March 2012. It was thought that termites had damaged the Jatropha seedlings in a nursery in Binh Thuan Province, Vietnam. Roots are attacked and stems too, often at soil level. An identification was requested and methods of control.
The photos do not show termite damage, but rather that caused by the larva of a click beetle (Coleoptera family Elateridae). For a precise ID adults are required. The name click beetle is because of the sound made when the beetle is placed on its back: it can “snap” its body into the air and right itself. The colour of the adult varies from tan to black, and it is about 1-2.5 cm long.
There is always the chance that the damage was done by the false wireworm (larva of a Tenebrionid beetle such as Gonocephalum), but the structures at the posterior are against that.
Both types of insect damage plants below or at the soil surface, which fits the description. As for control, if the seedlings are in plastic ‘gro’-bags, and if the problem is not too extensive (numbers in the tens), open the bags with damaged seedlings and carefully replant them in fresh potting soil. Search for and obviously destroy the larvae. If the damage is more extensive, then use an insecticide, one that breaks down quickly once applied. It would be best to apply a largish volume of a low dilution to each each bag and follow up – perhaps next day with a normal watering.
The question remains where the beetles came from? It could be beetles flying in and laying eggs in the attractive environment of the nursery or the eggs and/or larvae could have been in the propagation mixture when the seeds were planted. It is important to thoroughly compost the manure before using it as a potting medium.
A member who grows Jatropha commercially in Cambodia said that it is important to establish an economic threshold before deciding on treatment. For large nurseries producing seedlings to establish 100 ha of Jatropha or more, it is not economical to treat them if there is less than 5% damage. Simply remove the damaged bags. If more than 5%, then treat with an insecticide. Some comments were made about the use of carbofuran, and its toxicity. It is not a chemical that can be recommended, and it has been banned in may countries because of its efect on wildlife generally, but birds in particular. It is a very toxic carbamate compound.