CABI - Biocontrol News and Information 24(2) June News

Biocontrol News and Information

June 2003, Volume 24 No. 2

IPM Systems

This section covers integrated pest management (IPM) including biological control, and techniques that are compatible with the use of biological control or minimize negative impact on natural enemies.

Creating Semiochemical Diversions to Control Sugarcane Borers

Sugarcane has been cultivated for well over a century in KwaZulu-Natal and more recently in the Mpumalanga low-veld in South Africa. Currently, areas under sugarcane exceed 400,000 hectares, which feed a total of 16 sugar mills. It is grown commercially, creating revenue (about 2 million tonnes of sugar is produced annually) as well as employment opportunities (about 1 million people are dependent on the sugar industry) for the country. Around 66% of sugarcane produced is grown commercially by 2000 large-scale commercial growers, with the balance produced by 45,000 small- and medium-scale growers.

The pyralid moth, Eldana saccharina, originates from indigenous wetland and large-grass habitats in Africa, and is now the primary insect pest in sugarcane in South Africa. The females lay well-hidden eggs, so that they are protected from predators and parasitoids. The larvae bore into sugarcane stalks where they feed and ultimately pupate. Here too, they are well protected, not just from natural enemies, but also from the influence of insecticides. The larvae are the damaging life-stages, and as they bore into the sugarcane stem, they not only cause feeding damage, but also create entry points for fungal and bacterial infection into the sugarcane stalk, thereby further decreasing the sugar yield from the crop.

Specimens of E. saccharina have been collected in Africa since the early 20th century. Walker in 1865 first described it from sugarcane in Sierra Leone, West Africa. In West Africa it is a well-known pest of maize, sorghum and millet, but only a minor pest in sugarcane. In 1954 it was observed as a pest of sugarcane in East Africa. The first signs of E. saccharina in sugarcane in South Africa appeared in 1939-1940 near Umfolozi, KwaZulu-Natal, but the infestations dissipated about 5 years later. During 1970 a severe outbreak occurred near Hluhluwe, KwaZulu-Natal and E. saccharina regained its pest status. Since 1970 it has become widespread in the KwaZulu-Natal, Mpumalanga and Swaziland sugarcane belt. It is now the most damaging insect pest in sugarcane, causing on average 1% loss in sucrose for every 1% of internodes bored. The most recent estimates made in 1995 suggest that E. saccharina causes a revenue loss of R60 million [~US$7.7 million] per annum.

Various control methods are under investigation, but all have some limitations. One of the biggest constraints of insecticidal control is the cryptic nature of E. saccharina . The least cryptic life-stages are the adult moths or neonate larvae that have just hatched. These have been targeted in this approach, as they are exposed and vulnerable while mating and seeking feeding sites respectively. E. saccharina is multivoltine and occurs all year round, which would require continual applications of insecticides and/or more persistent insecticides, both of which are not always ideal because of cost and potentially negative environmental effects.

Cultural control of E. saccharina has proved successful to some extent. Measures include avoiding 'stand-over' cane (cane allowed to grow longer than the recommended 12-14 months in coastal conditions), whenever possible. In addition, cane is cut at or below ground level so as to prevent larvae in the stumps infesting ratoon crops; after cutting, all residue stalk material is removed, and exposed cane stumps are covered with soil to kill eggs and young larvae. Plant resistance research has produced varieties of sugarcane that are more resistant than others to E. saccharina infestation under South African conditions, but there are currently no varieties that are completely resistant to this pest.

The biocontrol programme at SASEX (South African Sugar Experiment Station) has investigated two sources of parasitoids to test against E. saccharina. One source is indigenous parasitoids of this insect in its natural habitats within South Africa and other African countries, ('translocation of natural enemies'). The other is parasitoids from related borers of graminaceous crops from other continents at the same latitudes as the southern African sugarcane belt ('new associations').

The ultimate aim is to find a parasitoid, from either source, which will accept and establish on E. saccharina in southern African sugarcane. Problems, however, have included climatic incompatibility of parasitoids introduced to South Africa, and the cryptic nature of E. saccharina in sugarcane, which places it out of reach of a large proportion of parasitoids collected from indigenous habitats. In addition, E. saccharina's ability to encapsulate parasitoid eggs and larvae, and the apparent inability of introduced parasitoids to remain in and search the sugarcane habitat for any length of time for the large number of hosts present in the crop are further compounding establishment attempts with parasitoids. Another constraint has been the recent discovery of the potential existence of biotypes of E. saccharina, the west and central African populations forming one group, and the east and southern African populations forming another. Parasitoids from one population may not perform as well on the other population.

The indigenous host plants of E. saccharina are large-stemmed grasses and wetland sedges in the genus Cyperus. It is believed that the host shift to sugarcane and other introduced crops occurred because these crop plants were planted into the wetland areas. The sedge habitats, in comparison to sugarcane, have high levels of parasitism of E. saccharina, and it is assumed that these parasitoids have not yet associated sugarcane with harbouring their insect hosts, and therefore do not readily forage in sugarcane. This has led Dr Des Conlong, from the Entomology Department at SASEX to initiate investigation into the potential of Stimulo-Deterrent Diversion (SDD) systems to control E. saccharina in sugarcane. This intercropping system has shown great success in Kenya by controlling Chilo partellus in maize (Dr Z. R. Khan, International Centre for Insect Physiology and Ecology, Kenya) and may have application in sugarcane.

SDD systems rely on trap plants that attract ovipositing females of pest insects away from the crop, and deterrent plant species that repel pests from the crop. By taking this system to the next trophic level, one can include plants that attract parasitoids of the pest into the crop areas. Parasitoids are believed to respond to chemical cues produced by the habitats of their insect hosts as long-range kairomones, before responding to short-range cues emanating directly from their hosts. This two-tiered effect can be exploited in an SDD system.

Our study has thus far focussed on potential trap plants for E. saccharina, investigating various grasses in comparison to sugarcane, tested in cage and field trials. Oviposition behaviour of female moths was investigated in cage trials by comparing egg batches and egg numbers laid on non-crop plants in comparison to sugarcane in two way choice tests. Initially, five grasses were tested and although eggs were laid on all of them, three species (Panicum maximum, Bothriochloa insculpta and Hyparrhenia dregeana) had an average of less than two egg batches and 20 eggs laid on them, in comparison to averages greater than three and 100 respectively in associated sugarcane plants. These three grass species are therefore not attractive to egg-laying females. Two grasses, wild sorghum (Sorghum bicolor) and Napier grass (Pennisetum purpureum) , received about 50% oviposition in comparison to sugarcane, and may make more adequate trap plants. This study reveals that ovipositing females show some level of host plant selection. Non-crop plants had either around 50% of total egg load in the cage or negligible numbers, which suggests that females may have a yes/no response in selecting hosts for oviposition, and for this reason repulsion may be more important than attraction in controlling E. saccharina.

In the field trial, where the same five grass species and sugarcane were planted in a quasi-complete Latin-square, E. saccharina larvae were recovered predominantly in sugarcane and wild sorghum, and only very rarely in P. purpureum. This indicates that female host plant selection is not related to offspring feeding sites. Survival of larvae fed on P. purpureum has been shown to be very low, which may explain the negligible numbers found on P. purpureum in the field. However, dead larvae were not collected, and borings were seldom noted in P. purpureum, which may indicate that the plant kills larvae at very early, pre-boring, stages or larvae disperse from P. purpureum. This finding initiated research into host selection in neonate larvae. Research is currently underway and is focussed on sugarcane and two sedges (Cyperus dives and C. papyrus), which are known indigenous hosts of E. saccharina, and the two grasses mentioned above.

Cage trials showed that E. saccharina females oviposited almost exclusively on dead leaf material (>99%) in all plants tested. However, when neonate larvae were offered a choice of dead or green leaf material in bioassay trials, they almost exclusively selected green plant material of all host plants tested. This reinforces the hypothesis that oviposition sites are not related to feeding sites. This finding is of importance, as the general belief was that young larvae fed in decaying material behind older leaf sheaths and/or in the soil before boring into the stalk at a later stage. In addition, neonate larvae in choice trials are showing a preference for the leaf material of their indigenous host plants (C. papyrus and C. dives) rather than sugarcane. What needs to be determined is the type of response of the larvae, whether it is to tactile or volatile chemicals. There is much conflicting data on dispersal capabilities of neonates, which still needs to be clarified so that testing can commence on larval preferences in cage trials. This research is promising with regards to trapping the insect pest, and shows the importance of not neglecting other mobile life-stages when considering an SDD to control insect pests.

Parasitoid response and activity to stalk borers in sugarcane and the five grass species mentioned above was also investigated. In the quasi-complete Latin Square field trial, natural parasitism of stalk borers was monitored. Parasitoid activity was greatest in wild sorghum, which was to be expected, as this grass is a natural host of E. saccharina and Chilo partellus. What is interesting is that parasitoids did not limit their foraging to wild sorghum, but parasitism was frequently observed in borer species collected from plant plots neighbouring wild sorghum. This is promising, especially as natural parasitism was observed in sugarcane plots neighbouring wild sorghum on two sides. Natural parasitism in neighbouring control sugarcane monocrops has been negligible. Sugarcane in the study site also had comparatively lower percentage internodes damaged in comparison to neighbouring control sugarcane, an effect that can be partly explained by parasitoid activity and partly by the variable habitat of alternate hosts for E. saccharina.

Laboratory studies, using two-way olfactometer and two-choice cage trials, were restricted to the ichneumonid wasp Xanthopimpla stemmator, a new association parasitoid from Chilo sacchariphagus in sugarcane in Mauritius, and the tachinid fly Sturmiopsis parasitica, a translocated E. saccharina parasitoid collected from maize in Benin, West Africa. Various Melinis species (Graminae) and silver-leaf desmodium, Desmodium uncinatum (Leguminosae) appear to be attractive to both parasitoids. Bothriocloa insculpta, a grass on which E. saccharina oviposition was lowest and which may be repellent to the moth, was attractive to S. parasitica. The effects of these grasses in the field still need to be evaluated. However, thus far indigenous parasitoids are attracted to certain indigenous hosts, and exotic parasitoids have shown responses to various grasses, which may aid in enticing parasitoids into our sugarcane environment.

The SDD system certainly has potential to control E. saccharina in southern African sugarcane. Eldana saccharina moths appear to show some selectivity of host plants for oviposition, and they are highly selective with regards to actual oviposition sites on those plants, choosing dead and senescing material and very cryptic sites. This study showed that some plants might have deterrent effects on female moths, although this still needs further research. Olfactometer trials showed female moths responded least to molasses grass (Melinis minutiflora - South African seed source), and this plant is currently being tested under field conditions as a potential repellent to the pest. Repellent plants need to be effective at the orientation stage of host searching by moths, not at a landing stage, because the aim is to repel the moths away from the general vicinity of the crop.

Plants that do repel moths almost certainly do so through the release of volatile chemicals, and this opens opportunities for synthesizing the active chemical components and utilizing these in the field. Perhaps a moth and parasitoid attractant may be discovered from indigenous host plants that could be synthesized and used, for example in baited traps for the pest. Once candidate push-and-pull plants have been identified, they need to be tested together under field conditions. Implementation of such a system in big commercial farms will also need some consideration. Possibilities of utilizing some of the plants in green manuring, or ensuring that non-crop plants utilized have other uses and values (preventing soil erosion, nitrogen fixing, cultural uses such as thatch grass, alternate crops, etc.), may aid in its application on farms. Integrated approaches and multiple benefits, such as these, will make the concept more appealing to farmers, and will hopefully together cause increases in sucrose yield, through reduced pest incidence and longer cropping cycle, and gain greater return per hectare from the land.

By: B. Kasl, D. E. Conlong and M. J. Byrne

Contact: Barbara Kasl, Postal Private Bag X02, Mount Edgecombe, 4300,
KwaZulu-Natal, South Africa
Email: Xentbk@sugar.org.za/Zanbk@yahoo.com

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