Telenomus remus and Trichogramma spp. as a tool for controlling Fall Armyworm
By: Lucas Fonseca Lacerda, Aloisio Coelho Jr., José Roberto P. Parra
The fall armyworm Spodoptera frugiperda (J.E. Smith, 1979) (Lepidoptera: Noctuidae) is a highly polyphagous pest reported from more than 80 host plants, many of them economically important, and has caused enormous damage to agriculture for many years in the Americas and more recently in Asia and Africa. Its high biotic potential allows S. frugiperda to have a great capacity for selecting population resistance to insecticides and transgenic plants, the main control techniques used today. The numerous reports of population resistance to these techniques and the invasiveness of S. frugiperda in regions with many small farmers that often lack access to insecticides and transgenic technology make controlling this pest extremely difficult.
One alternative to traditional control methods for S. frugiperda is the use of egg parasitoids in an Augmentative Biological-Control (ABC) program. Egg parasitoids are natural enemies of the target pest and use its eggs as hosts to complete their life cycle, preventing the larvae from hatching and eliminating the pest before it can cause any damage to the crop. Their use in an augmentative biological-control program consists of collecting the parasitoid from the target pest, mass rearing and quality control, and release of large numbers of the parasitoid (on the scale of millions) in order to maintain the pest populations below the level of economic damage.
To establish an efficient ABC program for S. frugiperda using egg parasitoids, it is necessary to understand the biology and behavior of the target pest and the parasitoids being used, so that they can be reared properly with no genetic deterioration and retain all the characteristics of an efficient parasitoid, such as good flight capacity and capability to find and parasitize as many hosts as possible in the field.
Among the several parasitoids that exploit S. frugiperda eggs, Telenomus remus Nixon, 1937 (Hymenoptera: Platygastridae) and some species of the genus Trichogramma Westwood (Hymenoptera: Trichogrammatidae), especially Trichogramma atopovirilia Oataman & Platner, 1983 and Trichogramma pretiosum Riley, 1879 are often identified as potential agents of ABC.
Telenomus remus is an egg parasitoid from Asia that was introduced into the Americas for classical biological control of S. frugiperda, because of its high biotic potential and specificity to hosts of the genus Spodoptera Guenée (Lepidoptera: Noctuidae), which includes several species that are important agricultural pests. Trichogramma egg parasitoids, on the other hand, are highly generalist and can use members of several lepidopteran families and species as hosts. Species of Trichogramma are already being used in large areas worldwide as augmentative biological-control agents.
Trichogramma pretiosum is already commercially registered as a biological-control agent for S. frugiperda in Brazil. However, some specialists are critical of its use, believing that it cannot parasitize overlapping egg layers, but can reach only the eggs that are exposed in the outer layers of egg clutches. There is also a supposition that Trichogramma spp. have difficulty in penetrating the scales that usually cover S. frugiperda egg clutches. In contrast, T. remus can penetrate these scales and parasitize the whole egg clutch, including the internal egg layers that Trichogramma spp. cannot parasitize. In any event, Trichogramma spp. are well-known parasitoids that are successfully mass-reared and used in ABC programs on millions of ha worldwide.
At first it appears that T. remus is a better candidate for controlling S. frugiperda than Trichogramma spp.; several studies have proven its effectiveness against S. frugiperda in laboratory conditions and there are some reports of its successful use in open fields. However, in many countries, such as Brazil, an ABC for S. frugiperda using T. remus has not yet been successfully established. Also, studies assessing the viability of using T. remus in the field often report highly disparate data. For example, in Venezuela, Linares (1998) determined that releases of 4,000 to 6,000 adults of T. remus per hectare would be necessary for efficient control of S. frugiperda; while Cave (2000), in a review, reported that in Honduras at least 75,000 to 105,000 parasitoids per hectare per week should be released. For use in Brazil, Figueiredo (1999) estimated that releases of approximately 200,000 adults of T. remus per hectare would be necessary. Another important factor is that most of the studies testing the efficiency of T. remus in controlling S. frugiperda in field and semi-field conditions were carried out using eggs of S. frugiperda laid in laboratory conditions, without taking into account the possible morphological and chemical differences from eggs that are found naturally in the field. These differences may affect the control efficiency of T. remus, considering that eggs of S. frugiperda can vary widely in color, number of eggs and number of eggs layers by egg clutch, and the amount of scales deposited on the egg clutches by the adults, all characteristics that can strongly influence parasitism behavior.
One potential alternative would be to use T. remus and Trichogramma spp. together in the field, acting as complementary agents and compensating for each other’s limitations. But to assess this possibility and establish an efficient biological-control program using T. remus and Trichogramma spp. to control S. frugiperda, it is still necessary to better understand certain ecological and biological aspects of the pest and its parasitoids.
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