The impact of predators on maize stem borers in coastal Kenya

M. Bonhof

Research output: Thesisexternal PhD, WU


<p>Damage caused by Lepidopteran stem borers is one of the most important constraints to maize production in East and southern Africa. Of the stem borer complex, <em>Chilo partellus</em> Swinhoe is the most abundant species in lowland areas. Although control strategies exist, many are not effective or feasible for small-scale subsistence farmers to practice. Consequently, stem borers are rarely actively controlled. Although natural mortality of stem borers may be high, this is not sufficient to keep densities at low levels. Disappearance is the major mortality factor in all life stages, often being as high as 80 to 90%. Disappearance is generally attributed to predation, although little quantitative information is available to support this assumption. In this thesis, the impact of predators on stem borer populations and their role in the disappearance of eggs, larvae and pupae was examined in the southern coastal area of Kenya.</p><p>In the first chapter, background information on maize cultivation and constraints to maize production in the coastal area of Kenya is presented. The biology and pest status of stem borers, as well as methods to control them, are also discussed.</p><p>Interviews held with small-scale farmers indicated that stem borers were considered a major constraint for maize production in the study area (chapter 2). Despite this, farmers had limited knowledge of the life cycle of stem borers, on possible sources of infestation and on stem borer control practices. Chemical application was the only well-known control method and was regularly used by 30% of the respondents. Most farmers were not aware of the beneficial activity of predatory arthropods, although ants, spiders, praying mantids and 'crickets' were sometimes mentioned as predators of stem borers. There is a need to create awareness among farmers of the biology and ecology of pests and the role of beneficial insects in cultivation. Effective control methods should be developed locally by farmers and researchers together, for example by using the 'Farmer Field School' approach.</p><p>Information on the natural enemies of stem borers in East Africa is reviewed in Chapter 3. A large variety of natural enemies was identified with fifty-seven parasitoids, fifteen predators and fifteen pathogens being recorded. The review discovered that most emphasis was placed on the parasitoids of stem borers. Conversely, information on predators and pathogens is scarce.</p><p>Stem borer infestation starts at an early crop stage (Chapter 4). Infestation levels in our trials varied between 0 and 10%, with occasional peaks of up to 28%. Most plants carried only one egg batch with an average of 20 eggs. Egg mortality was highly variable between sites and seasons, and ranged from 18 to 78%. Parasitism was the most important mortality factor by far, accounting for 6 to 59% of the total mortality. Between 5 and 10% of eggs disappeared and 2 to 8% was visibly preyed upon. Between 2 and 7% of eggs failed to hatch.</p><p>Studies on the incidence of predator and stem borer populations at the Kenyan coast are described in Chapter 5. These indicated that <em>Chilo partellus</em> and <em>C. orichalcociliellus</em> were the predominant stem borer species although <em>Sesamia calamistis</em> was occasionally found. Thirteen parasitoid species were collected: two from eggs, nine from larvae and two from pupae. Egg parasitism was fairly high (38 to 75%) but parasitism of larvae and pupae was less than 5%.</p><p>Ants, spiders and earwigs were the most common potential predators. Rarely encountered predators included coccinellids, lacewings, bush-crickets and rove. Ants were the most abundant and widespread predators by far and were represented by a rich variety of species. Spiders were found at all sites and occurred throughout the growing season, but numbers were relatively low. Earwigs were fairly common at two sites but virtually absent at the other sites. Predator numbers were low in the beginning of the growing season, when maize is most vulnerable to stem borer attack. Their numbers were generally not found to be related to the highly variable numbers of stem borer eggs and small larvae.</p><p>The important predators stem borer eggs, larvae and pupae were identified through laboratory and field studies (Chapter 6). When prey was offered in Petri dishes, eighteen predator species consumed eggs and fifteen consumed small larvae. Late instar larvae and pupae were only taken by gryllids. Of the potential predators tested, earwigs and cockroaches had a high acceptance and consumption rate of both eggs and small larvae. The activity of cockroaches was unexpected, since they are normally not seen as predators of insects. When eggs and small larvae were offered on plants, earwigs consumed (a few) egg batches and small larvae but cockroaches did not. An observation study showed that egg predation was low: only 3% of egg batches were attacked. Ants (three species) and adult flies of the family Chloropidae were seen to prey on eggs.</p><p>To evaluate the impact of predator populations on each stem borer life stage, a series of predator exclusion experiments were conducted (Chapter 7). Disappearance of eggs was 23 to 29% from plants accessible to predators (control plants) and zero to 0.2% from exclusion plants, thus showing that predators play an important role in egg disappearance. Few visibly preyed-upon or sucked eggs were encountered. Disappearance of small larvae was high but could not be attributed to predators. The impact of predators on disappearance of large larvae was small. In some trials, significantly more pupae were recovered from predator exclusion than from control plants. Thus, predators have an impact on the disappearance of eggs and, in some cases, of pupae, but have hardly any effect on larvae at the coast.</p><p>During field observations, egg batches were seen to disappear by curling away from the leaf and being blown off. To evaluate whether solar radiation played a role in this, disappearance of egg batches was compared between solar-exposed and shaded plants (Chapter 8). Disappearance was found to be four times higher on the exposed plants than on the shaded plants after four days. Non-viability of eggs was also significantly higher on the exposed plants.</p><p>Predators did not influence the disappearance of small larvae and therefore other factors must have played a role. Rainfall can lead to high mortality of small larvae feeding in the leaf whorl, with 50% (first trial) and 65% (second trial) of larvae disappeared from plants exposed to rainfall (Chapter 8). From the shielded plants, 32 and 25% of larvae disappeared, respectively. Cannibalism also caused some disappearance among small and larger larvae, but this is not expected to be an important mortality factor under natural conditions. Most of the larval disappearance is thought to be caused by dispersal, and subsequent desiccation and predation.</p><p>The research described in this thesis leads to the conclusions that:</p><ol><li>Predation plays a small role in the mortality of stem borers in plants, but may cause high losses among dispersing larvae;</li><li>Predator attack only partially explains the disappearance of stem borers;</li><li>Parasitoids cause high mortality in the egg stage but not in other life stages of stem borers;</li><li>Solar radiation and rainfall may cause considerable mortality of stem borer eggs and larvae.</li></ol>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • van Lenteren, Joop, Promotor
  • van Huis, Arnold, Promotor
Award date6 Sep 2000
Place of PublicationS.l.
Print ISBNs9789058082626
Publication statusPublished - 2000


  • maize
  • zea mays
  • insect pests
  • stem borers
  • lepidoptera
  • chilo partellus
  • chilo orichalcociliellus
  • sesamia calamistis
  • predators
  • natural enemies
  • kenya

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