False Fire Worm Alert

 Pest Biology 

 The Kutsaga Plant Clinic has received a growing number of false wire worm cases in the past week. Wireworms are the larval stage of several species of “click” beetles. There are two sub-categories of wireworms; True wireworms which belong to the family Elateridae and False wireworms belonging to the family Tenebrionidae (Fig 1 and 2). Historically, both true and false wireworms were not common pests on tobacco in Zimbabwe. If observed, they only caused minor damage although tobacco is a susceptible host. However, in the past two seasons, the occurrence and damage caused by wireworms has become a significant concern. 

False wireworm damage 

Infestations of the small false wireworm can be as high as hundreds of larvae per square meter, although densities as low as five larger false wireworm larvae per square meter can cause damage under dry conditions. The larvae of the small false wireworm are mostly found damaging seedlings shortly after germination or transplantation. They feed on the seedling stem, at or just below the soil surface. This causes the stem to be “ring-barked”, and eventually the seedling may be lopped off or it wilts under warm conditions (Fig 3 and 4). 

NB: Blue arrows point at pests 

False wireworm management 

In Zimbabwe, two crop protection agents are recommended for the control of false wireworm; 

1. Thiamethoxam 25 WG is applied using a rate of 125 g/100L water then apply one 30ml cup into the planting hole separately from the planting water. Alternatively, a pre-mix can be diluted with planting water using a rate of 3.7 g/100L of water and apply one litre mixture into the planting hole. 
2. Imidacloprid 200 SL can be applied using a rate of 220 ml/100L of water then apply one 30 ml cup into the planting hole separately from the planting water. Alternatively, a premix of 6.5 ml/100L in planting water and apply one litre into the planting hole. 

Mode of action of Thiamethoxam and Imidacloprid 

Thiamethoxam 25 WG and imidacloprid 200 SL are neocotinoids with a systemic mode of action. In addition, Imidacloprid also has a contact mode of action. Thus, if these products are applied correctly at planting they offer protection to early plant damage. Thiamethoxam 25 WG and imidacloprid 200 SL accumulate in the immediate rhizosphere of the plant protecting the roots of the plant from false wireworm damage. Imidacloprid 200 SL and Thiamethoxam 25 WG are taken up by the roots of the plant and travel through plant tissues following application. In the event that the false wireworm manages to chew on the plant, imidacloprid and thiamethoxam bind irreversibly to the nervous system of the false wireworm larvae thereby causing paralysis. This binding also triggers an antifeedant toxicity effect to the false wireworm larvae causing them to die of starvation. If gap filling is done, farmers are recommended to apply insecticides again to avoid damage. 

Use of unregistered products 

The only registered products recommended for use against false wireworm on tobacco in Zimbabwe are Thiamethoxam 25WG and imidacloprid 200 SL. While there are a number of insecticides such as cyantraniliprolle and pymetrozine that are presumed to be effective against false wireworm, farmers are discouraged from using these products as yet for false wireworm control as they are still under evaluation. In the past some unregistered products have been found to be ineffective and, in some instances, phytotoxic to tobacco. 

The Pesticide Approval Scheme Service (PASS) is working tirelessly to evaluate the efficacy of new formulations for the control of false wireworm larvae on tobacco. PASS is a grower advisory and pesticide registration system involved in the search for safer and green labelled agrochemicals for use on tobacco since 1964. PASS evaluates new and registered active ingredients to validate their efficacy on control of specific pests and to ensure that the efficacy data is consistent with the registered standard. Agrochemicals registered for use on tobacco in Zimbabwe are mandated to comply to PASS guidelines. Non-Compliance may result in revocation of the PASS certificate. All tobacco farmers are therefore urged to only use agrochemicals that are recommended by the PASS office. 

If you have any queries regarding the above, or require further information, please feel free to contact, Kutsaga Research Station’s Plant Health Services Division on VOIP 08688002604 or Email: kutsaga@kutsaga.co.zw or visit Kutsaga Research Station, Airport Ring Road, Harare.