May 2011. Adult Papuana are a problem in seedling nurseries in PNG. Efforts are made to avoid the use of insecticides, so the question asked was are there alternatives? what was wanted was the following:
- a good and reliable source of a pheromone
- a strain of Metarhizium for use against the larvae
- aplication rates for insecticides such as imidacloprid and bifenthrin for individual plastic bags
A member who was previously with the PRAP taro beetle project said the following:
The project was unable to find a pheromone for taro beetle. It seems beetles are attracted to the plants and then the mating occurs at the feeding site. Adult beetles soon after emerging from breeding sites were recorded in high numbers in taro fields in mark and recapture studies. Seventy percent (70%) of males remain in the plants feeding, while only 30% of females remained. The females were likely to visit breeding sites as they reach sexual maturity after one month, and then over the next three months lay 75% of their eggs. This is also the time when they are actively feeding and causing damage. A single mating is adequate, and fertilised females continue to lay eggs without further mating for another 3 months. Therefore pheromones may not be effective.
There are several strains of Metarhizium collected from Papuana and stocks are kept at CSIRO Entomology. The contact person was Dr Richard Milner but he has retired and hopefully colleagues in CSIRO have continued his work. With regards to PNG, two strains (Lowland strain collected at Ramu and Highlands strain collected from Kerowaghi) were purified and the lowland strain was cultured and tested in laboratory and field trials at NARI Kerevat. Application of Metarhizium in breeding sites was more effective as it is effective again all stages except the eggs. The adults can also move the fungus to other breeding sites and into the feeding sites and infect other beetles. On one occassion, 22 beetles were found under one plant, so it is possible for fungal transmission at such high densities.
Trials were conducted only on taro where chemicals are applied twice, once at planting and then again after 3 months. Rates can be obtained form NARI Kerevat where the trials were done.
A member from FAO mentioned that imidacloprid is a neonicotinoid with documented serious adverse effects on non-target insects – in particular bees and other pollinators. Many countries have taken action to restrict or ban imidacloprid. Its toxicity is WHO class II (hazardous pesticide), and is certainly not a good choice.
[Editor: it has to be noted that in this instance, imidacloprid is used as a soil drench on a plant that seldom flowers naturally; however, its adverse effects on bees, in particular, is proven, and has been thought a component of the worldwide decline in bee numbers known as Colony collapse disorder].
In a supplymentary question to PestNet, a member asked about the life cycle of Papuana beetle compared to Holotrichia, beetles that are common in India. This is the reply:
Taro beetle larvae (Papuana spp.) are most likely to be found in and around rotting organic matter. This means in and under rotting logs, under fallen palm fronds or banana plants, inside compost heaps, etc. The implication is that when farmers clear their fields of organic ‘trash’ and pile it around the field they create ideal propagation sites for taro beetle. The beetles attack the taro (corms). The females are apparently able to migrate freely between feeding and oviposition sites. (Question: is it possible to set up artificial oviposition sites – piles of organic trash – around fields and destroy the larvae that develop therein before they become adults? I believe this was investigated in Keravat, PNG). It would seem logical to treat these piles of trash with Metarhizium – potent strains were isolated in Canberra by Dr. Milner and assayed well in PNG (see Roy Masamdu’s message). The white grubs which are of concern in India, live in soil and feed on the living roots of plants – and have a particular liking for groundnut and sugar cane. Adults typically emerge at dusk at the time of the monsoon rains. They aggregate around or on certain trees, feed and mate and then return to the fields to lay eggs. As you will know synthetic pheromes (or caged calling females) can be placed in trees and interfere with mating by ‘confuing the males’. This can be combined with strategic insecticide applications or with the physical destruction of beetles (the idea is that community members beat the trees to which the beetles have been attracted with sticks to dislodge the beetles which are swept up and perhaps fed to the chickens). Interestingly, males and females of one of the Holotrichia spp. are attracted to the pheromone. In the case of taro beetles – it has been suggested that it may also be possible to interfere with the mating process through the judicious application of a pheromone (following successes with the rhinoceros beetle over the years). However, a pheromone has not been identified and synthetised as yet.
Sadly, farmers across the tropics, are not benefiting from this information because it has yet to become part of their ‘knowledge banks”.
A member also asked about the rate pf imidacloprid to use for seedlings in polybags. A leaflet produced by SPC is in the Gallery below.