PARASITOIDS
Introduction
• Parasitoids – organisms that spend a large part of their lifecycle
attached to or within the organism (in a relationship that is essentially
parasitic).
• However, unlike a true parasite, a parasitoid ultimately sterilises, kills
and/or consumes its host.
• Most beneficial insect parasitoids are wasps or flies.
• Unlike predators that immediately kill or disable their prey, pests
attacked by parasitoids die more slowly.
• Some hosts are paralysed, while others may continue to feed or even
lay eggs before they die.
• Parasitoids often complete their life cycle more quickly and increase
their numbers faster than many predators.
• Parasitoids can therefore be the more dominant and effective natural
enemy of some pest insects, but their presence may not be obvious.
• To determine the extent of parasitism, insect pests may need to be
dissected or reared to see if adult parasitoids emerge.
Characteristics of parasitoids
• Often highly specialised and host specific
• Tend to be smaller than the host
• Only the female searches for the host to lay eggs. Eggs are usually laid
in, on, or near host
• Immature parasitoids remain on or in host; adults are free-living and
mobile
• Sometimes multiple parasitoids may emerge from one pest.
• Highly susceptible to insecticides , especially adults
• Adult forms often require an alternative food source such a pollen or nectar
Importance of parasitoids in insect
management
• Pest management with parasitoids costs nothing
• At low pest densities, parasitoids can suppress infestations to below economic
thresholds
• Parasitoids reduce the number of pests surviving to the next generation
• They are compatible with other biological control agents (diseases and predators)
• Parasitoids are host specific
• Some parasitoids affect feeding behaviour e.g. parasitised larvae eat less than
healthy ones
• Parasitoids are efficient host searchers – they can find hosts even when pest
densities are low
Parasitoid life cycle and terminology
• Parasitoids are described according to the life stage of the host that
they attack. For example, egg parasitoids parasitise eggs. Life stages
and parasitoids include:
• Egg parasitoids e.g. Trichogramma & Telenomus wasps
• Larval parasitoids e.g. Netelia producta (Orange caterpillar parasite) &
Tachinid flies
• Larval – pupal parasitoids e.g. Heteropelma scaposum (Two-toned
parasite)
• Pupal parasitoid –Ichneumon promissorius (Banded caterpillar parasite)
Trichogramma
• Larval Parasitiod (Netelia product)
• Larval – pupal parasitoids e.g. Heteropelma scaposum (Two-toned
parasite)
• Adult Parasitiod (Healthy Aphid on left and mummified aphid on right)
• Some parasitoid species attack one stage but do not emerge from
their host until much later.
• An example of this is the larval-pupal parasitoid Heteropelma wasp
that lays its eggs inside the helicoverpa caterpillar, but the adult wasp
does not emerge until after the caterpillar has pupated.
Benefits of parasitoids
• They are usually extremely well adapted to their natural host and are
very good at finding them even when densities of the host are
relatively low
Limitations of parasitoids
• They are often (but not always) host-specific and often will attack only one
species of pest.
• They are generally more delicate than predators and hence more vulnerable
to pesticides.
• Because parasitoids spend most of their life cycle developing within their
prey, they are less visible than predators, and their performance may be
underestimated as a result.
• Parasitoids can be parasitised by other parasitoids. This is known
as hyperparasitism – a natural and common occurrence which can reduce
the effectiveness of some beneficial species. Little can be done to manage
hyperparasitism.
Evidence of parasitism
• Look for evidence of an exit hole in the host
• Dissect samples (may be difficult with very small insects)
• Rear pests in an insect proof cage to see if parasitoids emerge
• Check for deformed insects e.g. caterpillars or mummified/bloated
insects such as aphids
• Look for wasp cocoons near caterpillars
Parasitoid susceptibility to pesticides
• Parasitoids are often more susceptible to insecticides than predators.
• Adult parasitoids are usually more susceptible than their hosts.
• Immature parasitoids, especially if protected within the egg of their
host or in their own cocoon, may tolerate pesticides better than
adults, but immature parasitoids will usually die if their host is killed.
• To improve the abundance and diversity of beneficial insects –
consider native vegetation on the farm as a part of a pest
management strategy.
