Wireworms: a threat to stand establishment in the High Plains cotton

 As we approach the planting season, one of the first groups of insects that we need to start thinking about are wireworms. Wireworm issues are on the rise with increased adoption of conservation tillage practices and potentially the reduced use of aldicarb, a broad-spectrum insecticide over the last decade or so. Although cotton is not a preferred host for wireworms, they still can inflict serious damage to cotton seedlings especially in fields following grain crops.

What are wireworms?

Fig. 1. Click beetle (photo: Mike Quinn)

There are two types of wireworms. True wireworms are larvae of click beetles (Fig. 1) (Family: Elateridae), while false wireworms are larvae of darkling beetles (Fig. 2) from the Tenebrionidae family. Adults are highly variable in size and shape from species to species. Larvae (Fig. 3) in general look alike and are difficult to distinguish. They are smooth skinned, elongated, cylindrical, and up to 1¼ inches long. They are creamy white to yellow or light brown. Their heads are typically darker, and they have small true legs clustered near the head.

Wireworms (photo: Pat Porter)

Darkling beetle (photo: Mike Quinn)

What is the nature of wireworm injury/damage?

The larvae damage cotton by feeding on the root, hypocotyl (stem of the germinating seedling), and cotyledon (seed leaves) of emerging plants (Fig. 4). Root feeding can kill plants but usually results in stunting. The most severe damage occurs when the hypocotyl is severed, killing the plant, and reducing the stand. The larvae also feed on the growing point of the plant, slowing the growth of the main stem.

Wireworm injury to cotton seedlings (photo: Pat Porter)

 Are certain fields at higher risk of wireworm injury?

In general, fields with continuous vegetation cover allow more wireworms to survive if they are present or the field has history of wireworms. Wireworm attacks on cotton tend to be most severe when the cotton is planted following grain crops (especially sorghum), weedy ground, or in reduced-tillage systems.

 Does cultural practices such as crop rotation help?

Soil tillage in late spring and late summer when larvae or eggs are in the upper soil layers to enhance their death by desiccation, mechanical injuries or predator exposure can help reducing the wireworm load. Crop rotation is ineffective due to a much longer life cycle (2-8 years depending on species) and broad host range.

How to monitor wireworms and is there an established threshold?

Wireworms are difficult to monitor as larvae are strictly soil-dwellers and not seen unless removed from the soil. After planting, inspect emerging plants for any visible chewing damage to roots and stem and monitor the plant stand. There are no rescue treatments for wireworms, but regular field scouting will help make timely re-plant decisions based on the extent of stand loss and the size of skips.

Bait trapping a few weeks prior to planting can help monitor wireworms. Although the results of this technique have been inconsistent, it can help detect the presence of wireworms. To do this, dig several holes the size of a softball and fill them with soaked wheat or oats. Cover the hole with soil and examine the baited holes after about a week to determine if wireworms are present.

 What are the management options for wireworms?

There are no rescue treatments. Foliar insecticides targeted at adults are rarely needed. However, if adults are present in large numbers, causing evident plant clipping and probable unacceptable stand reduction, foliar insecticide application can be made. Treat wireworm larvae preventively. Insecticide (e.g. imidacloprid, thiamethoxam) seed and at-plant treatments are the most effective means of minimizing wireworm damage.

 Where to look for more info on wireworms?

Checkout our fact sheet on wireworms

Short video on wireworms

Assessing wireworm damage

 

The 2023 Version of the Handy Bt Trait Table for Corn is Posted

The new and revised 2023 version of the Handy Bt Trait Table for US Corn Production has been posted. Two of the seed companies are changing the names of their product lines this year, and the table now includes the new and old names. We chose to continue listing the older Bt products that have been removed from the market, in part so people can go back to older planting records, seed guides and research results. 

More hybrids from different companies now have the RNAi product dvSnf7 added to rootworm Bt corn, and this is reflected in the table. We have expanded the herbicide tolerance column to reflect the new Enlist technology.

Expansion of the table meant that we had to go to two pages to list everything, but the good news is that the font is larger!

Pat Porter

Managing insects in ThryvOn cotton

 Suhas Vyavhare and David Kerns

We have had a few inquiries around ThryvOn technology in cotton which has recently been fully commercialized in the US.

What is ThryvOn?

ThryvOn is a genetically engineered cotton expressing Bacillus thuringiensis (Bt), toxin mCry51a2. This protein has insecticidal activity (mostly deterrence/repellency) against thrips and plant bugs. Cry51 has an excellent activity on thrips. Field research indicates that ThryvOn cotton will not need insecticidal seed treatment (e.g., imidacloprid) or a foliar insecticide application for thrips in cotton. We do find adult and immature thrips in ThryvOn cotton but significantly fewer than the non-ThryvOn and with little or no injury to seedlings.

What would be the recommendation for thrips control in ThryvOn cotton?

We do not recommend insecticide treatment for thrips in ThryvOn cotton. Seed treatment (imidacloprid) will be optional when purchasing seed in West Texas but not in the East region where ThryvOn is packaged with insecticidal seed treatment. Contact your seed rep for more info on seed availability and seed treatment options.

How good is it on Lygus bug?

Not as dramatic as it is on thrips. There seems to be little or no activity on adults, but overall, there would be fewer and relatively smaller nymphs in ThryvOn cotton compared to the non-ThryvOn. Plant bug populations develop relatively slower in ThryvOn cotton which will provide some flexibility in terms of crop management (e.g., product choice, application timing) but will not necessarily eliminate the need for foliar application. Until we have a better understanding on ThryvOn on plant bugs, we are recommending following the current action threshold for plant bugs in both ThryvOn and non-ThryvOn cotton. Plant bug thresholds are based on combination of insect numbers and the level of fruit retention.

Does it have any activity on cotton fleahopper?

Cotton fleahopper will not be included on the ThryvOn label as a target pest. However, bioassays conducted by Bayer suggest mCry51Aa2 has activity on cotton fleahopper. Research conducted by Dr. David Kerns at Texas A&M indicates that ThryvOn cotton exhibits some activity on cotton fleahopper under field conditions and in cage studies, and slightly reduces the number of large nymph survival. However, these effects will most likely not prevent the need for foliar insecticide applications. Although cotton fleahopper numbers may not vary much between ThryvOn and non-ThryvOn cotton, ThryvOn cotton has consistently shown better square retention than non-ThryvOn at varying levels of cotton fleahopper infestation. Similar to Lygus bugs, we are recommending treating cotton fleahoppers in ThryvOn cotton using our current action threshold.

Does this technology have any impact on beneficial insect populations?

Considering the relatively broad-spectrum activity of mCry51Aa2 compared to the older Bt proteins in cotton, this is a concern among several. We have not noticed any negative impact on the beneficial populations that will affect the natural control at field level. Research conducted by Bayer indicated that mCry51Aa2 is unlikely to pose much risk to a number of beneficial insects in cotton including lady beetles, lacewings, minute pirate bugs, big-eyed bugs, damsel bugs and assassin bugs. Thus, we anticipate any impact on beneficial insects will be little to none.

In summary, ThryvOn is yet another tool in the toolbox that will further improve insect pest management in cotton. It will reduce the insecticide applications targeted at thrips and maybe to some extent against plant bugs. Regular field scouting, however, still remains critical to monitor plant bugs and other insects to make timely management decisions.