10. Fertilizing: Nitrogenous fertilizers in some soils tend to increase the susceptibility of sorghum to chinch bug. Rapidly growing cotton is more attractive to bollworms, leafworms and fleahoppers. Barley, wheats and oats can withstand greenbug infestation better when fertilized and irrigated.
11. Flooding: It is sometimes possible to destroy pests by flooding, as in the rice fields it destroys many migrating sugarcane pests like armyworms. Plowing followed by flooding kills pink bollworms in cotton.
12: Trap crops: A trap crop is a small planting of the susceptible crop made earlier than the main crop for the purpose of diverting the insects from the main crop. The plant in trap crop should be very attractive to the insect. For example broom grass is planted around wheat fields in Canada, against wheat stem sawfly, Cephus cinctus. The adult sawflies oviposit in broomgrass stem, usually in large numbers. The wheat crop when planted escapes oviposition. The emerging larvae cannibalize one another and only one larva per stem has a chance to develop, which is usually parasitized. The trap crop thus harbors parasites, which migrate to the wheat crop. Trap crops have been tried in rice against planthoppers.