Considerations for Thrips and Weed Management as It Relates to Seedling Vigor (Collins, Edmisten, York, & Reisig)

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Seedling vigor (seedling size, health, and growth rate) is the product of several factors related to genetics and environmental influences, and can also be manipulated through management. Naturally, seed size and performance in warm and cool germination tests can be indicators of how vigorous seedlings might be. Soil temperatures at planting and soon thereafter can also influence seedling vigor. Lastly, improvements in vigor can be achieved through adequate protection from early season pests such as thrips and occasionally the use of starter fertilizers (in a 2×2 placement).

As mentioned above, seed size can be one predictor of vigor as larger-seeded varieties tend to be more vigorous. It is important to note however, that variety decisions should be based on factors such as lint yield and yield stability, fiber quality, technology packages etc. Smaller-seeded varieties can also be high yielding varieties, however extra care may sometimes be necessary to ensure vigorous seedling growth, including adjustments in seeding rates, waiting until planting conditions improve, or taking action to protect seedlings from factors that can slow seedling growth.

So why is this important now? Over the years, seedling vigor has always been important in stand establishment, especially in the early part of our planting window when conditions can be relatively cool. However, when planting in excellent conditions, yes, growers may still notice small differences in high and low vigor seedlings, but these differences often disappear later in the season and may not translate into differences in yield. But if the truth be told, this was more common back when we still had Temik and glyphosate resistance was not an issue.

In modern times, there are two major factors that, in my opinion, interact to elevate the value of vigorous seedling growth. These factors are the loss of Temik, and thus increased challenges in controlling thrips, and the increased reliance on residual herbicides for managing glyphosate-resistant weeds.

Injury from thrips is exacerbated by any factor that slows seedling growth. Reduced growth rates of seedlings allow thrips to feed on developing terminal leaves for a longer period of time and also prolong the time between seedling emergence and the 4-5 leaf stage (point at which seedlings are generally safe from thrips) which intensifies the adverse effects on yield and maturity. With the loss of Temik, there is a greater need to promote rapid seedling growth to minimize these losses, through planting high-vigor seed and planting in warm, moist soils when conditions following planting are expected to favor vigorous growth. This is especially important in modern times as many of our thrips control products may expire before seedling reach the thrips “safe stage” if growth is slowed too much. Secondly, as we all know, the success of modern weed control programs relies heavily on effective use of residual herbicides to prevent pigweed emergence. An occasional side effect of such is herbicide injury, which can also slow seedling growth and most commonly occurs when experiencing rainfall at cracking or when seedlings imbibe a high concentration of herbicide as they are trying to emerge. The effects of herbicide injury alone (i.e. in the absence of all other stresses) is generally minimal and seedlings can overcome that injury given a little time. However, the effects of herbicide injury can be exacerbated when seedling growth is slowed due to cool weather or if proactive thrips control measures are not taken.

Let’s be clear on something….the use of residual herbicides is as important now as it ever was. Weed control programs that are too lax (reduced rates or no residual herbicides) will result in poor weed control which will more than negate any improvements in seedling vigor or thrips control. In the current period of low cotton prices, some growers have expressed concern over the cost of some herbicides, especially when they have experienced severe injury in previous years. The effects that herbicides have on seedling vigor and thrips control can be overcome by timely management and are miniscule in comparison to the costs of dealing with weedy fields. Finally, it should be noted that herbicide injury can sometimes look very similar to thrips injury. Be sure you know if thrips are present before you manage what is actually an herbicide problem.

So what can we do?? Given the current challenges we face, the potential impact of planting high vigor seed in conditions favorable for rapid seedling growth is greater now than it has ever been. Therefore, as simple as it may seem, a little extra attention to germination tests and avoiding planting during periods of cool, wet conditions can help alleviate many of these issues. The effects of both thrips and herbicide injury should be lessened to some degree when doing so. Secondly, if herbicide injury is observed, research across the belt has shown that a well-timed foliar spray of acephate when the first true leaf barely appears between the cotyledons (15-20 days after planting depending on growth rate) can remove the added stress from thrips and can prevent adverse effects on yield. It is important to note that the timeliness of this foliar spray is extremely important to the success of this approach. It is understandable that growers want to combine sprays when possible to reduce trips across the field. However, if the preemergence herbicide is working, thrips control will often be needed prior to time for the first postemergence herbicide application.

With regard to thrips management practices, many growers have had good success with an insecticidal seed treatment followed by a well-timed foliar spray to 1-leaf cotton. Recent concerns regarding the documented resistance to thiamethoxam-based seed treatments (Cruiser, Acceleron N, Avicta, others) across the belt, including North Carolina, have encouraged some growers to use imidacloprid-based seed treatments (Aeris, Acceleron I, others) moving forward. In 2015, growers should have the option to choose which seed treatment they prefer, whether the seed is treated by the manufacturer or downstream at local distribution outlets. As with any seed treatment, a well-timed foliar spray should be expected in most cases. It is important to note, that a higher rate of acephate or Radiant plus surfactant (Radiant rates of 1.5 oz/A or above, with better control expected at higher rates) may be necessary in controlling western flower thrips. In many cases, the presence of high numbers of western flower thrips is only realized once the foliar spray has failed and another application is needed.

In recent years, there has been a growing interest in applying liquid Admire Pro (or like product) in-furrow in addition to a seed treatment. There has been a great deal of success with this approach, as data have illustrated prolonged control of thrips on occasion. This approach may also provide additional value in situations where a timely foliar spray cannot be made or is later than ideal, due to equipment limitations or size of a grower’s operation. However, the success of this program relies heavily on achieving adequate coverage or direct contact with the seed as it is planted. There are several methods used to make liquid in-furrow applications of Admire Pro, however research in North Carolina has shown the greatest success with the 9.2 oz/A rate of Admire Pro at application/carrier volumes of 5 to 9 gallons per acre applied through an orifice which provides a direct stream of the insecticide contacting the seed as it is planted, especially when using Rebounder seed covers. Other techniques may also be effective, however growers should make adjustments to any method they adopt to ensure direct contact of insecticide to the seed.