the Big Belt Mountains , then in othe r areas of Montana and down into Ida ho the fores ts looked
as though they had been s corched. It was evident that this s ummer of 1957 had brought the
most extensive and spectacular infestation of spider mites in history. Almost all of the s prayed
area was affected. Nowhere else was the damage evident. Searching for precedents, the
fores ters could re me mbe r other s courges of s pider mites , though les s dramatic than this one.
There had bee n s imilar trouble along the Madis on River in Yellowstone Park in 1929, in
Colorado 20 years later, and then in New Mexico in 1956. Each of these outbreaks had followed
forest spraying with insecticides. (The 1929 s praying, occurring before the DDT era, employed
lead arsenate.)
Why does the s pider mite appear to thrive on ins ecticides? Bes ides the obvious fact that it is
relatively insensitive to them, there s eem to be two othe r reas ons. In nature it is kept in check
by various predators s uch as ladybugs , a gall midge, predaceous mites and s everal pirate bugs ,
all of them extremely sensitive to insecticides. The third reason has to do with population
pres s ure within the s pider mite colonies. An undis turbe d colony of mites is a dens ely s ettled
commu ni ty, huddled under a protective webbing for concealment from its enemies. When
s prayed, the col onies dis pers e as the mites , irritated though not killed by the c hemicals , scatter
out in s earch of places where they will not be dis turbe d. In s o doing they find a far greater
abundance of s pace and food than was available in the former colonies. Their enemies are now
dead s o there is no need f or the mites to s pend their ene rgy in s ecreting protective webbing.
Ins tead, they pour all their energies into producing more mites. It is not uncommon for their
egg production to be increas ed threef old—all through the be neficent effect of ins ecticides.
In the Shenandoah Valley of Virginia, a famous apple-growing region, hordes of a small insect
called the red-banded leaf roller arose to plague the growers as soon as DDT began to replace
ars enate of lead. Its depredati ons had neve r bef ore been important; s oon its toll ros e to 50 per
cent of the c rop and it achieved the s tatus of the mos t des tructive pes t of apples, not only in
this region but throughout much of the Eas t and Midwes t, as the us e of DDT increas ed. T he
s ituation abounds in ironies. In the apple orchards of Nova Scotia in the late 1940s the worst
infes tations of the codling moth (caus e of ‘wormy apples ’) were in the orchards regularly
s prayed. In uns praye d orcha rds the moths were not abundant enough to caus e real trouble.
Diligence in spraying had a similarly unsatisfactory reward in the eastern Sudan, where cotton
growe rs had a bitter experience with DDT. Some 60,000 acres of cotton were being grown
under irrigation in the Gash Delta. Early trials of DDT having given apparently good res ults ,
s praying was intens ified. It was then that trouble began. One of the mos t destructive enemies
of cotton is the bollworm. But the more cotton was s prayed, the more bollworms appeared.
The uns prayed cotton s uffered les s damage to fruits and later to mature bolls than the s prayed,
and in twice-s prayed fields the yield of s eed cotton dropped s ignificantly. Although s ome of the
leaf-feeding insects were eliminated, any benefit that might thus have bee n gained was more
than offs et by bollworm damage. In the end the growers were faced with the unpleas ant truth
that thei r cotton yield would have been greater had the y s aved thems elves the trouble and
expense of s praying.
In the Belgian Congo and Uganda the res ults of heavy applications of DDT agains t an ins ect pes t
of the coffee bush were almost ‘catastrophic’. The pest itself was found to be almost
completely unaffected by the DDT, while its predator was extremely sensitive. In America,
farmers have re peatedly traded one ins ect ene my for a wors e one as s praying ups ets the
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