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The grubs damage the roots mechanically, leading to stunted plant growth and reduced vigour. The leaves begin to dry; the stems are thin with short internodes. In case of a severe attack, the root system is impaired enough to allow the canes to be easily pulled out of the ground. Ratoon cane is less able to withstand Phyllophaga attack compared with plant (virgin) cane with older rations being weaker and more heavily attacked with the progressive buildup of soil infestation with each successive ratoon.
The control of Phyllophaga smithi in Mauritius was one of the biggest projects in the entomological history of the island, and has been reviewed by Moutia and Mamet (1946), Greathead (1971), and summarized by Williams (1961).
Introduced in Mauritius in 1906, this insect was, until 1954, the subject of intense scientific and technical investigations, farmer and public interest and participation.
Economic Threshold Levels
Infestation of sugarcane by P. smithi depends upon several factors: larval populations, cane variety, soil type, soil fertility, soil moisture, geographical region, agronomic practices, age of cane, and the incidence of other pests and diseases. All of these factors can modify the result of larval attack, and hence estimation of economic threshold levels is not easy. In general, a population of about 50,000 larvae/acre has been considered to be the level that can result in yield reduction (Williams, 1961).
A range of control methods including mechanical, chemical, cultural and biological were studied to bring down pest numbers. Even at present, inspection and monitoring are carried out regularly to check on grub and adult populations, not only as a preventive measure for protecting Mauritian agriculture, but also in the context of the quarantine regulations between Mauritius and Reunion.
Hand collection of adult beetles and digging out grubs from soil were the earliest control measures adopted in Mauritius. An astounding 4,006,620,699 beetles and grubs were collected and destroyed between 1911 and 1938 (Moutia and Mamet, 1946).
Other measures attempted included ploughing, forking, trash layering, trash burning before harvest, subsoiling, but the degree of success in these cases was negligible (Moutia, 1936). Use of plant oils and extracts, extracts of Phyllophaga female sexual glands as attractants, light traps and even the early insecticides did not appear to make any significant difference to the pest infestation (Edwards, 1928b as quoted by Moutia and Mamet, 1946; Jepson and Moutia, 1939).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
P. smithi is an insect of quarantine interest in Mauritius, with efforts made to prevent its spread to the neighbouring island of Reunion. However, it is worth recording its history in Mauritius and the economic impact of this pest in the island in the early part of the twentieth century.
The first outbreak of P. smithi in Mauritius was in 1911, 5-6 years after its accidental introduction from Barbados, probably in soil associated with rooted planting material. Over the next 20-30 years, the insect spread to approximately 60% of the sugarcane acreage, and by 1944 was recorded over practically the whole island. During the early stages of the pest spread, up to 200 larvae per cane stool was not uncommon, giving infestation figures of about 300,000 larvae/acre, with 100% yield loss in severely infested fields. By the mid-1940s this figure had fallen to about 80,000 larvae /acre with yield losses of 10-15% in terms of cane per acre.
A combination of natural and anthropogenic factors (biological control, improved cane varieties, improved cultivation methods) helped to bring down Phyllophaga populations during the 1940s, and 1950s, and by the mid-1950s, Phyllophaga was no longer considered to be a serious pest of sugarcane in Mauritius.