Biology of the babushkas
Words: Samantha Ward
As the name suggests, at the Pest & Environmental Adaptation Research Group we are interested in pest species and enjoy investigating novel approaches to sustainably control such organisms. I began my PhD at PEARG in 2016 studying parasitoid wasps; A group of wasps that lay their eggs inside or onto other arthropods. Parasitoids differ from parasites because they kill their host during the process of larval development. Think Ridley Scott’s ‘Alien’ circa 1979. This may sound like something straight from a horror movie, but in an agricultural context they can be incredibly beneficial (not to the host of course) by controlling detrimental pest species.
Symbiosis is the process of two different organisms living in close proximity for an extended period of time. This process can be beneficial (‘mutualistic’), neutral (‘commensalistic’), or deleterious (‘parasitic’). Parasitoids fall into the latter category, completing their lifecycle by feeding on a host. Predominantly found in the insect order Hymenoptera (wasps), parasitoids can also be found in several other orders including Diptera (flies) and Coleoptera (beetles). For most, if not all, pest species there is at least one wasp that can parasitize it. The parasitoids I’m interested in lay their eggs, or ‘oviposit’, into aphids: tiny sap-sucking insects of the order Hemiptera. You may have seen your beloved vegetable plots or rose stems teeming with a sea of green at one time or another. These will be aphids: a gardener’s or farmer’s nightmare.
Aphids are among the most destructive pests on cultivated plants, but also to the grains industry in Australia. This is a major problem as wheat, Australia’s major winter crop, is one of the country’s most profitable exports. Aphids can be particularly devastating to crops as they can transmit viruses, cause direct damage to plants from feeding on their sap, and excrete ‘honeydew’, a sugar-rich sticky liquid that can cause sooty moulds. Furthermore, aphids can reproduce asexually through a process known as ‘parthenogenesis’. A female, known as a ‘stem mother’, produces female clones of herself that are themselves pregnant, much like an aphid version of a Russian matryoshka or ‘babushka’ doll, as more commonly known in the west.
These offspring can develop wings to migrate and infest further crops. Males are involved in sexual reproduction, however the ability of aphids to reproduce asexually enables the population to grow exponentially without being slowed down by the need to meet a potential mate. You can see why they can be problematic! Aphids have been on Earth for millions of years and can be found around the globe, however they mostly affect temperate regions. Their wide distribution is in part down to natural, passive means of dispersal such as being carried by wind or debris floating in the oceans, but also because of us. Intercontinental flights, cargo ships and cars travelling from one town to the next can carry these insects from one place to another. International trade is also a culprit, particularly when plants, flowers, fruit and vegetables are transported. They can even hitch a ride on our gumboots and clothes.
So, how can we beat these pesky bugs? For the last century we have used all sorts of chemicals to eradicate them from farms however, as most people are aware nowadays, these substances can produce undesirable effects. Agricultural chemicals can leach into waterways, into soil, and in high doses can be detrimental to humans. Within Australia, although we have strict regulations in place to ensure these issues do not occur there are still other factors to bear in mind. These insecticides are not selective when it comes to the organisms they kill and so, rather than targeting only pest species, they can also eradicate the useful ones such as ladybeetles, lacewings, and aphid parasitoids. Additionally, the insects that persist after an insecticide application do so due to a resistance to the chemical and will continue to reproduce, creating a population that is proportionally more resistant. To meet the expected level of pest control chemicals may be repeatedly used, causing this cycle to continue until the chemical is rendered useless.
An alternative control method is required and that is where Integrated Pest Management (IPM) comes in. IPM is an environmentally sensitive and sustainable method of pest control and does not solely rely on using agricultural chemicals. This strategy involves monitoring pest species before using a combination of cultural, physical, genetic and biological controls. Chemical methods are not completely ignored, except in this case less toxic options are selected and used as preventative measures, or as a last resort. I won’t spend too much time explaining the other control methods however crop rotation, genetic engineering of pest-resistant crops, the use of barriers and traps, and cultivation techniques are all options regularly employed within IPM. The area I’m interested in, however, is biological control and predators, pathogenic fungi and parasitoids that are either naturally occurring or introduced are used as tools to achieve this. These groups of organisms are termed ‘natural enemies’ of pests and have been used, particularly in horticulture, for several decades. Parasitoid wasps themselves have been utilised to control whiteflies, which are pests of tomatoes and other horticultural crops, for almost a century.
Aphid parasitoids include microscopic wasps within the subfamily Aphidiinae and the family Aphelinidae. Adult wasps usually feed on nectar, pollen or the honeydew secreted by aphids or other Hemiptera, as shown in the photograph below.
Female wasps lay their eggs using a modified sting called an ‘ovipositor’ to pierce into the aphid host and begin the process of egg and larval development. As a defence mechanism, aphids will kick at the wasps often resulting in an unsuccessful oviposition. Sometimes aphids have guardian angels in the form of ants protecting them from wasps in order to feed on the honeydew themselves.
Before beneficial organisms can be used to control a pest in a specific region we need to understand the species occurring in that environment. Aphid parasitoids have been well-studied in other countries, but little is known about them in an Australian context. Australia has highly varied climate zones, and the productive southern grain belt is a temperate region which is ideal for aphids to thrive. This is where I’m currently studying, collecting aphids and their parasitoids across several states with the purpose of investigating their distribution, activity and relationships with native flora, which can produce the food and shelter required by adult parasitoids. The objective is to produce data that will be useful to assist in the implementation of parasitoids within IPM programs in this country.
Sounds rather complex I know, however this is just the beginning of a long and complicated process. This network of interactions stretches a tad further because parasitoid wasps do not just parasitize aphids. They also parasitize each other. Another groups of wasps, called hyperparasitoids parasitize the primary parasitoids of the aphid pests. No, it’s not a tongue twister, but it is a story for another day…