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Sigastus Weevil

Sigastus Weevil was initially found infesting macadamia orchards on the Atherton Tablelands in Far North Queensland in 1994/5. Fay et al in their preliminary report Sigastus Weevil – An Emerging Pest Of Macadamias in North Queensland (QDPI) stated that nuts may be attacked until shell hardening stage, and that larvae consumed whole kernel with larval duration being shorter in larger nuts. They went on to say that crop loss in an unsprayed orchard may be up to 30%.

It was noted the weevils were susceptible to Methidathion (Supracide®/Suprathion®) and Carbaryl (Nufarm Flowable Carbaryl 500 Insecticide®) applied as a control measure for Fruit Spotting Bug/Nut Borer. It was also noted that Beta- Cyfluthrin (Bulldock®25 EC) was less effective than other chemicals and took longer to control weevils. It was recommended that fallen nuts be swept into a windrow and mulched to provide a mechanical control for the larvae/eggs in the nuts in the September to December period.

Sigastus Weevils have been detected in the Clunes/Eureeka area for the past four years. In the 2014 growing season they were found across the Northern Rivers in isolated pockets. The reason for this expansion in territory is unknown. Some growers suggest it may result from the major storm event in January 2013 – with wind moving them around and the out of season flowering resulting from this event, giving the weevils access to a breeding source all year round.
Growers in the Clunes/Eureeka area whose orchards were affected by weevils have used a combination of insecticide applications (in conjunction with their Fruit Spotting Bug/Nut Borer control) and mulching of fallen nuts. There is variability in the success of the chemical control measures used and it is apparent timing and coverage is critical for success.

The winter of 2013 was very mild which produced multiple flowerings in the Northern NSW growing area. There were at least three distinct flower sets for the 2014 macadamia crop – which provided a constant supply of developing nutlets for the Sigastus Weevils to lay eggs in and feed on. Nutlets from about a 5 cent size were seen to have been attacked, with larvae inside.

Sigastus weevil on a macadamia nut showing scarified areas in the husk from weevil feeding (source NSW DPI)

The female weevil scarifies an area on the husk as shown in Fig 2. through which it inserts its ovipositor into the husk (above the unformed shell) or onto the surface of the developing kernel. After an egg is deposited, the female weevil chews into the nut stork and this usually causes the nut to fall from the tree about three days later. In some cases the nut doesn’t fall.

 

Fig 3. Sigastus larvae in husk/shell.

The egg hatches within the developing nut and the larvae proceeds to consume the kernel before emerging up to six weeks later as a fully developed weevil. The weevil then flies off into the orchard to recommence the cycle. Once shells of macadamia nuts harden the female weevil stops laying eggs into nuts as it is unable to penetrate the hard shell. They do however continue to feed on the husk of nuts. Although not fully understood, it is believed adult weevils live for many months. The difficulty with managing the weevil is little is known about it’s lifecycle. We seem to have continual feeding and egg laying which causes multiple generations to be present at the same time. Added to this the eggs are laid in the husk and the developing weevil larvae are protected by the husk. This makes the timing of insecticide applications difficult. With these difficulties in mind, MPC has been working with the NSW DPI Entomology team for alternative control options. This has involved a small pilot project with an affected MPC grower to ascertain if the Sigastus Weevil is susceptible to a naturally occurring fungus known to attack insects and to see if this can be used to give population control. All work in this pilot project is through the contribution of time and effort by the grower, MPC staff and NSW DPI.

This project is not funded by Horticulture Australia Limited (HAL) or the Australian Macadamia Society.


Growers story on his orchard Sigastus Weevil experience

Mark Bulmer

Mark and his father Eric operate a family owned orchard in the Clunes area of NSW. They have seen an increase in the population of Sigastus Weevils in their orchard during the past two seasons, with a significant increase running up to the 2013 season.

“It appears that A4 is particularly attractive to Sigastus Weevil in our orchard as it has had almost continual flowering and nut set cycles for the past two or three seasons. There are always nuts for the Sigastus Weevil to lay eggs into. We were keen to get control of this insect pest as its numbers were increasing particularly in 2013 and we were concerned by the damage levels. We applied sprays of Carbaryl, Suprathion and Acephate to control Fruit Spotting bugs/Nut Borer last season hoping we would also control this weevil. We swept nuts into wind rows and mulched them on a regular basis hoping to achieve control by mechanical means.

Unfortunately all these approaches didn’t work as we had hoped and we didn’t appear to get any real control until shell hardening, when the pressure eased off”, Mark said.

“When all the varieties we have set nuts in January 2014, during the drought, we noticed once again we had significant numbers of active Sigastus Weevils”, Mark said.

Through consultation with MPC, Mark collected 100 Sigastus Weevils in March 2014 and these were provided to NSW DPI. These weevils were sprayed with a naturally occurring fungus that is known to attack insects and placed in cages in the field to see what happens. After seven days it was noticed they had died and the weevils had the fungus growing out of them. A number of these dead Siguastus Weevils were then placed in containers with fresh live ones, so a new batch of weevils could be infected. As these weevils showed symptoms of infection, they were then released into the orchard. The aim being that if they were infected, they would come into contact with other weevils and spread the fungus.
Craig Maddox from NSW DPI said: “the fungus requires moist, humid conditions to work. So we waited until we had the right weather conditions before we commenced this trial”.

Mark Balmer said: “Half the collected specimens were returned to the orchard just after they were inoculated with a fungus. The first samples of weevils were watched for signs of them dying for seven days before I saw any effect. It started raining on day five and by day seven, seven had died. The increase in humidity appeared to trigger the growth of the fungus. It is easy to see the white fungus growing out of the insect’s body. The remaining infected Sigastus Weevils were released, three cadavers were pinned to trees and four were retained for further inoculation of future collected specimens. Several days later a Sigastus Weevil was found dead and covered with white fungus several hundred metres from the release site.

“A second release of weevils occurred in early April during a showery period; however this lot started to die within four days of being inoculated”.
Craig Maddox from NSW DPI said,: “The catch, inoculate and release of the weevils is just a start to be sure the fungus used is effective in killing the weevils. If it continues to show promise, we will look at a larger scale field trial to see if it gives results”. A blend of commercially available fungus with fungus of the same species used in the initial trial may be used in the orchard to test for control of Sigastus Weevils in the future.

Mark has also made many other observations of Sigatus Weevil. “The weevils have an interesting habit of hiding or rolling up and dropping off the trees when I approach the trees they are in. They also appear to be strong flyers,” Mark said.

Mark has also undertaken some of his own research. “I did a trial in my refrigerator at home to try to simulate a cold winter night to see if the weevils would be adversely affected by the cold. The weevils were placed in the refrigerator at 1ºC for 11 hours over night. I took them out in the morning and placed them out side in the open. They were all pretty sluggish to start with but by 10.00am when the temperature was about 22ºC they were once again looking pretty lively. So from my observation it might take more than a cold winter to stop these things!” Mark said.

It is very early days but the results from the research into the use of pathogenic fungi looks promising. The fungus used in this research is potentially a risk to anyone who is Immuno compromised, so precaution must be taken with its use – and the appropriate PPE must be worn.