Page 6 - Vol.38-No.6

CROP PROTECTION

COTTON PESTS REQUIRE CONTROL

FROM THE VERY START

are soil-inhabiting larvae of particular moth sp), thrips (Thrips and Caliothrips), cotton
species that spend daylight hours curled whitefly (Bemisia tabaci), mirids (Lygus sp)
up in the ground near the cotton roots and and the mosquito bug (Helopeltis).
coming out at night to feed. They feed on The speed with which these attack and
a wide range of plants including crop and the damage caused will depend on a num-
weed species. ‘Dedicated’ cutworm spe- ber of factors which include
cies include Agrotis ypsilon (black or greasy • Prevalence of weeds which are alter-
cutworm), but the larvae of moth species native hosts,
more usually associated with leaf damage
(e.g Spodoptera sp, the lesser armyworm • Presence other crops which support
and cotton leaf worm) will occasionally these pests (tobacco and cassava for
behave as cutworms. Cutworm damage whiteflies)
DR. TERRY MABBETT often goes unnoticed until severe, by which • Soil water relations and the rapidity
time the cutworms are older, bigger and with which the seedling can grow into

Growing cotton can come at a price, from notoriously difficult to kill with sprays of a well- established young plant with a
the cost, hard work and worry of defending insecticide. self-sustaining canopy of leaves
cotton plants from an almost interminable Best approach is to clean-weed the • The frequency and intensity of rainfall
list of insect pests. They attack in sequence field before sowing, because many prefer which has a controlling effect on some
from the moment the seed is sown until to oviposit (lay eggs) on weeds, and to of these pests.
the cotton lint is safely gathered in. Cot- then spray insecticide at the first sign of As such cotton plants may need pro-
ton crops are so ‘pestered’ by insects that plant damage. Drench the rows of plants tecting against these pests just days af-
farmers virtually need to be at field edge, with a medium/coarse droplet (large noz- ter emergence (at the same time as for
with a sprayer primed, charged and ready to zle) high volume spray of an appropriate cutworms), or after several weeks when
go, for the moment when cotton seedlings broad-spectrum insecticide that is well the plants are already tall and bushy and
push through the soil. established for control of a wide range of approaching the ‘squaring’ or flower bud
Where possible, farmers should invest in soil dwelling insect pests. formation phase. Failure to act promptly
two knapsack sprayers, one for insecticides The classic early-season insect pests against sucking pests can lead to an early
and fungicides and the other for herbicides. are sucking pests comprising a range of and complete loss of the crop. A relatively
With all the care in the world, including sap-sucking bugs that includes cotton small number of sucking pest insects on
thorough sprayer cleaning, accidents can aphid (Aphis gossypii), jassids (Amrasca very small plants can do sufficient damage
and do happen. The price paid for applying to kill plants, especially if the ground is dry
insecticide spray to cotton plants using a and the plants can’t ‘grow-away’ quickly
machine that contains herbicide residue enough. The threat is especially acute with
can be very costly indeed. jassids because the insects inject a toxin
into the leaf as they feed.
Those farmers preferring to use hand-
held spinning-disk (CDA – controlled Sucking pests such as aphids, thrips,
droplet application) sprayers will automat- jassids and whiteflies tend to congregate
ically have separate applicators. That is a and feed on the undersides of leaves, and
fast-spinning, small-droplet atomiser for for good reason. It is here that they have
ULV (ultra-low volume) application of in- easy access to the raised leaf veins with
secticides and fungicides and a slow-spin- their sharp mouth-parts and where they
ning, large-droplet atomiser for safe, min- are better protected from hot sun and the
imum-drift application of herbicides. washin-off effect of heavy rain. But this
under-leaf position can work to a disad-
Farmers may not even see the first in-
sect pests which attack cotton. These are vantage for sessile insects like aphids in
cutworms, so called because they feed on colonies near to the soil. Heavy rainfall
the soft cotton stem and sever it at ground Spraying cotton with a hand-held, splashes up mud, which embalms the in-
level causing collapse of the plant. They spinning-disk (CDA – controlled sects and provides as effective control as
drop application) sprayer
(Picture courtesy Micron Sprayers) any insecticide. Thrips pupate in the soil

4 Vol. 38 No. 6