Suhas Vyavhare, Associate Professor and Extension Entomologist
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?
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.
Fig. 2 Click Beetle Fig. 1 Darkling Beetle
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.
Fig.4 Wireworm injury to cotton seedlings
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: agrilife.org/lubbock/files/2017/05/Wireworms_ENTO-068.pdf