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Cotton insect pressure has shifted

Stink bugs in the Southeast and plant bugs in the Mid-South have created problems for cotton growers who once thought these little critters were gone for good. The reasons for the dramatic rise in sucking bug pressure are varied, according to entomologists across the South.

In the Southeast the major problem is with stink bugs, though other plant bugs are sporadic pests of cotton. In the Mid-South, plant bugs, including lygus, tarnished plant bugs and cotton fleahoppers are the primary problems, with little pressure from stink bugs.

So, why the steady increase in pressure from these insects?

The Boll Weevil Eradication Program allowed many growers to grow cotton and has been one of the biggest technological breakthroughs in cotton production in the last quarter century. However, eliminating the boll weevil eliminated widespread use of broad spectrum insecticides that had virtually eliminated stink bugs and plant bugs as pests of cotton.

Since 1996 the use of Bt-containing cotton varieties has steadily increased. Along with the increased use of Bt cotton came a further reduction in the use of broad spectrum insecticides.

Virtual elimination of broad spectrum insecticides provided an ideal environment for stink bugs and plant bugs to flourish. In general, states where boll weevil eradication was the most successful early in the program, now have the heaviest pressure from plant and stink bugs. Georgia, for example, was declared boll weevil free in the early 1990s, and growers there have seen a steady increase in stink bug pressure.

In addition, a number of specialized insecticides to help control tobacco budworm have been widely used. These so called soft chemistries have little impact on plant bugs and stink bugs.

Then in 2003 and 2005 Bollgard II and WideStrike came to the market place, further reducing the use of broad spectrum insecticides.

In addition to fewer insecticide applications, stink bugs and plant bugs have developed resistance to a number of the insecticides used to control tobacco budworms. Plant bugs in the Mid-South, for example, appear to have developed resistance to a number of pyrethroids.

Clemson University Entomologist Jeremy Greene says in addition to the reduced use of insecticides with activity on plant bugs and stink bugs, dramatic swings in weather have had an impact on the rise of bug populations.

Among the 10 warmest years on record, for example, were 2005, 2006 and 2007, which had a dramatic effect on over-wintering of insects, Greene says. “For insects it’s often a numbers game. They go into the winter just trying to survive until springtime, and warmer weather certainly creates a better environment,” he adds.

In 2007, the Mid-South had near record rainfall, while the Southeast was in the midst of a record drought. The Mid-South had a tremendous problem with plant bugs, but plant bugs and stink bugs were virtually not present in the Southeast. Greene contends the rise in bug populations is directly correlated to rainfall.

In many cases CRP land, increases in corn production and early-planted Maturity Group III and IV soybeans can create an ideal nursery for plant bugs and stink bugs. The big increase in corn production in 2007, for example, likely had an impact on the rise in plant bug populations in the Mid-South, according to Greene.

Plant bugs can reproduce in the tassles and silk of corn and young corn plants are a favorite feeding site, especially for stink bugs. In general corn is an excellent host for plant bugs.

North Carolina State Entomologist John Van Duyn says the big increase in wheat acreage in the upper Southeast is likely to cause problems with stink bugs. Wheat, Van Duyn says, is an ideal reproductive host for stinkbugs. Wheat is typically harvested at a time when summer crops are just coming up and are ideal food sources for sucking bugs.

Despite all the cards being stacked in the favor of plant bugs and stink bugs, cotton growers still have a good arsenal of materials to control these yield-damaging pests. Carbine, for example, has proven highly successful in controlling plant bugs. Carbine is from a new class of chemistry called pyridinecarboxamide that has a different mode of action from the neonicotinoids, organophosphate or pyrethroid insecticides commonly used against lygus or plant bugs and aphids.

In general plant bugs are a problem in the Mid-South and stink bugs are a problem in the Southeast. However, entomologists warn that as more areas are declared boll weevil free and as the use of Bt cotton continues to grow, there is a threat that this could flip-flop.

The primary stink bugs that affect the Southeast are green, brown and southern green. The southern green stink bug adult is green in color, and the nymphal stage has white spots on the back or abdomen. The green stink bug is also green but the nymphal stage has a striped abdomen. The brown stink bug is brown and closely resembles another predaceous stink bug, the spined soldier bug (a beneficial insect). They can be distinguished one from another by the very sharply pointed "shoulders" on the spined soldier bug.

The primary plant bugs that attack cotton are lygus bugs and tarnished plant bugs. The tarnished plant bug adult is a brownish-yellow mottled insect of approximately 3-6 mm in length. It is oval in shape with a distinctive white triangle located just behind the "shoulders". Nymphs are yellow-green in color and have 4 distinctive black dots on the back.

Tarnished plant bugs feed by piercing the plant with very small mouthparts and sucking out the plant juices. The damage is mostly inconspicuous and is not evident to the eye. Damaged plant tissue will often wilt and may abort.

In cotton, damaged pinhead squares may show a slight discoloration of the infected anthers. In fruits and vegetables, damage may appear as indented or cat-faced injury.

Tarnished plant bud injury is often associated with poor seed germination. This is especially important in seed production systems like cotton.

Lygus bug adults are about 0.25 inch (6 mm) long and 0.1 inch (2.5 mm) wide, and flattened on the back. They vary in color from pale green to yellowish brown with reddish brown to black markings, and have a conspicuous triangle in the center of the back.

Lygus bugs can threaten a cotton crop from earliest squaring through cutout and final boll set. Lygus bugs pierce squares and damage anthers and other tissues. When squares are less than 0.2 inch (5 mm) long, they shrivel, turn brown, and drop from the plant. Damage to larger squares may be to anthers, styles, and stigma, and may interfere with fertilization.

Stink bugs damage cotton by sucking sap from squares and bolls. Affected bolls have a small sunken black spot on the outside. Internal evidence of feeding may be seen when bolls are opened by hand and lint/seeds/carpel walls are examined for signs of feeding injury. The seeds and lint usually turn brown as a result of stink bug feeding. Often, wart-like callus growths are present on carpel walls, marking the feeding wounds and the plant's response to injury.

The consensus of entomologists across the Southeast is that plant bugs and stink bugs are likely to continue to increase in severity in years in which weather is conducive to over-wintering and early development of these insects.

As long as growers in the Mid-South continue to spray early for plant bugs, stink bug numbers will likely remain low.

Scouting for these insects will be critical because even low levels of these insects can significantly reduce both yield and quality of cotton.


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