CHAPTER 2: Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) colony
2.1 Introduction
Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) is a serious pest in Asia and southern African countries (Arabjafari and Jalali, 2007). The main host plants of C.
partellus include crops such as maize (Zea mays L.), sorghum (Sorghum bicolor L.
Moench), rice (Oryza sativa L.) and pearl millet (Pennisetum glaucum) (Sallam and Allsop, 2002). C. partellus can infest the crop from seedling stage up until maturity and affects the entire plant except for the roots. C. partellus has adapted to
sugarcane in North Africa (Assefa and Conlong, 2009), and is present in the South African sugarcane agro-ecosystem (Hutchison et al., 2008). It may represent a threat similar to the one once posed by Eldana saccharina Walker (Lepidoptera: Pyralidae) before it adapted to feeding on sugarcane after which it became a major pest
(Rutherford, personal communication).The fast spread of C. partellus from Malawi, and the associated crop losses and damage due to this borer prove how serious this pest can be if left uncontrolled (Conlong and Goebel, 2002). Chilo sacchariphagus Bojer originated in south East Asia and has been a serious insect pest of sugarcane since the 19th century in Reunion, Mauritius and Madagascar (Rochat et al., 2001;
Conlong and Goebel, 2002). The potential for an invasion by C. sacchariphagus from Mozambique into South Africa poses a great risk to the South African sugarcane industry (Goebel, 2006; Bezuidenhout et al., 2008).
The use of host-plant resistance is one of the main methods of control for harmful borers such as C. partellus and C. sacchariphagus (Songa et al., 2001). The behavior of the borers is unpredictable and their numbers vary with changing
seasons. As a result, field trials depending upon natural infestations of the borers are
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unreliable for resistance screening studies. Artificial rearing of the target pests is therefore required. Incorporating leaf material of different sugarcane varieties into the artificial diet of insects can be used to test for the presence of constitutive antibiosis resistance mechanisms in different sugarcane varieties, which can be useful in
arthropod research (Blanco et al., 2009). In order for successful resistance screening studies to take place, a large supply of insects in sufficient numbers is required, which requires a suitable artificial diet for rearing and maintaining insects to be used in resistance screening studies (Songa et al., 2001). Thus, colonies need to be
established in a controlled environment. The success of the research depends on the supply of the insect in sufficient numbers and at the correct stages of its life cycle (Songa et al., 2001). Mass rearing can be defined as “the production of insects in numbers per generation exceeding 10 thousand to 1 million times the mean productivity of the native female population” (Taneja and Nwanze, 1990). The
maintenance of the colony involves periodically incorporating wild populations of the species into the laboratory colony. This is done in order to preserve and maintain the heterozygosity in the pest population and to avoid deviation from the natural pest behaviour (Onyango et al., 1994). A suitable rearing facility is required in order to establish colonies successfully. Requirements in the facility are sufficient laboratory space, equipment, diet components, trained staff and different rooms are required to perform specific functions in rearing insect colonies. The rooms where larvae and moths are kept should have conditions (temperature, humidity and light) suitable to those of the insect and representative to conditions in the field (Tende et al., 2010).
There have been various artificial diets used for rearing C. partellus, and that have been adapted and improved over time (Figure 2.1) (Taneja and Nwanze, 1990).
Generally, artificial diets of insects should contain portions of nitrogen, lipids,
carbohydrates, vitamins and minerals (Cohen, 2004). A list of the most important diet components and their functions in insects is shown in Table 2.1 (Cohen, 2004). It is important that the components in an insect’s diet be available in sufficient amounts, or else the insect will feed at a slower rate and less efficiently. In a number of cases, where economically important pests have been reared solely on artificial diet,
reduction in fitness and reproduction has occurred, which in turn leads to slower developmental times and reduced fecundity (Kega et al., 2010). This is often referred to as bottleneck stress, which occurs when insects are taken out of their natural
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environment and have factors imposed on them which did not occur in nature.
Stresses that can impose themselves to insects in a rearing environment include crowding, nutrition, temperature, humidity, antimicrobial agents and lack of feeding choices (Cohen, 2004).
C. sacchariphagus is not yet found in South Africa and therefore cannot be used to carry out studies in this country, unless under quarantine. The concept of ‘surrogate insect resistance screening’ will be explored, whereby C. partellus could be used as a potential surrogate for C. sacchariphagus in host-plant resistance studies. The concept is based on the fact that both these pests feed on the whorl of the plant, and both are top borers, and therefore similar resistance mechanisms within the plant may act against them equally. The concept of ‘surrogate insect resistance screening’
has proven successful within the borer genus Ostrinia on maize (Overman, 1994).
Cytochrome oxidase I subunit (CO1) barcoding allows for the identification of organisms by looking at the similarity of their DNA sequence to a set of reference taxa (Habeeb and Sanjayan, 2011). It allows for the discrimination of closely related species of lepidopterans and can therefore be used to select which target insects can be best used as surrogate insects in resistance studies (Herbert et al., 2003).
The objectives of this chapter were to (a) survey the sugarcane agro-ecosystem in KwaZulu-Natal for the presence of C. partellus and to identify its various hosts; (b) discriminate between borers using cytochrome oxidase I subunit barcoding for the selection of target insects for use as surrogate insects in resistance studies (c) describe rearing methods used for establishing and maintaining C. partellus at the South African Sugar Research Institute (SASRI); and (d) establish a new artificial diet, to carry out constitutive resistance studies on sugarcane varieties, with an increased proportion of cane leaf powder, without having a detrimental effect on the nutrient composition within the diet. Inoculation and rearing methods were also compared in order to establish which method would have the least negative impact on larval survival and growth.
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Figure 2.1 Diets used for mass rearing Chilo partellus (Taneja and Nwanze, 1990)
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Table 2.1 Important diet components and their functions in insects (Cohen, 2004)