by Schalk Schoeman, Rersearch Extension Manager, SAMAC
The polyphagous shot hole borer was positively identified on macadamias in the South Coast during March this year. Various bark borer species were observed on macadamias since 2014 and their incidence has been increasing every year since then. This situation is not unique and other countries with subtropical climates such as Australia and the USA (Hawaii) recorded similar increases. Climate change and more particularly small but incremental increases in mean ambient temperatures have been blamed for this phenomenon. In countries such as Canada, the effects of global warming are much more noticeable in the form of receding glaciers and increases in CO2 emissions due to thawing of permafrost and peat bogs. In this context beetles that normally require 2 years to complete their life cycles can now do so in less than a year. This has led to exponential increases in the numbers of an indigenous species with the concomitant destruction of thousands of hectares of pine forest.
The danger posed by these insects is that some species can transmit an ambrosia fungus which rapidly aids in killing a tree. This fungus blocks the vascular bundles of a tree and if sufficient beetle numbers are available, they will overwhelm the defences of the trees leading to rapid death. While the fungus/beetle association is of high interest to academics as it is possibly the first steps in the evolution of social behaviour in beetles most farmers are less exited when these uninvited guests show up on the farm. Nature will in time balance this situation out but what can we in the meantime do in South African macadamia orchards to minimise the impact of these destructive beetles.
- It is said that these insects prefer stressed trees, so keeping trees as healthy as possible should be the first and obvious defence method. Stressed trees, especially in the early stages are not easily discernible and drones equipped with stress sensing cameras could provide an appropriate solution for early detection.
- According to literature, some beetles are attracted to ethanol or turpentine, but due to the volatile nature of these compounds, they are unfortunately unsuitable for long-term surveillance.
- Scouting teams should be specifically trained to be on the lookout for these pests and the best method will be to look for secondary symptoms such as dead or dying leaves as well as fine sawdust particles collecting in the crotches of the main branches as well as around the base of these trees. Noodle like projections consisting of fine sawdust can often also be seen on basal stem portions while some beetles make more open holes that is often very difficult to see. These openings are very small (and are therefore easy to overlook (± 1mm) and are scattered throughout the trees. Tree mortality can be very rapid as there is possibly an ambrosia fungus associated with at least some these insects.
- In the case of a positive identification, sanitation measures in the form of tree destruction should immediately be carried out. Trees should not be dragged through the orchards as beetles could escape into previously undamaged parts of the orchards. Affected stems should be cut into smaller pieces and immediately destroyed by burning. Stems of surrounding trees should be sprayed with a mixture of a general fungicide such as propconazole as well as an insecticide such as chlorpyrifos as a preventative measure.
- Growers with infested trees should immediately contact SAMAC as it is important to keep record of all infected locations to ensure that this problem is timeously and efficiently addressed. An important strategy for this group of pests is early detection and rapid response time as established colonies are often very difficult to manage once they are established in an orchard.