CHAPTER 13 Potential for Biological Control in the Management of Cassava Pests* Elsa Liliana Melo and Carlos Alberto Ortega1 Biological Control direct methods for using, manipulating, or conserving natural enemies. In the first phases, the fundamental Biological control, in the ecological sense, as a phase aspects studied are taxonomy, biology, physiology, of natural control, may be defined as the regulation of genetics, ecology, demography, behavior, and nutrition the population density of a pest organism by natural of pests and their enemies (DeBach 1975). enemies (parasites, parasitoids, predators, or pathogens) at a level that would be equal or greater If necessary, a pest and its enemies are identified by than would have been reached by means of another a specialist, as the organism’s name and classification alternative. Applied biological control supposes are key to all existing knowledge on it, and thus professional activity or human manipulation that understanding and controlling it if it is pest, or using it if promotes the effectiveness of natural enemies it is a natural enemy (Cave 1995b). (DeBach 1977). Natural Enemies In a broad sense, biological control may also be defined as mortality or suppression of pest organisms In biological control, various natural enemies participate, by any biotic factor. In this wider sense, it is the direct for example, parasites, parasitoids, predators, and action of parasites, parasitoids, predators, and microorganisms. These organisms must be able to pathogens (i.e., natural enemies) and of competition respond quickly to the pest’s population dynamics, so with other species for natural resources (i.e., that proportionately more natural enemies are present antagonists) that regulate an organism’s population as the pest population increases. This characterizes the density to a level lower than it would have been in the theoretically ideal natural enemy, as well as certain absence of that control. It does not include plant biological and ecological criteria (Cave 1995c). resistance, interference with the pest by semiochemicals (e.g., pheromones, allomones, Predators kairomones, and synomones), genetic engineering of the pest, natural chemical extracts, or mechanical Predators are carnivorous organisms that, in either control by humans. It does include the manipulation immature or adult state, actively seek and capture of natural enemies and antagonists through, for numerous prey, consuming them either partially or example, importation, mass-rearing, and release totally. Perhaps half of all insects and mites are (Cave 1995a). predators. As they are so numerous, determining the most effective predators is difficult. They are classified Research on biological control includes baseline as either generalists or specialists, according to eating surveys of any application of the method. These do habits and behavior. The principal arthropod predators not necessarily report immediately useful results or belong to the following orders: Odonata, Orthoptera, Dermaptera, Hemiptera, Neuroptera, Coleoptera, Diptera, Hymenoptera, Araneae, and Acari. Families that * This chapter is a revision of the version originally published in the stand out are Mantidae, Labiduridae, Pentatomidae, Proceedings of the XXVII Congress of SOCOLEN, 2000. Chrysopidae, Carabidae, Staphylinidae, Coccinellidae, 1. Advisors in Plant Health, Biotechnology Laboratories and Annexed Services (LABIOTSA), Quito, Ecuador. Elateridae, Cecidomyiidae, Syrphidae, and Phytoseiidae E-mails: [email protected] and [email protected] (Banegas and Cave 1995). 277 Cassava in the Third Millennium: … Parasites controllers. According to Castillo et al. (1995), the expression “microbiological control” refers to the use of A parasite is an organism that lives at the expense of microorganisms (which, in the broad sense, includes another organism—the host (Australian Museum nematodes) to control pests. 2005). In general parasites share the following features: Recently, the use of microorganisms to effectively • Parasites are usually smaller than their host. control insect pests in different crops has increased, Parasites use both invertebrate and vertebrate most likely as a result of the discovery and development hosts. of new species and strains of entomopathogens (Lacey • Adult parasites may live on the host (e.g., lice), and Brooks 1997). Insects associate with in the host (e.g., tapeworms) or feed on a host microorganisms in diverse ways such as symbiosis, occasionally (e.g., mosquitoes). mutualism, and parasitism. Mutualism abounds among • Parasites generally do not kill the host but may insects; an example is the association of protozoa with harm the host indirectly by spreading termites, although the former are not pathogenic to the pathogens. This may affect the host’s behavior, host insects. In contrast, entomopathogens cause metabolism or its reproductive activity. infections, parasitism, or toxemia in insects (Lacey and • Many parasites have hooks, claws or suckers to Brooks 1997). attach to their host. Generally parasites have either a sucker (e.g., leeches) or piercing and Five principal groups of microbiological agents sucking type mouthparts (e.g., fleas) for exist: viruses, fungi, nematodes, bacteria, and protozoa. feeding. • Both adults and young can be parasitic. In Viruses. Entomopathogenic viruses are infectious some cases the young are parasites but the entities whose genome, constituting nucleic acid, DNA, adult is not. or RNA, is replicated in host tissues. A major viral family for insect control is the Baculoviridae. Parasitoids Baculoviruses contaminate the insect through the oral pathway. Normally, virions (infective units of viruses) A parasitoid is an organism that has young that develop are found on plant leaves and stems, and are ingested on or within another organism (the host), feeding on a by the insect as it eats. The first cells to be affected are single host and killing it at the end of their cycle. The the epithelial ones of the intestine. The virus then adult state lives free and is not parasitic. Among the attacks other tissues such as fatty bodies, intestinal characteristics that make parasitoids promising for epidermis, hemocytes, trachea, and silk glands. biological control are: Infected larvae become lethargic, stop eating, and finally become paralyzed. Dead insects become the • Specificity (e.g., Aphelinidae and Encyrtidae) most important sources of inoculum for maintaining • Ease of breeding in large quantities (e.g., the epizootic (Castillo et al. 1995). Trichogramma spp., Cotesia flavipes, Encarsia formosa, and Telenomus remus) Fungi. Entomopathogenic fungi kill the host • The power of flight, which facilitates dispersion relatively quickly by penetrating and proliferating within • High fertility, short generational time, and its body. The insect dies as the fungus either deprives it evolutionary rates that are comparable with of soluble nutrients from the hemolymph, invade or those of the pests digest its tissues, or releases toxins that poison it. Not all fungi associated with insects are pathogenic. Some Parasitoid species can be found in five of the insect pathogens are obligate, but most are facultative. orders, but most are in two: either the Hymenoptera or Saprophytic and other symbiotic fungi also exist Diptera (Díaz and Hanson 1995). Important families (Ferron 1985). include Aphelinidae, Platygastridae, Eulophidae, and Encyrtidae. More than 700 species of entomopathogenic fungi exist, distributed across different taxonomic groups, Entomopathogens and all with potential for use in regulating insects (Hajek and St Leger 1994). The most widely accepted As their name says, entomopathogens cause diseases classification system of fungi was proposed by in insects, and are grouped as microbiological Ainsworth in 1973 (cited by Tanada and Kaya 1993). It 278 Potential for Biological Control in the Management of Cassava Pests separates fungi into two divisions: Myxomycota, which spiracles, or penetrates the insect’s cuticle to reach the are plasmodial (i.e., asexual, pseudopodial, and hemocele, where it releases the symbiont. The producing masses of multinucleate protoplasm that bacterium proliferates and produces enzymes (lipases resemble amebas); and Eumycota, which are and proteases) that degrade the host’s tissues. nonplasmodial and frequently mycelial in form. Together, the nematodes and bacteria kill the insect Entomopathogenic fungi are found in the division within 48 h. The juveniles then consume the bacteria Eumycota and in the following subdivisions: and the insect’s degraded tissues. • Mastigomycotina: mobile cells or zoospores; Nematode development is favored by the perfect stage as oospores bacterium producing antibiotics, preventing the • Zygomycotina: cells not mobile; perfect stage proliferation of secondary pollutants. Between one and as zygospores three generations of the nematode are produced in the • Ascomycotina: perfect stage as ascospores host insect, depending on its size. All individuals of • Basidiomycotina: perfect stage as Steinernema are sexually differentiated. In basidiospores Heterorhabditis, however, those of the first generation • Deuteromycotina: cells not mobile; no perfect are hermaphrodite and those of the second are stage differentiated. Most entomopathogenic fungi are found in the The bacteria Xenorhabdus and Photorhabdus are subdivisions Zygomycotina, class Zygomycetes, order Gram-negative bacilli of the family Enterobacteriaceae Entomophthorales (e.g.,