South Carolina Cotton: Aphids on the Rise, Look Out for Spider Mites

    Aphids on cotton leaf.

    We are leaving thrips in the rear-view mirror and dealing with aphids, spider mites, and plant bugs head-on. As the crop starts to set squares, we will observe increasing populations of plant bugs, aphids, and spider mites.

    As you might be aware of, spider mites are arthropods that can be an issue throughout most of the season. Severe infestations prior to bloom can result in significant yield loss, so be sure to scout for spider mites early to ascertain their status.

    Cotton can host low levels of spider mites and withstand some injury without yield loss. It is difficult to conduct research on treatment thresholds for spider mites, as uniform populations are difficult to establish in test plots. The best “product” for spider mites in cotton remains a good heavy rain.

    Our recommendations for spider mites read as:

    “Spider mites are occasionally a problem in South Carolina cotton. Infestations of mites are often flared by extremely hot and dry weather conditions. Applications of insecticides (e.g. acephate) for other pests can also flare infestations of spider mites by reducing the numbers of beneficial arthropods that prey upon them.

    “Initial infestations occur from spider mites moving from wild host plants or other crops into border rows of cotton. White-to-yellow speckling on the upper surfaces of leaves (in proximity to petiole attachment) will be an indication of a mite infestation. As mites continue to feed on the undersides of leaves, the upper surfaces will become reddened.

    “Early recognition of these symptoms and spot treating infested areas will often prevent spider mites from spreading throughout a field.”

    You will want to look under leaves to where the petiole connects to the leaf at the base to see spider mites. You can see spider mites and their (round) eggs with magnification. If treatment for spider mites is necessary, we have various options for chemical control; however, you need to be sure the stress is too much, and something must be done.

    In the past, I have seen fields covered up in spider mites where I intended to put out efficacy trials go from severely infested to hardly any spider mites detectable due to a heavy rain over the weekend. So, know the forecast before you treat.

    Plant bugs will include several species, such as the tarnished plant bug (TPB), Lygus lineolaris, the clouded plant bug (CPB), Neurocolpus nubilus, and the cotton fleahopper (CFH), Pseudatomoscelis seriatus.

    The plant bug species of most concern in the Coastal Plain of South Carolina is the tarnished plant bug. I have only observed clouded plant bug in the Upstate area of South Carolina, and cotton fleahopper is widely distributed but rarely an issue in cotton. Most of what we deal with regarding plant bugs is TPB.

    AgFax Weed Solutions

    Plant bugs feed on pre-floral buds (squares), blooms, and small bolls. I provided photos of these species in the newsletter last week, but here are a few different photos. The first antennal segments of CPB are enlarged, and CPB are a little larger than TPB.

    Immatures of TPB are green and can look like fast-crawling aphids as small instars and have 5 black dots on the dorsum as late-stage nymphs. Adults of CFH are a pale color and smaller than adults of TPB. If the weed host cutleaf evening primose is prevalent, you can see more CFH in cotton.

    Centric or Transform would be good choice initially, if cotton needs treatment for TPB.

    Tank-mixing in 6 fl oz of Diamond can help control nymphs. In the mid-southern states, where TPB is the number one insect pest of cotton, they have routinely used this approach and rotated in the more broad-spectrum insecticides, such as Orthene and Bidrin, later, when they need “the big guns” for control.

    Cotton aphids have been slow to build so far this season, but they are increasing now, and pockets are visible in the field. The species we have colonizing cotton is the cotton or melon aphid, Aphis gossypii.

    Historical and combined data sets on yield effects and cotton aphid indicate that aphids rarely cause significant yield loss. In other words, insecticide use for cotton aphid rarely pays for inself in protected yield. That being said, early and severe infestations of cotton aphid can result in stunted plants and yield loss, but it is rare.

    However, when additive stresses (heat, drought, insects, etc.) are on a cotton crop, killing aphids is often the only thing that can be done to relieve plants of some stress.

    Furthermore, lingering populations of aphids that have covered the plant with honeydew are often sprayed because it allows consultants to get into the field and observe other problems that can be difficult to see with inundative populations of aphids in the way. While this is understandable and sometimes perceived as the best strategy, most data support not spraying for aphids in cotton.

    As we learn more about Cotton Leafroll Dwarf Virus (CLRDV) that is transmitted by infected cotton aphids, this might change, but only time will tell. So far, CLRDV has not been an economic problem in cotton. As I mentioned last week, aphids are food for beneficial arthropods, allowing them to build up and provide control of other pests when the naturally occurring fungal organism Neozygites fresenii decimates populations of aphids and leaves predators hungry for other arthropods.

    Consider all of this before spraying insecticides for aphids in cotton.

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