Farmer’s Weekly – 02 August 2019

vegetable production

do so; it is economical,

convenient and effective.

For this application, take

the whorl plate out of a cone

nozzle; this will produce a jet

with large droplets, which are

more attractive to the flies.

The jet will also travel much

further, saving you from moving

through the land with a tractor,

which crushes runners.

This setup, using plain water,

is also useful for dousing fires.

A DIAPHRAGM MODEL

When choosing a knapsack,

don’t go for the cheapest

model. Choose one with a

diaphragm; this is easy to

replace and more efficient than

a piston. You can also make

a replacement diaphragm

from an old car inner tube

that will last for years.

• Bill Kerr is a vegetable specialist
  and a breeder of a range of vegetables.
  Email him at farmersweekly@
  caxton.co.za. Subject line:
  Vegetable production.
  ▪

FW

R

ecent years have seen the

introduction of ever-more

advanced crop sprayers,

many of which are air-assisted to

improve penetration into foliage.

Nonetheless, there is still a place

for the humble knapsack sprayer.

For one, it’s ideal for selectively

spraying a young crop that

initially covers only a small

percentage of the land. The

knapsack is also more effective

in certain circumstances.

A case in point is a cabbage

crop that has just entered the first

stage of head formation. If aphids

are present at this time, many

will migrate into the loosely

developing head, where they are

extremely difficult to control.

A boom sprayer cannot

reach the inner leaves of this

crop. Products registered for

post-planting aphid control

are ‘trans-laminate sys t e m i c ’:

when sprayed on a leaf, they are

absorbed into it. But the chemical

is not carried to unsprayed leaves,

so the aphids on the inner leaves

remain untouched. It is therefore

pointless applying more spray.

At this critical stage, a knapsack

sprayer can be highly effective

and economical. Begin by mixing

the product in a large tank or

vessel so that you don’t have

to mix a small quantity each

time you refill the knapsack.

Go down each row, directing

the nozzle to slightly above

where head formation is starting

and where aphids forage. Move

at a slow walking pace from

head to head. This gives far

more effective penetration of

the product into the developing

head than is possible with a

boom sprayer. About 50 plants

can be sprayed per minute.

A word of caution: it is natural

to want to move the nozzle to

the adjoining row and try to

spray more than one row at a

time. Avoid doing this; it actually

slows down the process and

you risk missing some plants.

Using a knapsack sprayer in this

way, you can spray 3 000 plants

per hour or about half a hectare

per day. You will save insecticide,

as well as fuel, because you

don’t have to use your tractor.

Moreover, as noted, a knapsack

is more effective. Proper aphid

control in cabbages has become

very important since the arrival

of brassica stunting disorder.

OTHER USES

The knapsack sprayer can also

be used for spot spraying where,

at a later stage, single plants

might house aphid colonies.

This saves you from having

to spray the whole land.

If you need to apply a fruit

fly bait to cucurbits, use

a knapsack sprayer to

ABOVE:

Penetrating the

developing head

is important

for effective

aphid control.

bill kerr

You will save

on insecticide

and fuel

The knapsack sprayer: still useful!

Bill kErr

46 farmer’sweekly 2 August 2019

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