Chilo partellus

AuthorsW.D. Hutchison

Professor, Department of Entomology, University of Minnesota, St. Paul, MN 55108

R.C. VenetteResearch Biologist, Northern Research Station, U.S. Forest Service, St. Paul, MN 55108

D. BergvinsonBill and Melinda Gates Foundation, Seattle, WA 98101

J. van den BergProfessor, North-West University, Potchefstroom, South Africa

IntroductionChilo partellus (Swinhoe), also known as the spotted

stem borer (SSB), likely invaded Africa from India some-

time before 1930 (Overholt et al. 1996). The pest now

inhabits much of eastern and southern Africa. SSB is

highly invasive, having partially displaced a related

stem borer species, C. orichalcociliellus in several areas,

including the low altitude and coastal maize regions of

Kenya (Ofomata et al. 1999a; Ofomata et al. 2000; Mbap-

ila et al. 2002). SSB also co-exists in many areas with

another economically important maize stem borer, Bus-

seola fusca (Lepidoptera: Noctuidae), particularly in the

moist mid-altitude and moist transitional agroecological

zones of Kenya (van den Berg et al. 1991; Polaszek 1998;

Abate et al. 2000; De Groote et al. 2003). Interestingly,

Kfir (1997) reported a partial displacement of B. fusca

by SSB, in the eastern Highveld region of South Africa,

at an elevation of 1600 m.

Known DistributionSSB is currently known to occur in India, Pakistan, Af-

ghanistan, Nepal, Bangladesh, Sri Lanka, Thailand, Laos,

Vietnam, Yeman and portions of Indonesia (CABI 2007).

In Africa, SSB is found in most eastern and southern

sub-Saharan countries, including Botswana, Cameroon,

Eritrea, Ethiopia, Kenya, Malawi, Mozambique, Soma-

lia, Sudan, South Africa, Lesotho, Swaziland, Tanzania,

Uganda, Zambia and the island, Comoros; SSB is also

found in Madagascar (CABI 2007; De Groote et al. 2002;

2003; Haile and Hofsvang 2001; Kfir et al. 2002; Tefera

2004). There is one report of SSB in Australia (Ampofo

and Saxena 1989). SSB distribution is highly influenced

by altitude and moisture gradients. In Kenya, for exam-

ple, SSB populations are most common in the dry mid-

altitude and dry coastal areas, but the pest also occurs

in the moist-transitional and moist mid-altitude (ca.

<1500 m) agroecological zones (De Groote et al. 2003;

Muhammad and Underwood 2004).

PEST DISTRIBUTION PROFILE

©2008 HarvestChoiceDeveloped in cooperation with the HarvestChoice Workshop, CIMMYT (June, 2007)

Pest Profi le #HC001V05 | Pest Name: Chilo partellus (Swinhoe) (Lepidoptera: Crambidae)

C. partellus early-instar larva. (© J. van den Berg, North West University, Potchefstroom, SA)

C. partellus late-instar larva. (© J. van den Berg, North West University, Potchefstroom, SA)

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Description and BiologySSB exhibits complete metamorphosis, including egg, larval, pupal and adult stages. SSB will complete 1, 2 or more generations per year, depending on the location and the number of maize crops or other hosts avail-able throughout the year (Muhammad & Underwood 2004). One aspect of SSB biology, that is relevant to understanding its population dynamics and geographic distribution, is the strategy for diapause or the “resting period” (e.g., Kfir et al. 2002). In warm low-altitude re-gions, and with ample crop and grass hosts to maintain larval populations, SSB will cycle throughout the year. SSB will initiate diapause, in the larval stage, at higher elevations or during dry seasons (Kfir et al. 2002).

Female SSB moths typically prefer whorl (or “funnel”) stage plants for oviposition. Depending on temperature, eggs hatch in 7-10 days, larvae complete development in ca. 28-35 days, and pupae require ca. 8-10 days. SSB eggs are oval-shaped, flattened and laid in clusters, with total fecundity averaging 100-166 eggs/female (Ofomata et al. 2000). Most eggs in maize are found on the lower surfaces of the leaves, often near the midrib. Upon hatching, early instars move upwards on plants and into the whorl, where they begin feeding on leaf surfaces deep inside the whorl. Mid to late-instars bor-ing into the midrib and stalk, respectively. They may also move down the outside of the stalk, and bore into the stalk just above an internode. In older plants larvae will sometimes feed on the developing tassel.

In many ways, the larvae look similar to those of B. fusca (maize stalk borer). The larvae have a cream to pink col-oration, with dark spots along the dorsal surface; the head capsule is brown. When mature they are about 25 mm long. SSB larvae can be distinguished from B. fusca by the hooks on the prolegs. In C. partellus, the hooks are arranged in a complete circle, whereas in B. fusca the hooks are arranged in a crescent. SSB larvae pupate within the maize stalk. Prior to pupation the fully grown larva cuts out an exit hole to allow the adult moth to emerge from the plant.

Primary Host Crops and PlantsMaize (Zea mays L.), Sorghum (Sorghum bicolour L.), Rice (Oryza sativa); Sugarcane (Saccharum officinarum) and several millets including Pearl millet (Pennisetum glaucum); several grasses including Sudan grass (Sor-ghum vulgare sudanense), Napier grass (Pennisetum purpureum), and Sorghum arundinaceum (Devs.) Stapf (CABI 2007; Kahan et al. 2000; Kfir et al. 2002; Matama-Kauma et al. 2008).

C. partellus life cycle (Push-pull.net, International Centre of Insect Physiology and Ecology [ICIPE]), African Insect Science for Food and Heath, Kenya.

Death of the tassel area caused by C. partellus, often referred to as ‘deadheart.’ (© Ethiopia-IPM Program 2008)

Damage caused by C. partellus feeding on whorl stage corn. (© J. van den Berg, North West University, Potchefstroom, SA)

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Damage and Economic Impact to MaizeSSB injury to maize includes leaf feeding, tunnelling within the stalk, disruption of the flow of nutrients to the ear, and subsequent development of “deadhearts” (see photo). The first symptoms of SSB damage is the appearance of “shot-hole” injury to whorl leaves. “Dead-hearts” result from larval feeding injury to the growth point of maize plants; this damage is most important during the first 2-3 weeks after seedling emergence. Yield loss is attributed to the physiological effects on fi-nal ear size, lodging or the complete loss of ears. Larval tunnelling within the stalk may also predispose plants and ears to infection by fungal pathogens, further com-promising the long-term storability, and quality of food products (Polaszek 1998; Kfir et al. 2002).

Yield loss estimates for SSB vary greatly depending upon the country, season, maize variety and fertiliza-tion (e.g., Overholt et al. 1996; Kfir et al. 2002; De Groote et al. 2003). However, in studies with SSB alone, yields in east Africa were reduced by 15-45% (Seshu Reddy and Sum 1992). In South Africa, yield losses in maize and sorghum have exceeded 50% (Kfir et al. 2002). More re-cently, following an extensive survey of farmer fields in Kenya, with and without insecticidal control, De Groote et al. (2003) found that all stem borer species caused av-erage annual losses of 13.5%, valued at US$ 80 million. Of this total, losses to SSB were estimated at ca. US$ 23 million/year; the majority of other stem borer losses were attributed to B. fusca. The analysis was based on maize prices of $193/ton.

Potential Geographic Distribution of C. partellus using CLIMEXParameter EstimationCLIMEX is a flexible modelling and mapping tool for integrating plant or animal species biology and ecology, relative to temperature and moisture regimes that support or limit population establishment and growth (e.g., Sutherst et al. 1999; Venette and Hutchison 1999; Venette and Cohen 2006). CLIMEX can also be used to “match” the climate of a given species current distribution, with the climate of one or more additional countries of inter-est, for example to assess the risk of exotic pest introductions (e.g., Koch et al. 2006).

The CLIMEX map for SSB was developed using a two-step process. We first used the known distribution of SSB throughout the diverse

agro-ecological zones in Kenya to assess the potential distribution for all African countries. We then relied on the extensive literature for SSB biology and ecology, to “fine-tune” parameter estimates for temperature-de-pendent developmental rate, and sensitivity to mois-ture and heat stress (Ochieng et al. 1995; Polaszek 1998; Ofomata et al. 1999a; Ofomata et al. 1999b; Ofomata et al. 2000; Kfir et al. 2002; Mbapila et al. 2002).

CLIMEX Maps, ResultsThe maps generated in CLIMEX are based on known or inferred relationships between population growth and

Fig. 2. CLIMEX Environmental Index (EI) map for C. partellus in Southern Asia.

Fig. 1. CLIMEX Environmental Index (EI) map for the known C. partellus distribution in Kenya, and projections for all African countries and Madagascar.

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temperature, soil moisture, heat, cold, and drought, us-ing 30-yr average monthly temperature and rainfall data available for each country (Venette and Hutchison 1999). Within CLIMEX, an Ecoclimatic Index (EI) >24 is consid-ered highly favourable “on average” for year-round per-sistence of a species in a given geographic area. As the EI increases, the potential population size also increas-es. The EI for C. partellus in Africa is shown in Fig. 1. As expected, we found good agreement with the projected EI results for Kenya’s agro-ecological zones and recent survey results (De Groote et al. 2003). The EI values are greatest in Kenya for the coastal, low-altitude, dry mid-altitude, moist transitional areas, with some EI in the highlands.

The model was also validated qualitatively by reviewing the predicted distribution range for SSB for most south Asian countries (Fig. 2). These results did indicate SSB presence in India and Pakistan, and further south to In-donesia. The Asia map also suggests potential distribu-tion areas within China.

The results for several eastern and southern African countries are in agreement with known distribution re-cords (e.g., Kfir et al. 2002; De Groote et al. 2003; Tefera 2004; Matama-Kauma et al. 2008). Interestingly, the CLI-MEX model also projects possible future expansion of SSB into several western African countries, where the pest is not currently known to occur. These results are similar to those of Overholt et al. (2000), who used a GIS model to predict future distributions of SSB in west Africa, based on the climate of the known SSB locations. Additional analyses are planned using individual years of weather data to further explore the variance associ-ated with the CLIMEX results over time.

In addition to the CLIMEX maps for Africa and Asia, we provide a global map for the expected distribution of C. partellus using the same parameter estimates for climate-matching and pest biology. The global analysis does indicate that many additional countries could be conducive for stem borer establishment in North, Cen-tral and South America, the Caribbean and Europe. Ad-ditional aspects of this analysis will be published in fu-ture Harvest Choice publications.

Fig. 3. CLIMEX Environmental index (EI) map for C. partellus, worldwide.

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