Plant Bug Management in Cotton
Cotton must be managed differently pre- and post-bloom. First of all, we need to use the correct sampling method. The importance of this has been covered previously. Briefly, adults colonize cotton pre-bloom and the sweep net is great for capturing them- not so great for capturing nymphs. Reproduction tends to happen post-bloom and the drop cloth is great for capturing nymphs- not so great for capturing adults. Second, we need to choose the correct insecticide. Both will be covered below.
Before bloom, our goal is to keep 80% of the squares on the plant. If squares are dropping off pre-bloom, it’s a good bet that plant bugs are to blame. However, use a combination of plant bug presence (8 plant bugs in 100 swooshes of the net) plus 80% square retention as a threshold to trigger sprays. Specific scouting details can be found in the tarnished plant bug sampling section in the NC Cotton Insect Scouting Guide.
We are now past the point in the season where neonicotinoids (Admire and Centric) should be applied alone. In recent years, tarnished plant bug infestations have extended far into the blooming period. We have pyrethroid-resistant plant bugs in the northeastern part of the state and this should serve as a wake-up call to rotate insecticides. Try to avoid using more disruptive insecticides (e.g., Bidrin, pyrethroids [including bifenthrin], Orthene [acephate], etc.) until later on in the season when we need to manage both stink bugs and possibly plant bugs. Beneficial insects are a residual management strategy after insecticide applications (think of them as bio-residual). If we preserve them in the field, their populations should continue to build and eat pests as they show up. Now is the time to use insecticides like Transform (2 oz and above, unless tank mixed). Admire or Centric can be tank-mixed with either of these insecticides to add an additional mode of action to target plant bugs. Finally, if nymphs are present, Diamond (4 to 6 oz) should be tank-mixed to provide extended control. Diamond is an insect growth regulator and only directly active on immatures (hatch rate is reduced in eggs from adult females exposed to Diamond), so it needs to be paired with an insecticide for knock down of adults.
In general, a product that is killing a plant bug will likely kill related beneficial insects such as minute pirate bug and insidious flower bug, damsel bugs, assassin bugs, and big-eyed bugs. However, these products are still much less harsh on the system than pyrethroid and organophosphate-class insecticides. Don’t forget that an apparent “spray failure” early season could be a situation where you are chasing adults. That is to say that adults could have re-invaded into the field after spray, making it look poor, when in fact it worked. Check sprays as soon as it’s safe to enter the field.
When cotton blooms, it’s time to switch sampling and thresholds for plant bugs. This previous article covered why you should scout differently in pre-bloom and post-bloom cotton. There is no magic switch point, but once cotton blooms for a couple weeks, monitoring square retention becomes a less reliable way to make treatment decisions, as does the sweep net. For this reason, we recommend a threshold of 2-3 plant bugs per drop cloth sample (~0.5 per row foot) during the bloom. During early bloom, consider using both a sweep net and a drop cloth for sampling.
Remember that plant bugs will feed on larger squares and small bolls during bloom. The feeding on the large squares can result in dirty blooms. You should never treat based on the presence of dirty blooms, but they are an indicator that you should use your drop cloth in the field and scout. You can visit the cotton scouting guide for detailed information and a video for how to use the drop cloth correctly. Recently published work done in North Carolina supports the fact that net profits will be higher when these thresholds are used.
A number of different insecticides are effective post-bloom, but growers should choose carefully given their individual scenario. Questions to ask are: how far am I out from a bollworm flight, how many times (and what) have I sprayed this season, how many times do I expect to spray this season, and do I normally battle stink bugs during bloom? These questions can help guide a specific insecticide plan. To reiterate, neonicotinoids (Admire and Centric) should be not applied alone, but can be tank mixed with other insecticides post-bloom. Finally, as we’ve learned time and time again, pesticide rotation is a major key to long-term success.
Transform (2 oz, unless applied in a tank mix) is the insecticide that growers should reach for prior to a bollworm flight. A huge advantage of this insecticide is that it is softer on beneficials than many other products- don’t be tempted to tank mix other chemicals with this insecticide that might hamper this benefit!
Other insecticides and mix combinations are helpful post-bloom and later season, such as Bidrin, Vydate, and tank mixes of pyrethroid and Orthene, pyrethroids, and neonicotinoids, etc. However, they are harsh on our bio-residual (beneficial insects).
Similar to pre-bloom cotton, Diamond can be tank mixed if nymphs are present. Research results from 2020 showed that spray intervals can be extended if this product is used at the correct time.