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Assessing The
Lygus Debate | 16
By Stefan Long, Dietrick Institute
Lygus is probably the most feared pest in
cotton because it can be a major cause of square loss. But
does it make sense to base treatment decisions solely on
Lygus counts? Let’s consider some other factors…First,
not only the numbers of Lygus are important, but also the
numbers in each lifestage should be counted since different
ratios of adults to nymphs reflect different situations.
High numbers of adult Lygus with no or few nymphs may indicate
these are new immigrants or transients. Second, Lygus prefer
alfalfa to cotton and will migrate from cut alfalfa through
a cotton field to another alfalfa field.
Comparing Populations
The presence of nymphs tells a different story, however.
Nymphs are of greater concern not only because they represent
a breeding population in the cotton but also because this
stage causes more damage per insect. Another important factor
to consider is the number of each lifestage of natural enemies
especially the Big-eyed bugs. Big-eyed bugs, damsel bugs,
Assassin bugs and minute pirate bugs (the good bugs) prey
on the eggs and nymphs of Lygus. Consistent samples taken
over time that contain a few adults, but are lacking Lygus
nymphs and contain many natural enemies are evidence that
these good bugs have eaten the eggs and nymphs thus preventing
the Lygus population from growing. The presence of “good
bug” nymphs in the sample indicates that their populations
are growing at the expense of Lygus. Thus, in order to get
a complete picture of the potential threat Lygus poses,
the number of each lifestage of both Lygus and at least
one natural enemy (usually big-eyed bugs) need to be counted.
To get a clearer picture of the Lygus situation, two samples
taken two to three days apart should be compared.
Acceptable Boll Loss
The most important factor to consider is the number
of squares on the plant that can actually produce bolls.
Nearly 2/3 of the squares produced will fail to produce
bolls with absolutely no pest pressure. In other words,
cotton is ‘programmed’ to set 10 bales worth
of squares but can physically only manage to support four
bales worth of bolls. That means six bales can be ‘lost’
to Lygus without affecting the yield potential of the plant.
Of course, a higher percentage of the early squares produce
a majority of the bolls. Even on plants with only 3-5 fruiting
branches, the treatment threshold is 73% retention implying
that some of these first squares can be lost without decreasing
yield.
Best Method
Counting square retention is the most unbiased way of making
a treatment decision, because it is based solely on the
yield potential of the plant. The number of Lygus and natural
enemies are irrelevant. Unfortunately, different varieties
have different square retention abilities which translates
into different yield potentials. To complicate matters more,
action thresholds for the same variety are different for
different geographic regions. The best that can be done
is to use the action threshold values as rules of thumb
to support or oppose a treatment decision based on the assessment
of both Lygus and its natural enemies.
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