SCARABAEIDAE (WHITE GRUB) DIVERSITY IN SUGARCANE FIELDS IN THE ZIMBABWE SUGAR INDUSTRY: PRELIMINARY RESULTS FOR THE 2011 CROPPING SEASON

Transcription

1 SHORT, NON-REFEREED PAPER SCARABAEIDAE (WHITE GRUB) DIVERSITY IN SUGARCANE FIELDS IN THE ZIMBABWE SUGAR INDUSTRY: PRELIMINARY RESULTS FOR THE 2011 CROPPING SEASON MABVENI AR 1 AND MGOCHEKI N 2 1 Zimbabwe Sugar Association Experiment Station, P/Bag 7006, Chiredzi, Zimbabwe 2 Stellenbosch University, Dept. Conservation Ecology and Entomology, P/Bag X1, Matieland, 7602, Stellenbosch, South Africa amabveni@zsaes.org.zw zee@sun.ac.za Abstract In Africa and surrounding Indian Ocean islands, several species of scarabaeids cause damage to sugarcane. Although many factors affect the number of ratoon crops obtained from sugarcane planting, scarabaeid infestations can drastically reduce the ability of the crop to ratoon. This factor is often neglected in assessments of scarabaeid damage. Despite the importance of white grubs as pests in sugarcane production, it is not known for certain how many species of white grub attack sugarcane in this region. White grub taxonomy is important for developing effective control tactics. This study seeks to identify and classify Scarabaeidae life forms collected from sugarcane fields. Pits were dug in sugarcane fields, across cane lines, uprooting sugarcane stools. Searches for all developmental stages of white grubs were done in soils around cane roots and soils removed from the pit. Larvae and pupae were transferred, singly, into labelled rearing vials containing moistened peat. The vials were incubated in the laboratory at ambient temperature and checked periodically to monitor development. Adults collected from pits and peat were kept in a freezer, in labelled transportation vials. Beetles were also collected from ultraviolet (UV) light traps at ZSAES, Triangle Ltd and Mwenezana estates. Four hundred and six samples were dispatched to Stellenbosch University for identification and classification. Some larvae collected from the field and reared in peat completed their life cycle under laboratory conditions. The scarabs included in the collection comprised chafer beetles, rose beetles, rhinoceros beetles and black maize beetles. Keywords: scarabaeids, taxonomy, sugarcane, UV light trap, peat, white grub species Introduction Sugarcane damage by scarabaeid (Coleoptera: Scarabaeidae) has become prevalent in sugar industries in Africa and surrounding Indian Ocean islands (Conlong and Mugalula, 2003; Dittrich et al., 2006; Mugalula et al., 2006). Scarabaeids damage sugarcane in the larval or adult stage or in both stages. In southern Africa, species associated with sugarcane belong to the Scarabaeidae subfamilies Melolonthinae, Rutelinae, Dynastinae and Cetoniinae (Scholtz and Holm, 1996; Way and du Toit, 1996). Scarabaeids are characterised by a lengthy larval life cycle and short pupal and adult periods (Wilson, 1969; Dittrich-Schroder et al., 2009). 103

2 Larvae are soil dwelling and feed on roots of plants sometimes used as food by the adult stage. In southern Africa, adult Heteronychus licas feed on young sugarcane shoot tissue and the larvae feed on sugarcane roots (Wilson, 1969; Way and du Toit, 1996). It is not known for certain how many species of scarabaeids (white grubs) attack sugarcane in southern Africa, despite the importance of this information for developing pest management strategies. The aim of this study was to identify and classify Scarabaeidae life forms collected from sugarcane fields. Materials and Methods Sugarcane fields where pits were dug were selected from lists supplied by industry estate Agronomists. These fields had a history of black maize beetle (Heteronychus licas) attack on young sugarcane during the rainy season (November to March). In 2010/2011 pits measuring (50x50x50 cm) were dug across the cane-lines, uprooting the sugarcane stools. The pits were positioned 50 m from the edge of the field and 70 to 100 m along the cane-line. Five pits were dug per 10 hectares. Four pits were along the edges of the field, while the fifth pit was at the centre of the field. Three to five fields were sampled per month. Searches for all developmental stages of white grubs were done in soils around cane roots and soils removed from the pit. Numbers of larvae, pupae and adults collected per pit were recorded. Larvae and pupae were transferred, singly, into labelled rearing vials containing moistened peat. The vials were incubated in the laboratory at ambient temperature and checked periodically to monitor development. Adults collected from pits and those that emerged from peat in the lab were kept in a freezer, in labelled transportation vials. Beetles were also collected from ultraviolet (UV) light traps at the Zimbabwe Sugar Association Experiment Station (ZSAES), and Triangle Ltd and Mwenezana estates. All samples were dispatched to Stellenbosch University for identification and classification. Results Thirty-five sugarcane fields were sampled over a six month period. Varied numbers of larvae, pupae and adults were collected from the pits (Figure 1). While larvae were constantly present in the soil, adults started to appear in the samples only during the last quarter of the year (October to December). From the larvae that were incubated in moist peat in the laboratory at ambient temperatures, 101 adult insects emerged in the vials. These included robber flies (Diptera: Asilidae) and several species of Scarabaeidae, as illustrated in Figure

3 No. of specimens per pit Mabveni ARS and Mgocheki N Proc S Afr Sug Technol Ass (2012) 85: larvae pupae adults june july aug sept oct nov dec month of sampling Figure 1. Average number of larvae, pupae and adults collected from soil pits dug in various sugarcane fields during a six month period in 2011, at Triangle Ltd and Hippo Valley estates. Rose beetle Black Maize beetle Robber fly Figure 2. Some of the insects recovered in the laboratory from rearing vials containing peat. 105

4 Discussion Scarabaeidae have long life cycles, normally one generation per year (Wilson, 1969; Scholtz and Holm, 1996; Picker et al., 2004). Larvae are consistently present in the soil, with adults emerging once a year for short periods (Conlong and Mugalula, 2003; Mugalula et al., 2006). In this study, adults emerged in the laboratory after larval incubation periods that ranged from four to six months, thereby confirming the long life cycle pattern. In the field, more adults were collected from the soil pits in the last quarter of the year (October to December). This also confirmed earlier indications that adult Scarabaeidae emergence is triggered by the onset of rains (Mugalula et al., 2006). Several species of Scarabaeidae emerged from larvae incubated in the laboratory (Figure 2). This is a strong indication that there is a species complex of scarabaeids in the soil in sugarcane fields in the Zimbabwe sugar industry. It has been reported that, while a diverse group of white grubs maybe found inhabiting the soil, only a few are pestiferous (Wilson, 1969; Scholtz and Holm, 1996). In Zimbabwe and Swaziland, the most damaging scarabaeid in sugarcane is Heteronychus licas (Carnegie, 1988), while in Tanzania it is Cochliotis sp and in Uganda it is Idaecamenta eugeniae (Conlong and Mugalula, 2003; Mugalula et al., 2006). In the South African sugar industry, Hypopholis sp. and Schizonycha sp. have been associated with sugarcane damage (McArthur and Leslie, 2004). Conclusion There is a diverse group of scarabaeids in soils in sugarcane fields in the Zimbabwe sugar industry. This group includes Heteronychus licas, which has been identified as a major pest of sugarcane. Three life stages were collected from the soil. Larvae were present during all months of sampling, while adults were found from October through to December. Further field collections and light trap catches will be used to elucidate the diversity of white grubs in sugarcane fields. Acknowledgement This study is part of a SADC sub-regional project: ACP Sugar Research Programme (RPR/009/07) EDF IX Project 2.1: Use of bio-pesticides for the control of sugarcane white grubs ( ). REFERENCES Carnegie AJM (1988). White grubs (Scarabaeidae) continue to cause damage to sugarcane in South Africa and Swaziland. Proc S Afr Sug Technol Ass 62: Conlong DE and Mugalula AM (2003). Idaecamenta eugeniae (Coleoptera: Scarabaeidae: Melolonthinae): A new species of white grub in Ugandan sugarcane. Proc S Afr Sug Technol Ass 77: Dittrich G, Conlong DE and Mitchell A (2006). Molecular identification of South African sugarcane white grubs (Coleoptera: Scarabaeidae). Proc S Afr Sug Technol Ass 80: Dittrich-Schroder G, Conlong DE, Way MJ, Harrison J du G and Mitchell A (2009). Identification key to Scarabaeid beetle larvae attacking sugarcane in South Africa using DNA bar-coding and integrative taxonomy. Proc S Afr Sug Technol Ass 82:

5 McArthur DG and Leslie GW (2004). Preliminary observations on the impact of white grub on sugarcane yields in the Midlands North region of the South African sugar industry. Proc S Afr Sug Technol Ass 78: Mugalula A, Tibakanya G and Conlong DE (2006). Recent developments in the control of Idaecamenta eugeniae (Coleoptera: Scarabaeidae: Melolonthinae) at Kinyara Sugar Works Ltd, Uganda. Proc S Afr Sug Technol Ass80: Picker M, Griffiths C and Weaving A (2004). Field Guide to Insects of South Africa. Struik Publishers, Cape Town, South Africa. 444 pp. Scholtz CH and Holm E (1996). Insects of Southern Africa. University of Pretoria, Pretoria, South Africa. 502 pp. Way MJ and du Toit MA (1996). White grub working group: Pest status, biology and rearing of white grub (Coleoptera: Scarabaeidae) pests in southern Africa: 6-7 June SASEX Technical Report, Mount Edgecombe, KwaZulu-Natal, South Africa. 23 pp. Wilson G (1969). White grubs as pests of sugarcane. pp In: JR Williams, JR Metcalf, RW Mungomery and R Mathes (Eds.), Pests of Sugarcane. Elsevier Publishing Company, Amsterdam. 107