Saturday, November 26, 2022

Sporadic Pests Of Seedling Cotton

Keep An Eye Out For The “Odd Balls” That May Threaten Young Cotton

⋅ BY SCOTT GRAHAM ⋅
AUBURN UNIVERSITY

One of the challenges of cotton insect pest management is also one of the things that makes it exciting: no two seasons (or even weeks) are the same. For this reason, there is no substitute for “boots on the ground” weekly scouting for odd critters that may show up.

Depending on weather, we may see some “relatively” common pests, such as spider mites or three-cornered alfalfa hoppers, or we may observe some real odd balls, like white-margined burrower bugs. Walking fields weekly, beginning at planting helps identify potential problems before they arise and can help ensure we maximize our yield potential from the start. 

Dry Weather Pests

Grasshoppers

Grasshoppers have been a sporadic pest of seedling cotton in Alabama for many years. Dry weather during the winter favors the survival of grasshopper eggs, which are laid in pods in the soil. Shifts to reduced and no-till systems have also led to increased grasshopper populations, as eggs are not mechanically killed during tillage.

In Alabama, immature grasshoppers begin emerging in late March and continue through June as soil temperatures rise and spring rains occur. Grasshoppers damage cotton by feeding on the mainstem of seedlings — often in the crook or cracking stage — resulting in stand loss.

Scouting for grasshoppers is difficult, as they are hard to capture in a sweep net. There are no current thresholds for grasshoppers and likely never will be due to their sporadic feeding nature and difficulty to sample. In fields with historic grasshopper problems, preventative approaches, such as adding an insecticide to burndown herbicides, may be needed. Border sprays are not effective since immature grasshoppers emerge throughout the entire field.

Since not all immature grasshoppers emerge at the same time, a long-lasting residual insect growth regulator insecticide could be used. Ultimately, preventative applications for grasshoppers are a judgment call and are based on the risk level an individual farmer is willing to accept. While most labeled insecticides provide control of immature grasshoppers at low labeled rates, adults, which develop later in the spring, are more difficult to control, even at higher rates.

For more information, visit the ACES publication Insect Pests of Cotton: Grasshoppers (https://bit.ly/3MfULI6) and the Alabama Extension IPM Guide IPM-0415 (https://bit.ly/36CieUA). 

Two-Spotted Spider Mites

Two-spotted spider mites are an increasingly common early season cotton pest in Alabama. As acres continue to shift to reduced tillage and cover cropping systems, this trend will likely continue, particularly in dry springs. Mite populations slowly build on existing winter vegetation and will move to cotton as it emerges in the spring. One way to mitigate spider mite outbreaks is to achieve good burndown. 

Historically, spider mites were mostly observed to be border pests as populations moved from weedy hosts in field edges into cotton fields. Recently, outbreaks can be found in the middle of fields, often in areas where good burndown was not achieved. In hot, dry spells, spider mites can develop from egg to adult in as little as five days. Outbreaks are more likely in these conditions and following applications of broad-spectrum insecticides. 

The most efficient way to scout for spider mites is to look for symptoms of seedling injury. Mites cause damage by sucking juices from the bottom of leaf surfaces. Signs of feeding begin as white or yellowish stippling that develops into reddening or purpling of leaves along leaf veins. In seedling cotton, this can lead to defoliation and stand loss in severe outbreaks.

If early signs of spider mite damage are observed, look on the underside of the leaf for mites, which are small (0.3 mm) and yellowish with two black dots on their back. A 10x hand lens is helpful to see mites. Exposing the underside of the leaf may agitate mites, causing them to move and making them easier to see.

Determining when to treat for spider mites requires some professional judgement, as management decisions are not based on numbers of mites found. In addition to the number of plants showing symptoms, and the severity of symptoms, the immediate weather forecast should also be taken into consideration. If no significant rain chances are expected in the next few days, treatment may be warranted. Up-to-date thresholds and insecticide recommendations can be found in the Alabama Extension Cotton IPM Guide IPM-0415 (https://bit.ly/36CieUA).

Three-Cornered Alfalfa Hoppers

Three-cornered alfalfa hoppers (TCAH) are a sporadic pest of seedling cotton. Infestations tend to be worse on field borders and in dry springs when migrating adults move from weedy hosts to cotton fields. TCAH damage seedlings by feeding around the mainstem (girdling), resulting in a “knot” below the cotyledons that reduces the plant’s ability to move nutrients and photosynthate. Damaged plants will appear stunted, the leaf veins will turn dark red, and the leaves will turn a red-orange color. Typically, girdling is not a concern once plants reach the 6- or 7-node stage.

Although sampling for TCAH is possible, most infestations are observed once plants begin to show symptoms. Often, damage observed is not as bad as it looks and is almost always confined to field borders. There are currently no established thresholds for TCAH in cotton.

Unlike soybean, girdled cotton plants usually die, and neighboring plants compensate for any stand loss. TCAH can easily be managed with pyrethroids or organophosphates. However, economic losses are rare, and treatment is not often justified. In some cases, TCAH may reinfest fields soon after application, giving the appearance that the treatments did not work. Follow-up applications are more likely to flare spider mites than to save cotton from economic TCAH damage.

White-Margined Burrower Bug

White-margined burrower bug is another sporadic insect that can be found in cotton fields during dry springs. This insect may be confused with the peanut burrower bug, which is a sporadic, but important, pest of peanut. Little is known about the life cycle of white-margined burrower bug or its pest status in cotton.

Infestations, though rare, tend to occur in reduced till fields and in dry years. These insects feed on developing seeds and are particularly attracted to seeds in the mint family, such as henbit or purple deadnettle. Anecdotal observations suggest that these insects are highly susceptible to insecticide seed treatments and do not pose a threat to cotton seedlings. Thus, insecticide applications are not recommended as the potential to flare other pests is increased. 

False Chinch Bug

False Chinch Bug (FCB) are a rare pest of seedling cotton. This insect can be found on a wide variety of plants and weeds and builds up populations during cool, wet springs where weedy hosts thrive. Then, as conditions become hot and dry, and weedy hosts dry down, populations will move to cotton.

False chinch bugs damage cotton by sucking juices from the plants, and heavy infestations can kill cotton seedlings. Many times, infestations are confined to field borders where populations move from weedy hosts along ditch banks. But they may also be found throughout the field if high populations of weeds were present prior to a herbicide application. 

In the rare cases that FBC presents an issue, they usually occur during dry periods and after a herbicide application that kills weedy host plants so FCB have no other hosts to infest but cotton seedlings. Due to the rarity of infestations, there are no thresholds for FCB in cotton. Anecdotal observations suggest some at-plant insecticides, such as aldicarb, provide control of FCB. If foliar treatment is warranted, FCB can be difficult to control. In some cases, a tankmix of acephate and bifenthrin are needed to manage populations.

Wet Weather Pests

Slugs

Slugs are not an insect but are a growing pest of seedling cotton in Alabama. These critters, known as mollusks, are associated with cool, wet years and reduced-tillage systems. Infestations are more likely to occur following high residue crops, such as corn or grain sorghum. Slugs may damage cotton by feeding on cotyledons or true leaves, often leaving irregularly shaped holes on leaf edges. However, the greatest risk is stand loss from slugs feeding on the mainstem of seedlings. 

Slugs are often most active at night and difficult to find during the day. To scout for slugs, scratch the soil surface back and look for them. There are no thresholds for slugs and few effective options for control after planting.

In high-risk fields where tillage is not an option, setting row cleaners more aggressively can push plant residue back which may reduce the risk of injury. Additionally, ensuring the furrow is closed can help reduce the chances of slugs clipping cotton seedlings before they emerge. Baits containing iron phosphates or metaldehyde are available, but in many cases are cost prohibitive and not readily available.

Ultimately, the best management strategy for slugs is hot, dry growing conditions that are conducive for seedling growth and bad for slugs. It is important to note that insecticides have no activity on slugs and should not be used as a control measure.

Snails

Snails are another mollusk pest that are becoming more of a threat to cotton stand establishment. Although a more consistent issue in in south Alabama, infestations have been observed in central and in northeast Alabama as well. The most common species of snails currently observed in Alabama cotton feeds on detritus and decaying organic matter, not cotton plants. Damage is caused from the sheer number of snails that climb on seedlings, with the resulting weight breaking the plants and causing stand loss.

Unlike slugs, snails are easily found during the day and can be differentiated by their shell. Snails may be worse in fields with high calcium levels or in areas where lime was over applied, as they mine this nutrient for shell production.

Like slugs, we do not have thresholds or effective control options. Currently, tillage is the only option to manage them. High-risk fields or fields that have issues with snails in the past may benefit from at least light tillage prior to planting.

Cutworms

Several species of cutworms may be observed in wet springs. Vegetation in fields prior to planting, such as cover crops or weeds, attract moths to lay eggs that results in finding caterpillars as the cotton emerges. Cutworms are more common in reduced till fields and may be more likely to be found in low lying areas where effective burndown was not achieved.

Cutworms are more active at night and damage seedlings by clipping plants at the soil level then feeding on leaves below the soil surface. Thus, the presence of severed stems with no leaves may be an indicator of cutworm infestations.

To scout for cutworms, look for signs of stand loss then scratch below the soil surface and look for the “dirty” looking caterpillar. If herbicide burndown applications are made less than four weeks prior to planting and conditions are cool and wet, preventative control measures may be justified. Mid labeled pyrethroid rates often provide adequate control of cutworms when applied with burndown herbicides or within seven days before or after planting.

For up-to-date thresholds and insecticide recommendations for cutworms, consult the Alabama Extension IPM Guide IPM-0415 (https://bit.ly/36CieUA). 

True Armyworms

True armyworms are a sporadic pest of seedling cotton in Alabama. Infestations are almost always confined to reduced-tillage systems as moths are attracted to existing vegetation for egg laying.

These caterpillars are commonly found on grasses in the spring and may require treatment in wheat. Thus, infestations may be more likely behind fields with grass cover crops, such as cereal rye. The caterpillars are a light brown or tannish color and have characteristic black bands around their legs. Like other spring pests, the threat of damage is from stand loss by caterpillars clipping seedling stems.

Scouting for true armyworm is best done in the morning, as larvae hide under plant stubble during the heat of the day. There are no established thresholds for true armyworm in cotton. However, planting into fields with infestations is not advised. If populations are observed, mid labeled pyrethroid rates often provide adequate control when applied with burndown herbicides or within seven days before or after planting. 

Summary

Insect management is different from all other aspects of cotton production. The cotton insect situation changes from year to year, week to week and field to field in the approximately 20,000 cotton fields across Alabama. To manage key and sporadic insect pests effectively and efficiently, scouting is critical. Changes in the weather can affect which species of pests are most likely to infest fields.

Although pests, such as thrips, plant bugs and stink bugs are consistent threats, sporadic pests sometimes cause more headaches because many field advisors have less experience with them. The only way to know what insects are in fields and at what levels is to scout weekly. Familiarizing yourself with the “odd balls” is important to properly identify which insects are pests, which are beneficials and which are incidentals. 


Dr. Scott Graham is an Extension entomologist with Auburn University. He may be reached at shg0013@auburn.edu.

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