Naturally-occurring bio-control agents
The Fall Armyworm has many naturally-occurring ‘natural enemies’ or ‘farmers’ friends’. These biological control agents are organisms that feed on FAW.
In the Americas, and probably in Africa, these natural enemies can be active during all development phases of FAW, i.e. in the egg, larval, pupal and adult stage. Natural enemies have the potential to substantially reduce the FAW populations and hence the damage caused by FAW.
Their impact however depends on a number of factors including the diversity of organisms being active, their life-style, local presence, numerical and timely abundance, host specificity, agronomic practices, pest management methods etc.
A major challenge is to create conditions to exploit the potential of these beneficial organisms to their full extend. Broad spectrum pesticides kill many of the farmers’ friends. It is important that farmers recognize the pest in all its development stages, its associated natural antagonists, identity possible gaps to be filled in local natural enemy guilds and at the same time sustain their action by adequate management measures in an IPM context.
Biological control should be understood as an integral component of IPM and an important part of mutually compatible pest-suppressing methods aimed at generating higher profits whilst preserving the environment and human health.
Biological control agents (BCAs) include the following: 1) predatory insects and mites, which eat their prey; 2) parasitoids, which are insects with a free living adult stage and a larval stage that is parasitic on another insect; and 3) parasites and microbial pathogens, such as nematodes, fungi, bacteria, viruses and protozoa, which cause lethal infections.
Parasitoids of the FAW
Parasitoids are organisms whose adults lay eggs inside or attached to a single host organism. For their development, the resultant larvae feed on the tissues of the host until they are fully grown and pupate. The larvae of parasitoids always kill their host as the outcome of their development.
The majority of parasitoids known to be associated with the FAW are wasps, and less frequently flies. Species that have undergone an adaptation process to the FAW display a narrow host range.
Such co-evolved parasitoids can exert a strong impact on populations of the FAW and are thus good candidates for use in biological control programmes.
During inventories in the Western Hemisphere, about 150 different parasitoid species were found to be associated with the FAW in various crops.
The following are some of the most common parasitoids known to be well adapted to the FAW in the Americas:
Telenomus remus Nixon (Hymenoptera: Platygastridae)
- – Identification: minute wasp of about 0.6 mm size with black shiny body. The wings are transparent and have reduced venation. Female antennae have 11 segments whereby the last 5 are enlarged forming a club. Males have 12 antennal segments of equal size.
- – Behaviour: The species behaves as an egg parasitoid, i.e. females T. remus are attracted to FAW egg masses where they oviposit. Offspring of the parasitoid develop within eggs of FAW of which they then emerge as adults.
- – Life cycle: over their lifetime females are able to parasitize some 120-130 FAW eggs. The development of immatures takes about 10 days at 280C and thus about 40 generations are produced per year.
- – Importance: T. remus is reported to be highly effective in several South American countries with parasitism rates above 80 percent depending information sources.
Chelonus insularis Cresson (Hymenoptera: Braconidae)
- Identification: parasitoid of about 5 mm size characterized by a carapace-like abdomen. A white band medially divided can be observed at the base of the abdomen. Wings bear numerous veins. Antennae of both sexes are filiform and have 16 segments or more.
- – Behaviour: C. insularis is an ovo-larval parasitoid. Females oviposit in eggs of FAW but larvae start their development in later instars of the caterpillar. When mature, parasitoid larvae exit their host and build a silken cocoon to pupate.
- – Life cycle: each female can parasitize about 600 FAW eggs. At 28-300C the parasitoid is able to develop within 20-22 days and females can live for about 12 days.
- – Importance: C. insularis is the most common among FAW parasitoids in the Caribbean as well as in Central and South America.
Cotesia marginiventris Cresson (Hymenoptera: Braconidae)
- – Identification: Male and female average 3 mm in length. While the head and thorax of adults are black, the abdomen is tan. The antennae are long segmented and slighter shorter than the body length. Females can be recognized by a very short ovipositor at the tip of the abdomen.
- – Behaviour: C. marginiventris is a solitary larval parasitoid of noctuids. On the FAW, Cotesia adult females attack preferably 1st and 2nd caterpillar instars, on which a single egg is usually laid. Shortly before pupation the full grown parasitoid larva leaves its host and spins a white cocoon of 4 mm size, of which an adult wasp will emerge a few days later.
- – Life cycle: The parasitoid needs 12 days to develop from egg to adult at 300C. In total, 200 to 300 offspring are produced per female. Adults have a lifespan of 22 to 30 days.– Importance: C. marginiventris is less sensitive than other parasitoids in environments sprayed with chemical insecticides. It is adapted to subtropical and warm temperate areas. Attracted to host volatiles, it can persist at low FAW population densities using alternate hosts, thus it is a better competitor than Chelonus insularis.
Trichogramma spp. (Hymenoptera: Trichogrammatidae)
- Identification: There are numerous species of the genus Trichogramma known to develop inside the eggs of the FAW and of many other Lepidoptera. Typically Trichogramma spp. are tiny wasps less than 0.5 mm long. Adults are mostly orange, brown or even black. Antennae are short, clubbed in females and hairy in males.
- – Behaviour: Adult females lay their eggs inside FAW eggs. Along with the larval development they gradually turn darker and are almost black when the parasitoids pupate. Adults emerge by chewing an exit hole on the FAW egg.
- – Life cycle: The parasitoid completes its development in about 8 days at 280C. Females can parasitize up to 120 moth eggs and live for 6-7 days.
- – Importance: In Latin America Trichogramma spp., in particular T. pretiosum and T. atopovirilia, are commonly mass-reared on alternative hosts in local mass production units and commercialized for inundative field releases.
Fly parasitoids: Archytas, Winthemia and Lespesia (Diptera: Tachinidae)
- – Identification: Several fly species of the family Tachinidae are able to develop on FAW caterpillars. Attacks by such parasitoids can be detected either when small maggots are visible in presence of FAW caterpillars, or tiny white eggs are observed on their skin. Alternatively fly pupae can be found nearby dead FAW larvae.
- – Behaviour: For species that lay directly several eggs on the skin of their host, parasitism starts immediately upon penetration of the maggot into its host. Other species await pupation of FAW to intensify their host feeding and complete their development. Despite frequent superparasitism, only a single fly develops per caterpillar.
- – Life cycle: larvae Lespesia archippivora (Riley) completes their development within 13 to 17 days. Fly females can lay up to 204 eggs during their life time.
- – Importance: About one third of parasitoids inventoried from the Americas belong to the family Tachinidae. While these often target several species of Lepidoptera attacking maize including other noctuids, they are also found on diverse host plants of the FAW.
In Africa, because of the relatively recent introduction of the FAW on the African continent, data on native natural enemies are still very scanty. First field data demonstrate that a few parasitoids species have already accepted eggs and caterpillars of the FAW as a host.
As these data are preliminary, it remains to be verified whether these natural enemies have shifted from African stemborers or earborers to the FAW, or if they represent new associations from other hosts.
The following parasitoids were recovered from the FAW in West, Central and East Africa:
