Why Sesamia Nonagrioides (Lefèbvre) (Lepidoptera : Noctuidae) Is a Pest Only in Portions of Its Geographic Range: the Influence of Climate
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Why Sesamia nonagrioides (Lefèbvre) (Lepidoptera : Noctuidae) is a pest only in portions of its geographic range: the influence of climate. Pascal Moyal IRD, 32 avenue Henri-Varagnat, 93140 Bondy, France. [email protected] Abstract. The stem borer Sesamia nonagrioides (Lefèbvre) (Lepidoptera : Noctuidae) is a major pest of maize in Mediterranean Europe and in several countries of sub-Saharan West Africa but not in East Africa where it lives on wild host plants along wet places. Sampling of Sesamia species in maize fields in Côte d’Ivoire during five years revealed high densities of S. nonagrioides in the southern forest regions, and showed that it was absent in crops in the northern half of the country, a savannah region where Sesamia calamistis was the major pest. A regression analysis showed that rainfall and temperature explained 81% of the variation of the density of S. nonagrioides in maize crops. The model predicts no infestation in the cool and dry conditions of East Africa. This conclusion is in agreement with the results of the study of migration from mitochondrial genes that showed that the species moved highly only along wet places in East Africa. In southern France, where S. nonagrioides is a pest of maize, the model predicts no infestation. This contradiction may be explained by (i) the selection of individuals adapted to cold and dry climate, (ii) The diapause, that occurs only in the palearctic region and (iii) the possible diffusion by humans. Heavy damage in maize crops by this pest in France are mentioned only from 1836, and were not mentioned in a comprehensive book on maize by Parmentier in the late eighteenth century. The colder temperatures of the little ice age may explain that no infestation occurred in maize fields at that time. In West Africa, S. nonagrioides is absent in the countries East of Togo in spite of favourable climate, which is likely due to the Dahomey gap, a dry region that splits the forest. 1. Introduction botanephaga Tams & Bowden [1]. Later on Nye [2] observed that their morphology The maize stem borer Sesamia was nearly identical and named them two nonagrioides (Lefèbvre) (Lepidoptera : subspecies, a status that was recently still Noctuidae), has a wide and fragmented regarded as uncertain [3]. Molecular data geographic distribution that extends over enabled to conclude that these taxa do Mediterranean Europe, Northwest Africa indeed belong to the same species but that and equatorial and tropical regions of sub- they must not be regarded as sub-species Saharan West Africa and East Africa. [4]. Actually, in the sampled regions, The taxa on either side of the Europe and sub-Saharan Africa, the Sahara were first considered different species includes three geographically species, S. nonagrioides and Sesamia isolated populations. In sub-Saharan 2 Africa, the West and East populations were this species in Europe. Furthermore, S. separated a long time ago, estimated at 1 nonagrioides has apparently not always Myr from the mitochondrial distance of been a major pest of maize as it is 2.3%. The fragmentation between the currently. Indeed, in an exhaustive report African and European populations is much on maize in France in the late 18th century, more recent and it was shown that the most the famous agronomist Parmentier did not plausible hypothesis is that the species mention any borer damage [15]. The crossed the Sahara during the Last species was described only in 1827 [16], Interglacial, about 100 kyr ago [4]. This but its damage in the stems and cobs is was the first study that showed the easily visible and should have been considerable influence of abiotic factors on observed in case of strong infestation. the distribution of this insect, that was able Parmentier lists several other pests (insects to colonize new continents when climate and fungi) known by their local name only, became warmer and wetter. some of them less damaging than S. Sesamia nonagrioides is a rather nonagrioides. If this species was present in generalist species unlike the most part of Europe at that time, as is suggested by the other species of the subtribe Sesamiina genetic data [4], the question arises as to which are specialized on one or very few why it would not have infested maize in host plants [5-8]. Larvae of S. southern France, and particularly whether nonagrioides feed in the stems of many this might be due to climatic factors. Poaceae, Cyperaceae and Typhaceae [2, 9]. The aim of the present paper is to This nutritional plasticity suggests that host gain a better insight into the influence of plants must not have a major influence on these factors on the distribution, the distribution and abundance of this abundance, and pest status of S. species, contrary to more specialized nonagrioides. species which have, for the most part, a localized distribution [6-8]. In contrast, 2. Materials and Methods several observations suggest that the influence of abiotic factors on this widely 2.1. Population density in Côte distributed species might be strong. For d’Ivoire instance, S. nonagrioides is a major pest of maize in several countries of West Africa During five years, from 1986 to [10-12] but not in East Africa. For example 1990, the population density of Sesamia in Kenya, where maize is one of the main species has been estimated in maize fields crops, this pest is very rarely or not at all grown in the various ecological zones of found in fields [13-14]. These differences Côte d’Ivoire. In this country, rainfall in abundance might be a consequence of decreases from the southern forest region abiotic factors. In Europe, to be able to (about 2000 mm/year) up to the northern survive, this tropical species had to adapt savannah region (900-1000 mm/year) (Fig. to climatic conditions very different from 1) ( modified from [17]). Concomitantly, those in its region of origin. An example of there is a strong reduction in relative this adaptation is the diapause during cold humidity, from 85% to 65%. Temperature months, which does not occur in the decreases from East (annual average of tropics. Diapause enables a better about 26°C) up to West (24-25°C) (Fig. 1). resistance to hard winter conditions but The country is divided into two main within certain limits only. Sesamia ecological zones (Fig. 2). The Guinean nonagrioides is indeed restricted to the zone, roughly the southern half of the Mediterranean regions, at latitudes lower country, is a region of forests, and is than 45° North in France [9]. Temperature divided in three sectors. The southern part, therefore has likely a strong influence on the Ombrophil sector, is a region of 3 evergreen forest; the northern part, the estimated in maize crops planted in June- preforest sector, is a transition region and July, the only ones that can be grown at the comprises of a forest-savannah mosaic; same time in the North of the country, between both, the Mesophil sector is a where there is only one rainy season per region of semi-deciduous rain forest [18]. year, and in the other regions. Several The Sudanese zone, roughly the northern hundred plants were sampled in each half of the country, is drier; it is the location, between the soft dough stage and savannah region, which becomes drier maturity (at about 80 days after emergence, from its southern part, the sub-Sudanese 20 days before maturity), at the peak of sector, to its northern part, the Sudanese borer density. The insects collected after sector. Altitude is low in Côte d’Ivoire, dissection were reared until the adult stage less than 400 m, except along the western and identified. In two southern localities, border, from Man up to the North, where it San-Pédro and Abidjan, only the is between 500-900m, with peaks of more proportion of the different Sesamia species than 900 m surrounding Man [19]. was estimated because the maize crops The population density and relative could not be planted at the same dates as in proportions of Sesamia species were the other regions. 4 Figure 1. Annual rainfall and average temperature in Côte d’Ivoire (1961-1975) (modified from [17]). 5 2.2. Gene flow between populations The migrant number between In order to examine whether the populations per generation (Nm) was differences in maize infestation might calculated following Slatkin’s method [23] result from variable migratory ability, gene using Arlequin software ver 3.1 [24]: one flow between populations was estimated individual exchanged between two within West Africa and East Africa. In populations indicates a gene flow sufficient West Africa, three populations were to prevent the fixation of neutral alleles sampled (Fig. 4), in Côte d’Ivoire (35 [25]. individuals), in Ghana (21 individuals) and in Togo (four individuals). In East Africa, five populations were sampled in Kenya: three in close localities next to Lake Victoria (31 individuals), one in the Rift 3. Results Valley (three individuals) and one to the East of the Rift Valley (eight individuals) 3.1. Population density in Côte (Fig. 4). d’Ivoire Sequence data were obtained from Three Sesamia species were a fragment of the mitochondrial gene collected in maize fields in Côte d’Ivoire Cytochrome b (915 nucleotides). Total (Fig. 2). Sesamia penniseti Tams & DNA was extracted using a Qiagen Bowden was very rare; only some DNeasy tissue kit (Qiagen GmbH, individuals were found in Bouaké, in the Germany) and the gene was amplified by centre of the country. This species feeds in Polymerase Chain Reaction (PCR) using fact mainly in a wild host plant, the successive steps: initial denaturation Pennisetum purpureum [8] and is not a for 5 min at 92°C; 39 cycles of major pest of maize.