An integrated approach that relies on many different tactics in addition to chemical insecticides is the best way to effectively and economically manage tarnished plant bugs in the Mid-South, according to studies by Mid-South entomologists.

“Plant bug has become a very serious pest for Mississippi as well as other areas of the Mid-South,” said Mississippi State University assistant research professor Jeff Gore. “The number of applications for plant bugs increased dramatically in 2007 over the previous years.”

Reasons for the increased applications in 2007 include observations of widespread resistance to acephate in tarnished plant bugs in 2006. “Late in the season in 2006, we weren’t controlling plant bugs as well as we were in previous years. We had a very high population going into overwintering.

“We also had about a 60 percent reduction in cotton acreage from 2006 to 2007. Plant bugs were still out there in the landscape on wild hosts, and when plant bugs moved into cotton in July and August, we had a smaller cotton area for plant bugs to move into.”

According to resistance monitoring studies, plant bug resistance to acephate climbed in 2006, then fell slightly in 2007.

Efficacy studies also show that maximum percent control from organophosphate insecticides has been decreasing. “In 2007, it was around 60 percent.” Gore also pointed out that rates of those products have increased as percent control decreased.

To combat plant bugs, “we’re having to find innovative ways to control plant bugs in the Mid-South,” Gore said.

Scientists have found that mixing a pyrethroid with an organophosphate — even though the insects have resistance to both of these insecticides — shows a fairly significant level of control.

For example, improvement in control occurred when applying Orthene plus Brigade over Orthene alone. The scientists also found improvement in control when a pyrethroid is added to Trimax Pro or Centric. The study cited improved control with insecticides such as Endigo, a premix of Centric and Karate, or Leverage, a premix of Trimax Pro and Baythroid. “But this is not going to be something we can sustain. It’s helping us get over the hump right now.”

Scientists also looked at how application intervals affect control. The data indicated that an application of a half-pound of Orthene followed by another application of a half-pound of Orthene four to five days later “improved control significantly compared to waiting six to seven days for the second application of Orthene. After six to seven days, control went down rapidly.”

Rotating chemistries

In a study on the value of rotating chemistries, three sequential applications of Orthene were compared to sequential applications of Orthene, Centric and Orthene. Gore said plant bugs reached threshold after 11 days with Orthene as a second application. “But we were still below the threshold after 11 days where we applied Centric as the second application. So we’re encouraging our growers to rotate as much as possible.”

The research, funded by Cotton Incorporated, also found that crop maturity can have a fairly significant impact on yields. In 2007-08, scientists looked at early-maturing (DP 444 BG/R) and late-maturing (DP 555 BG/RR) cotton varieties under early-season control, season-long control and late-season control scenarios. Plots with season-long control were sprayed from pinhead square to cutout. Early-season control plots were sprayed from pinhead square to first flower, while late-season control plots were sprayed from first flower to cutout.

In 2007, a heavy year for plant bugs, scientists found that for the earlier-maturing DP 444 BG/RR, early-season control was the most important component of the sprays. For the later-maturing DP 555 BG/RR, both early-season and late-season control were very important.

In 2008, plant bug populations were less severe. “Because of lower plant bug populations, late-season control appeared to be more important than early-season control for DP 444 BG/RR, which was the exact opposite of 2007. On both varieties, season-long control improved yields compared to non-treated plots. On DP 555 BG/RR, both early-season and late-season were important and recorded the highest yield in 2008.

The study also looked at sequential applications of insecticide initiated during different weeks of flowering. “We started spraying at first flower and continued from the second through the seventh week of flowering. We had lower yields when we stopped spraying earlier in the flowering period, while we increased yields as we stopped spraying later in the flowering period.

“A lot will change according to when plant bugs move into the crop, but looking at the fruiting patterns of cotton, the third through sixth week of flowering is when we’re going to have the majority of fruiting structures that can be knocked off and cause the greatest yield losses.”

Sampling methods

Scientists also evaluated different methods for sampling for tarnished plant bugs “which showed that some of the plant-based measures like square retention and dirty squares provided the best correlation for yield versus some of the other methods.”

Gore said that observations of dirty squares, the result of plant bug excretions, “show that 10 percent dirty squares correlate very well with our recommended drop cloth threshold of three plant bugs per 6 row feet.”

Gore concludes that standard insecticides do not provide the same level of control they once did and that tank mixes and premixes “are going to be important in the short-term.”

Crop maturity does impact plant bug management, Gore says, and plant-based thresholds such as dirty squares and square damage are important “considering the current levels of resistance and for some of our new insecticides that have a slower activity in terms of mortality.”

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