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Insect Sci. Applic. Vol. 21, No. 4, pp. 395-402

CHANGES IN THE DISTRIBUTION OF LEPIDOPTERAN MAIZE STEMBORERS IN KENYA FROM THE 1950S TO 1990S

ZHOU GUOFA, WILLIAM A. OVERHOLT AND MOSES B. MOCHIAH

International Centre of Insect Physiology and Ecology (ICIPE), P.O. Box 30772, Nairobi, Kenya

Accepted 21 November 2001

Code Number: ti01049

ABSTRACT

Three hundred and ninety-two maize fields in the southern arable zone of Kenya were sampled for lepidopteran cereal stemborers from 1996-2000. Chilo partellus (Swinhoe) was the most abundant stemborer, and was found at all locations with elevations below 1500 m, and at some locations between 1500 and 2300 m. The highest density of C. partellus was in the semi-arid ecological zone of eastern Kenya. Chilo orichalcociliellus (Strand) was found in the lowland southern coastal area, and a few inland sites on the border of Tanzania in southeastern Kenya. Busseola fusca Fuller was dominant in highland areas. In the Lake Victoria Basin, which has an elevation of about 1100 m, B. fusca was dominant at some sites, but overall, C. partellus was more abundant. Sesamia calamistis Hampson was present at all elevations and all locations, but typically at low densities. Eldana saccharina (Walker) was found in two-thirds of the fields sampled in the Lake Victoria Basin. These results are compared with the distributions of the various stemborers in the 1950s to 1960s, as reported in the literature.

Key Words: stemborers, distribution, Chilo partellus, Busseola fusca, elevation

RÉSUMÉ

Trois cent quatre vingt douze champs de maïs ont été échantillonnés dans la zone arable du sud Kenya pour les lépidoptères foreurs de tiges entre 1996 et 2000. Chilo partellus (Swinhoe) est le foreur le plus abondant, il a été trouvé dans toutes les localités situées à une altitude inférieure à 1500 m, et dans certaines localités entre 1500 et 2300 m. La plus forte densité de C. partellus a été trouvée dans les zones écologiques semi-arides de l'Est du Kenya. On trouve Chilo orichalcociliellus dans les basses terres de la zone côtière sud et dans plusieurs localités de l'intérieur des terres près de la frontière tanzanienne au sud-est du Kenya. Busseola fusca Fuller est dominant dans les hautes terres. Près du bassin du lac Victoria, qui a une altitude d'environ 1100 m, B. fusca est dominant dans certaines localités, mais d'une manière générale, C. partellus est l'espèce la plus abondante. Sesamia calamistis Hampson est présent à toutes les altitudes et toutes les localités, mais à faibles densités. On trouve Eldana saccharina (Walker) dans deux tiers des champs échantillonnés sur le bassin du lac Victoria. Ces résultats sont comparés avec la distribution des foreurs de tiges des années 1950 et 1960, rapportée dans la litérature.

Mots Clés: foreurs des tiges, distribution, Chilo partellus, Busseola fusca, altitude

INTRODUCTION

Damage due to lepidopteran stemborers is a major constraint that limits the production of maize and sorghum in East Africa (Seshu Reddy, 1998). In Kenya, there are five major stemborer species: Chilo partellus (Swinhoe), C. orichalcociliellus (Strand) (Crambidae), Busseola fusca Fuller, Sesamia calamistis Hampson (Noctuidae) and Eldana saccharina (Walker) (Pyralidae) (Seshu Reddy, 1983). Among these, Chilo partellus is the only non-native-African species, having originated from Asia. This species has successfully invaded most maize-producing regions in East Africa, with infestations often causing a yield loss of 20-40 % (Seshu Reddy, 1988). Busseola fusca has been reported to cause 20-50 % yield loss (van den Berg, 1997), and the other three species are of lesser importance. Busseola fusca, C. partellus and S. calamistis are the most widely distributed species in Kenya, and have all been reported from both maize and sorghum (Nye, 1960; Seshu Reddy, 1983).

Nye (1960) published a detailed report on the distribution of maize and sorghum stemborers in East Africa based on surveys conducted from 1956 to 1958. On the basis of the distribution of the principal species, he divided Kenya into three regions - the coastal areas of up to 600 m altitude, the mid-elevation area from 600-1500 m, and the highland area, of over 1500 m. He found that in the coastal zone, C. partellus was the principal stemborer, followed in importance by C. orichalcociliellus. In the mid-elevation area, C. partellus was present, but B. fusca was the most abundant species. Chilo partellus was absent from the highland region, but B. fusca remained as the dominant species. Of the other borers, S. calamistis occurred at all elevations, but was particularly abundant in the highlands, and in more humid locations in the coastal and mid-elevation areas. Eldana saccharina was found in Uganda, where it was of minor importance, but was absent from Kenya.

In 1981 Seshu Reddy (1983) also conducted a countrywide survey of stemborers in maize and sorghum in Kenya, and reported distributions more or less similar to those of Nye (1960). Chilo partellus was found at elevations of 21-1676 m, while B. fusca was recorded at elevations >1088 m. However, in contrast to Nye's (1960) findings, Seshu Reddy (1983) found Eldana saccharina in southwestern Kenya, and only reported S. calamistis from the coastal zone and western Kenya.
There are now reports that C. partellus has become more important than B. fusca in some locations where B. fusca was previously the dominant species (Kfir et al., 2002; Songa, 1999). Moreover, E. saccharina has also been reported in Kenya since Nye's (1960) survey (Seshu Reddy, 1983). In this paper, we provide an updated distribution of stemborers in Kenya based on surveys conducted in the mid- to late-1990s, and compare the current distributions with those reported by Nye (1960) and Seshu Reddy (1983).

MATERIALS AND METHODS

Data on stemborer occurrence were collected in 269 maize fields in the southern arable zone of Kenya (Fig. 1) from 1996-2000, with the exception of National Parks and Reserves (Table 1). The total number of sampling sites in Table 1 is 392 since the same fields were often sampled in more than one season. Each field plot was visited at least twice during each sampling season. In each field and on each sampling occasion, 20 plants were randomly selected. Selected plants were taken to a laboratory for dissection. All stemborers found in dissected plants were identified to species where possible, and each species of stemborer in each plot was recorded as one data record. The sampling location (latitude/longitude) of each field was recorded (Fig. 1).

Table 1. Sampling regions, years and seasons, and number of sampling sites

Regions⁺

Year and season⁺⁺

No. of sampling sites

A

96 LR, 99 SR, 2000 LR

31

B

96 LR, 99 SR, 2000 LR

26

C

99 LR, 99 SR, 2000 LR

56

D

98 LR, 99 LR, 99 SR

87

E

98 LR, 99 LR, 99 SR

192

⁺ Refer to Fig. 1.
⁺⁺ LR represents long rains season, and SR represents short rains season.

The seasonal stemborer density was weighted since the number of sampling repetitions was not the same from season to season. We calculated densities from the maximum sampling repetition, then we weighted the seasons in which there were fewer sampling occasions. The weight was calculated according to a frequency distribution, i.e. using the following records to modify the stemborer density:

Sampling repetition: 1, 2, 3, ... ... Rmax
Plant stage: S1, S2, S3, ... ... SRmax
No. of stemborers: N1, N2, N3,... ... NRmax
Frequency (0 to 1): F1, F2, F3, ... ... FRmax

The frequencies were calculated based on at least 30 plots. For any specific sample plot, we checked the missing samples according to the plant stage. For any missing sample, we added the respective number of stemborers; i.e. if plant stage j was missing then Fj * total stemborers was added.

The locations where we found each stemborer species were mapped together with an elevation contour map using data from the Almanac Characterization Tool, version 2.0 (Corbett et al. 2001). The study area was classified into 5 regions based on topological, and climatic information (Fig. 1). The five regions were:

A. Upper Lake Victoria Basin
B. Lower Lake Victoria Basin
C. Highland/Rift Valley
D. Semi-arid Eastern Kenya
E. Lowland coast.

Regions C, D and E were basically the same as Nye's zones (1960) and Hassan et al.'s (1998) agroecological characterisation. Region D was the transitional maize-growing zone of Hassan et al. (1998). We divided the Lake Victoria basin area into two regions not only because of the climatic and geographical differences between the two regions, but also because of the differences in stemborer distributions.

RESULTS AND DISCUSSION

Stemborer species distributions

Spatial distribution of Chilo partellus (Fig. 2)
Chilo partellus was distributed throughout southern Kenya at elevations below 1500 m, but it was also found in some highland areas at elevations of up to 2300 m. During surveys conducted from 1956-1958 Nye (1960) found C. partellus in the plateau area at elevations of 600-1500 m, but not around Lake Victoria. Seshu Reddy (1983) found that C. partellus was more widespread than Nye's distribution, but reported a maximum elevation of 1676 m. It appears that the distribution of C. partellus has expanded to higher elevations since the earlier surveys. A similar elevational change in the distribution of C. partellus has been reported in South Africa (Kfir, 1997).

Spatial distribution of C. orichalcociliellus (Fig. 3)
Chilo orichalcociliellus was found in the coastal area at elevations below 600 m by Nye (1960), and further inland in Taita Taveta by Seshu Reddy (1983). Our results show that it remains primarily a lowland coastal species, and the distribution we found was similar to that reported by Seshu Reddy (1983).

Spatial distribution of Sesamia calamistis (Fig. 4)
Sesamia calamistis occurred at all elevations from sea level to 2400 m. Nye (1960) found it in highland areas, and in moist areas of the mid-elevation and lowland zones. We found that S. calamistis was distributed from the moist coastal region across the transitional zone to the semi-arid areas of Eastern Kenya. Thus, our findings do not suggest that S. calamistis is limited to moist locations. Seshu Reddy (1983) did not report finding S. calamistis in the Eastern or Central Provinces of Kenya (our zone C), whereas we found it at several locations in that area.

Spatial distribution of Busseola fusca (Fig. 5)
Busseola fusca is probably the only species for which the distribution has been more or less static since the 1950s, as it was found everywhere above 600 m. Nye (1960) believed that the distribution of B. fusca was limited by temperature, with warmer lowland areas being unsuitable.

Spatial distribution of Eldana saccharina
Nye (1960) did not find E. saccharina in maize or sorghum in Kenya in 1950s, but only in those crops in western Uganda. He also reported this species from sugarcane near Arusha in Tanzania. Similar to Seshu Reddy's (1983) findings, we recorded E. saccharina in many locations in the lakeshore area of western Kenya in 1996, especially in region B. The difference between Nye's results and the more recent surveys clearly shows that E. saccharina has expanded its distribution to include maize in mid-elevation areas of western Kenya. However, it does not appear to have yet become a serious pest of maize in western Kenya and, so far, does not appear to have spread further eastward.

Regional distribution of stemborers (Table 2)

Table 2. Stemborer distribution in regions A, B, C, D and E in Kenya in the 1990s (See Fig. 1 for information on regions)
	Mean density (larvae/plant)					Maximum density					% Sites infested
Borer species	A	B	C	D	E	A	B	C	D	E	A	B	C	D	E
Cp	0.74	0.83	0.15	3.22	2.71	2.86	4.21	0.83	21.64	12.80	90	85	30	100	100
Bf	0.33	0.51	1.45	0.07	-	1.82	1.71	5.5	0.85	-	72	73	85	23	-
Sc	0.13	0.10	0.18	0.47	0.47	0.57	0.27	0.37	1.67	2.00	65	67	23	96	92
Es	0	0.11	-	-	-	0	0.32	-	-	-	0	65	-	-	-
Co	-	-	-	0.06	0.49	-	-	-	0.20	2.13	-	-	-	7	95
Cp, C. partellus; Bf, B. fusca; Sc, S. calamistis; Es, E. saccharina; Co, C. orichalcociliellus

Region A
Three indigenous species, B. fusca, S. calamistis, and E. saccharina were found in region A at about the same low population densities of between 0.10-0.30 larvae per plant, and they occupied a similar proportion of the sampling sites (ca. 60-70 %). Chilo partellus was by far the dominant species in the region with a population density of about one larva per plant. It accounted for about 60 % of the total stemborer population and it was recovered from 90 % of the sampling sites. Interestingly, C. partellus was rarely found in the late 1950s in this area, but was the most abundant stemborer species during our surveys, having replaced B. fusca as the dominant species. It appears that the exotic stemborer has only reached this area during the past 40 years.

Region B
We found more B. fusca in region B than in region A, with B. fusca accounting for nearly a quarter of the total stemborer complex. The density of C. partellus remained the same as in Region A, but accounted for a smaller proportion of the total stemborer complex. More importantly, C. partellus was widely distributed. Eldana saccharina was also found at two-thirds of the sampling sites but its density was relatively low.

Region C
In region C, B. fusca was by far the dominant species, accounting for 92 % of the borer complex and occupying 85 % of the sites. Chilo partellus and S. calamistis were also found in about 30 % of the sampling sites in this region, but their densities were low.

Region D
Region D is a dry zone between the coast and mid-elevation areas. Both the highland stemborer, B. fusca and lowland stemborer, C. orichalcociliellus, were found. However, B. fusca was found only at high elevation sites above 800 m, and over 85 % of B. fusca-occupied sites were above 1000 m. Region D had by far the highest stemborer density which averaged almost 4 larvae per plant during the 1999-2000 survey period. We found C. partellus and S. calamistis almost everywhere. In contrast, both B. fusca and C. orichalcociliellus had very low densities. Busseola fusca occupied about 20 % of the sampling sites while C. orichalcociliellus was rarely found. This region was the furthest west we found C. orichalcociliellus in Kenya.

Region E
Three stemborer species, C. partellus, C. orichalcociliellus and S. calamistis were present in the coastal region, and all three were found nearly everywhere sampled. Chilo partellus accounted for about two-thirds of the total stemborer complex with a density of nearly 3 larvae per plant during the 1999-2000 survey period. Sesamia calamistis and C. orichalcociliellus had about the same population densities.

Countrywide stemborer distribution: Summary (Table 3)
Overall, C. partellus was the dominant stemborer species in the 1990s, rather than second in importance as was found by Nye (1960) in the 1950s. Chilo partellus occupied 236 out of the 269 sampling sites (Table 3a). The density of B. fusca ranked second, but it occupied only about one-third of the sampling sites, largely because our sampling was limited in the highland area. The other three stemborer species, namely B. fusca, C. orichalcociliellus and S. calamistis, occupied many of the sampling sites, but their densities were low.

Table 3. Countrywide stemborer distribution: Summary
a. By stemborer species
Species	Mean density (larvae/plant)	Maximum density	No. of sites infested
C. partellus	1.60	21.64	236
B. fusca	0.43	5.50	86
S. calamistis	0.28	2.00	213
C. orichalcociliellus	0.11	2.13	102
E. saccharina	0.05	0.27	27

b. By regions
Percentage infested
Region	plants (±SE)
A	74.90 ± 20.53
B	80.12 ± 16.74
C	55.28 ± 27.83
D	80.32 ± 31.81
E	74.89 ± 25.19

Stemborer infestation rates in maize ranged from 8-100 % in western Kenya (regions A and B) and eastern Kenya (D and E), and averaged 75-80 % in the four regions (Table 3b). The average percentage of plants infested was 55 % in highland central Kenya (region C), and was the lowest among the regions. All of the fields sampled were infested to some degree.

Chilo partellus was found everywhere below 1500 m, and at some locations with elevations as high as 2300 m; overall, it was by far the most abundant stemborer species. It occurred not only in the warm, dry areas of eastern Kenya (region D) but also in the relatively cooler area of central Kenya (within region C). The exotic borer has clearly adapted widely, and is one of the most serious constraints to maize production in Kenya. Moreover, its distribution appears to be expanding to higher elevations. Previous work has shown that C. partellus consumes more maize daily during its larval lifetime, than C. orichalcociliellus, which it has partially displaced in region E (Ofomata et al., 1999; Ofomata et al., 2000; Overholt et al., 1997). Likewise, in South Africa, studies have shown that C. partellus is more injurious than B. fusca (van den Berg et al., 1991), and has partially displaced the latter in some areas (Kfir, 1997). Thus, it appears that in Kenya, C. partellus is now the most serious insect pest of maize.

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