Predators and Parasitoids

Opender Koul, G. S. Dhaliwal, Opender Koul, G. S. Dhaliwal

• Publisher: CRC Press
• Year: 2003
• ISBN:
  9781134400713
• Edition: 1
• Topic: Sciences biologiques
• Subtopic: Entomologie

Their natural enemies largely determine the population size and dynamic behavior of many plant-eating insects. Any reduction in enemy number can result in an insect outbreak. Applied biological control is thus one strategy for restoring functional biodiversity in many agroecosystems. Predators and Parasitoids addresses the role of natural enemies i

1
PREDATORS AND PARASITOIDS: AN INTRODUCTION

Opender Koul ¹ and G.S. Dhaliwal ^2
1 Insect Biopesticide Research Centre, Jalandhar, India, ²Department of
Entomology, Punjab Agricultural University, Ludhiana, India

Introduction

The terrestrial communities comprise decomposers plus three trophic levels: plants, herbivores, and predators and parasitoids. In support of this model, Hariston et al. (1960) argued that plant sources are generally abundant and underexploited (the green world hypothesis). Under this model, biological control agents are found only at the top of the trophic web and are limited only by the availability of resources. However, predators and parasitoids are natural enemies that attack various life stages of insects, resulting in the regulation of herbivore numbers in a particular ecosystem. This regulation has been termed “biological control” and has been defined by DeBach (1964) as the action of parasites, predators, or pathogens in maintaining another organism’s population density at a lower average than would occur in their absence. In the applied sense, it may be defined as the utilization of natural enemies to reduce the damage caused by noxious organisms to tolerable levels (DeBach and Rosen, 1991). In a strict ecological sense, “applied biological control” can be considered as a strategy to restore functional biodiversity in agroecosystems by adding, through classical and augmentative biocontrol techniques, “missing” entomophagous insects or by enhancing naturally occurring predators and parasitoids through conservation and habitat management (Altieri and Nicholls, 1998).

The potential of biological control by predators and parasitoids has largely remained untapped because it has been underused, underexploited, underestimated, and often untried and, therefore, unproven (Hokkanen, 1993). In fact, the use of predators and parasitoids should be a primary consideration in any pest management program. Biological control is generally the best method of control on the basis of ecological and environmental considerations. Biological control is self-propagating and self-perpetuating. The successes achieved more than half a century ago continue to work to this day. Biological control agents possess the ability to search for their host (pest). It is perhaps for this reason that biological control has been especially successful in case of pests that are difficult to control otherwise. Nearly 66 percent of total successes have been obtained in case of homopterous insects, which are covered by a waxy layer and are not easily killed by contact insecticides. Another 18 percent of successes have been reported in case of Lepidoptera, a majority of which are borers and internal feeders (Dhaliwal and Arora, 2001).

Another aspect of biological control is hyperparasitism. In emphasizing the “bottomup” effects of hosts on their parasitoids, the first trophic level shows that both inter-and intraspecific plant variations can influence the ecology and behavior of the second trophic level of phytophagous insects, which in turn is one of the major determinants of the third trophic level of insect parasitoids (Craig, 1994). Thus there is a highly evolved fourth trophic level relationship that exists between entomophagous insects, and this is called “hyperparasitism.” It refers to the development of a secondary insect parasitoid or hyperparasitoid at the expense of a parasitoid. The hyperparasitoids can have an influence on “top-down” control of terrestrial herbivores by parasitoids (Rosenheim, 1998) and their action may have shaped the evolution of parasitoid foraging strategies (Mills and Gutierrez, 1996). This type of parasitism has been studied for many phytophagous hosts (Hawkins, 1994), but there is limited knowledge of life-history parameters and of foraging behaviors of hyperparasitoids. This knowledge is, however, crucial for understanding of the role of hyperparasitism in natural populations as well as in pest control (Sullivan and Völkl, 1999). The knowledge of hyperparasitoid ecology is of basic importance for the design of biological control programs. The aphid hyperparasitoid community as a model system to demonstrate typical patterns of selected life-history aspects and foraging strategies at the fourth trophic level has recently been dealt with comprehensively (Sullivan and Völkl, 1999). The “push–pull” approach is another strategy for controlling insect pests. This was described for the first time by Pyke et al. (1987) to control Heliothis sp. on cotton and subsequently followed by Miller and Cowles (1990) for protection of onions from the onion fly. However, in both cases no consideration was given to natural enemies and a chemical deterrent or toxin was used to repel or kill the pest. The latest strategy of the push–pull concept is exploitation of natural enemies through trap and repellent plants. The repellent plants reduce the pest attack and at the same time increase the level of parasitism on protected plants, resulting in significant increase in crop yield. For instance, the two most successful trap crop plants — Napier grass and sudan grass — attracted greater oviposition by stem borers than did cultivated maize. The intercrops giving maximum repellent effect were molases grass and a legume species, silver leaf. Push–pull trials using trap crops and repellent plants reduced stem borer attack and increased the level of parasitism of borers on protected plants (Khan et al., 1997; Khan, 2001).

On the whole, habitat manipulation, the use of behavioral chemicals, biological trait improvement, the use of feeding and oviposition attractants, relay cropping and the establishment of entomophage parks are the methods recommended for maximizing the effectiveness of the natural enemies. Differential performances of parasitoids and predators on cultivars with different morphological characteristics are well known.

Similarly, distinct variations in behavioral responses to chemical cues from different sources increase the level of parasitism (Paul and Yadav, 2003). Continuous misuse of chemicals has also led to elimination of potential bioagents. Therefore, it is imperative to find ways to protect them from extinction. An entomophage park is an interesting approach to conserving the biodiversity of natural enemies. In India an entomophage park of 0.2 ha area was established in Gujarat, where 28 species of arthropods were conserved in situ that were natural enemies to several species of insect pests (Yadav et al., personal communication, 2001).

Historical perspective

The use of natural enemies for combating phytophagous insects has a long historical background. The Chinese used nests of Pharaoh’s ant, Monomorium pharaonis (Linnaeus) to combat stored grain pests, while Oecophylla smaragdina (Fabricius) has been used to control foliage feeding caterpillars and large boring beetles in citrus trees since ancient times. Not only could these colonies be purchased or moved from wild trees, but placing bamboo runways from one tree to another facilitated the movement of ants between cultivated trees. The earliest record of this practice dates back to 324 BC (Coulson et al., 1982).

During modern times, the mynah bird from India was imported in 1762 for the control of red locust, Patanga septemfasciata (Serville) in Mauritius. By 1770 it brought about the successful control of the locust. In the 1770s the practice of creating bamboo runways between citrus trees was developed in Myanmar (Burma) to facilitate the free movement of ants between the trees for the control of the caterpillars (van Emden, 1989).

The first well-planned and successful biological control attempt was undertaken during 1887–88, when the developing citrus industry in California (USA) was seriously threatened by the cottony cushion scale, Icerya purchasi Maskell. No chemical treatments known at that time could control the scale. C.V. Riley, a prominent entomologist, suggested that the original home of the scale was Australia or New Zealand and that natural enemies of the scale should be introduced into the USA from these countries. The idea found immediate favor and, in 1888, Albert Koebele was sent to Australia for this purpose. He soon found a small beetle known as Vedalia, Rodolia cardinalis (Mulsant), attacking the scale in the Adelaide area and in November 1888 the first shipment of the beetles reached California. The beetles in this and subsequent shipments were liberated on scale-infested trees (DeBach, 1964).

Within a year, a spectacular and highly effective control of the beetle had been obtained throughout the citrus growing areas of the state at a total cost of less than US$200. To this day, the Vedalia continues to provide a completely satisfactory control of this pest in California and the savings it has brought to the citrus industry are incalculable. The resounding success of this venture and its successful extension to other parts of the globe where this pest was a problem, coupled with its permanency, simplicity, and low cost, generated enthusiastic support for biological control as a solution to other agricultural pest problems.

Since then, many successful examples of biological control by predators and parasitoids in different parts of world have been well documented (DeBach and Rosen, 1991; Hoy, 1994). The recent outstanding example is the control of cassava mealybug, Phenacoccus manihoti Matile-Ferrero, by a tiny wasp, Epidinocarsis lopezi (De Santis) in Africa. The edible roots of cassava are a staple diet in most of sub-Saharan Africa, providing up to half the daily calories of 200 million people. The mealybug was devastating the cassava plant in late 1970s, destroying as much as 80 percent of the crop in some areas and widespread famine was a real possibility.

A search was made in several South American countries to look for P. manihoti and its natural enemies. P. manihoti and several parasites and predators were found in Paraguay in 1981. One of these, E. lopezi, proved to be an immediate success. It was first released in Nigeria in 1981 and in less than one and a half years spread to areas as far away as 170 km from the original release sites. By the end of 1984, E. lopezi was found in almost 70 percent of cassava fields spread over an area of more than 200 000 km2 in southwestern Nigeria. Subsequently, the wasp was sent to several other countries, where it was extensively released from air in addition to numerous releases from the ground. It was established over a total area of more than 750 000 km2 in 16 African countries by the end of 1986. Over the next 7 years, the cassava mealybug problem was effectively eliminated from 30 countries (Herren and Neuenschwander, 1991; Herren, 1996; Bellotti et al., 1999).

DeBach and Rosen (1991) have summarized the number and extent of successes obtained from biological control importation projects against insect pests at global level. By the year 1988 it was found that natural enemies had been introduced against 416 species of insect pests and permanent control was achieved in 164 species (39.4 percent of pests). Of these, 75 species were completely controlled and another 74 were substantially controlled. However, 15 species showed only a partial control (reduction in pesticide application by nearly 50 percent). These successes were later extended to other countries by introduction of the same enemies and, taking these into consideration, a total of 384 importations were successful by 1988.

Biological characteristics

Biological control theory is still based on a three-trophic-level model of arthropod communities (Rosenheim, 1998). In analyses of higher-order predator and parasitoid prediction, with influences ranging from enhanced to disrupted biological control, terrestrial communities are unable to support more than three trophic levels and nonrandom selection of study systems may have promoted this view. To visualize the efficacy of biological control systems, we must understand the characteristics of these systems and know the factors that impede the top-down regulation of herbivores.

Predators

The insects that prey on other insects and mites occur in many insect orders, the most prominent being Coleoptera, Neuroptera, Hymenoptera, Diptera, Hemiptera, and Odonata. In addition, there are several species of mites and spiders feeding on a wide range of insects and mites. Some predators use biting or chewing mouth parts to devour their prey (e.g., praying mantids, dragonflies, and beetles), whereas others use piercing and sucking mouth parts to feed upon the body fluids of their prey (e.g., assassin bugs, lacewing larvae, and hover fly larvae). The sucking type of feeders often inject a powerful toxin that quickly immobilizes their prey.

Predatory insects feed on all host stages, i.e., egg, larval (or nymphal), pupal, and adult stages. Many species are predaceous in both larval and adult stages, although not necessarily on the same kinds of prey. Others are predaceous only as larvae, whereas the adults may feed on nectar, honeydew, etc., and among these it is often the nonpredaceous adult female that seeks the prey for her larvae by depositing eggs among the prey. This is because their larvae are sometimes incapable of finding the prey on their own. Many predators are agile, ferocious hunters, actively seeking their prey on the ground or on vegetation, as do beetles, lacewing larvae, and mites, or catching it in flight, as do dragonflies or robber flies. Certain hunters have specially adapted seizing organs, such as the barbed forelegs of mantids or the labial mask of aquatic dragonfly nymphs. Other predators may use various traps to catch their prey, of which the webs of spiders and sand pitfalls of ant lions are perhaps the best known (DeBach and Rosen, 1991; Beckage et al., 1993).

Insect predators are often embedded in a complex network of trophic interactions not only with their herbivorous prey but also with each other (Po...