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These unassuming beetles have been observed on macadamia nuts for some time, but numbers and concomitant damage levels were low. However, things changed this season when several processors simultaneously reported problems.
Damage and ecology
These beetles are not native to South Africa yet Carpophilus hemipterus have been observed in the country as long ago as 1979. Due to the presumed unavailability of natural enemies, population increases are rapid and infestations can be significant. Researchers of the Free State University recorded this beetle feeds on prickly pear and mention that it could be implicated as a vector of a range of plant diseases. No link between disease transmission and these beetles in macadamias has been found but little research on the subject has thus far been done.
The beetles are small (about 3mm long) broad and flattened with the wings shortened that leave between one and three segments of the abdomen exposed (Figure 1).
Although very little is known about these beetles, infestations originate in the field and are then transported to the processing facility where damage becomes significant. Females are lured to fermenting fruit and it was observed that oviposition may be preferred on damaged fruit. Although no dedicated study was done on the insect, Nelmak varieties were found to harbour large amounts of beetles.
This is presumably because nuts of this cultivar sometimes tend to split open along the suture line when approaching maturity. Larvae are approximately 6mm long and are cream coloured and the head capsule and end of the abdomen are dark brown. The larvae have three pairs of true legs as well as horn like structures on the anal end. Larvae can be found feeding inside the husk on decaying tissue and they can enter the shell either through the hilum or sometimes even through the shell (Figure 2).
Partially constructed holes in the shell can often be seen and in severe cases, a nut may have several holes. Once inside, the larvae consume the entire nut turning it into a meal like consistency.
These insects have been associated in other cropping systems with wet environmental conditions and it was assumed that current high numbers are associated with abnormally high rainfall experienced during the previous season. Prolonged wet conditions may have led to nuts remaining on the soil for longer periods.
These beetles have been observed on macadamia nuts in the past, but numbers were generally low.
However, the number of nuts laying on the ground due to the wet weather with concomitant decaying husk tissue may have exacerbated the situation considerably.
Management of the beetles
Early harvest, sanitation and short nut pick-up cycles will reduce the food source of these beetles in the field. These insects have a very wide host range and the presence of any over ripe or fermenting fruit in or near a macadamia orchard may lead to high beetle numbers.
Because these insects are lured to mouldy or fermenting fruit, bucket traps can easily be made on farms that will lure large numbers of insects. Plastic buckets can be filled with rotten fruit, a bit of water and some yeast. The buckets must be deeper than they are wide as the beetles will then find it impossible to escape. No research on the efficacy of these bucket traps has been done so far but it is speculated that it could be used for control (mass trapping) as well as for monitoring. – Macadamias South Africa NPC (SAMAC)