Lizzy Lowe We all know that having beneficial insects in crops is a good thing, especially natural enemies that help to reduce pest numbers. But how many species does it take to make a difference? And how can we measure the impact that natural enemies have on pests?
The answer is complicated. In this article we will cover some of the difficulties that researchers have when it comes to accurately measuring the impact of natural enemies, or even working out how many species are present in a field!
The range of beneficials in grains crops
It is becoming increasingly evident that sustainable cropping systems rely on healthy supporting ecosystems. In agriculture, most of the beneficial species we rely on are invertebrates: insects, worms, spiders and mites which all contribute to keep ecosystems ticking.
When we talk about beneficial species, this encompasses creatures with many different jobs, including pollination, decomposition, aeration of the soil, distribution of native seeds and of course biological control of pests by natural enemies.
In grains crops, generalist predators such as spiders, assassin bugs, carabid beetles, lacewings and ladybirds are all vital tools in any IPM tool kit. More information on the beneficial insects, mites and spiders present in grains crops can be found at IPM guidelines for grains and in our Doorstep Science video below.
In addition to predators, parasitoid wasps are some of the most valuable natural enemies in grains because they are very efficient (especially at controlling aphids), and some species are specialists which target specific pests.
But beneficial insects are notoriously hard to capture, count and identify (especially parasitoids). So how do scientists do it?
Counting the MANY species of natural enemies
Since there is such a huge array of natural enemy species, all with different host preferences and life cycles, working out which ones are present in a given area is a challenge.
One of the most effective tools is often the good old yellow sticky trap. The number of trapped parasitoids can be used to predict overall numbers in a crop. The other usual methods of sweep netting or beat sheets for critters in the crop foliage and pitfall traps for ground dwelling species can be used to get better coverage.
As an example see our PestFacts article about monitoring for natural enemies of green peach aphids.
Another accurate way to find different species of parasitoids is to take pests collected from the field into the lab and rear them until the parasitoids emerge. You can find more information here about a research project we are currently working on with the University of Melbourne which does just that.
Know your enemies
However, catching natural enemies is the easy part, proper identification is where it gets tricky. It takes a highly specialised entomologist to identify many species, and there are still countless species which have yet to be formally identified.
Parasitoid wasps in particular are amazingly diverse. Nobody knows the exact number of species in the world, but numbers are known to be in the hundreds of thousands (and that’s before we take into account hyperparasitoids, the wasps that lay eggs in the wasps that lay eggs in caterpillars). A recent study conducted by Dr Samantha Ward looked at parasitoids in grain crops found 23 species of aphid parasitoid in Victoria alone.
In addition, in Australia there are over 3500 species of spider, 1000’s of beneficial beetles, 250 species of lacewing and over 300 species of assassin bugs.
That’s a LOT of natural enemies, but how do we know if they’re actually doing any good?
The difficulties in assessing impact
It’s one thing to know how many species are present, but a different kettle of fish to measure the impact that these species have on pest control. This is such a complicated research question that it is rarely fully addressed.
In order to measure natural enemy impact, you need to know whether the pest is killed by the natural enemy, how many pests each natural enemy can kill (and how quickly), how natural enemy populations change when pest density changes and how the natural enemies search for prey.
And this is before we even consider how the actions of a natural enemy on a pest affect actual rates of herbivory, or how different environments might affect these interactions.
But the difficulty in calculating the exact levels of impact doesn’t mean that there aren’t useful ways to estimate rates of predation and parasitism.
Counting “mummies” to estimate parasitism rates
Parasitoid wasps are especially valuable biological control agents because they can play a large role in the control of pests, especially aphids and caterpillars. But how can we measure just how big an impact they have?
A simple survey of a field is not enough, because the adult wasps are often tiny and the number of adults will not reflect the number of pests impacted, since some species can lay eggs in many hosts.
When in the field, one of the best ways to estimate parasitoid loads is to look for the remains of infected pests. For example, parasitoids use the eaten-out husk of aphid hosts as a safe place to pupate, and these “mummies” are often visible in the field.
However, recent research by Dr Samantha Ward and colleagues showed that although rates of parasitism in green peach aphid can be quite high (up to 14%), the true rates of parasitism rates (calculated by hatch rates in the lab) can be two to four times the rates calculated from mummy counts.
This means that the actual impact of parasitoids is likely to be higher than what you are able to observe in the field.
Using “bait” to estimate predation and parasitoid rates
Another interesting way that scientists can get an idea of the impact of predators and parasitoids in an area is to use sentinel prey. This is where a pest, such as an aphid, is cultivated on plants within a greenhouse and then plants with known numbers of aphids are placed out in the field (that already has aphids present). The plants are monitored for predators and collected after a day or two to see how many aphids survived and how many are infected with parasitoids.
A good example of using sentinel prey comes from a study looking at the impacts of natural enemies on cowpea aphids, a pest found in a wide range of summer and winter pulse crops throughout Australia. Plants infected with cowpea aphids were placed in faba bean fields and after 48 hours, researchers counted the predators present and then reared out the parasitoid wasps in the lab.
Predators such as ladybirds, lacewings and hoverflies were found on 75% of the plants, and 56% of the aphids put out into the field were parasitised. This shows that under some conditions, predators and parasitoids are able to very quickly locate and begin to control prey.
You can find more information about this research at the Beatsheet.
The importance of natural enemy biodiversity
Ok, so we now know that it’s very difficult to put real numbers on the impact that natural enemies have on pest suppression, but what we DO know is that there are literally thousands of natural enemy species in Australia, and that given half the chance they can help reduce pest numbers.
Although natural enemies which are targeted towards specific pests are one of the most important tools in IPM, research shows that the best pest control outcomes occur when the number of different natural enemy species is high.
For example, a large-scale international research project looking at ecosystem services in crops found that biodiversity significantly improved the provision of biological control.
Because so much of the insect life present in natural systems is undiscovered, there are also likely to be species providing benefits that we’re not even aware of. This is why encouraging biodiversity for biodiversity’s sake is a fundamental part of sustainable farm management.
Natural enemies are generally very prevalent during spring and will play a key role in pest control when low to moderate numbers of crop pests are present. One of the best practices to support natural enemies in grain crops is to avoid the use of broad-spectrum insecticides.
Upcoming webinar
If you would like to know more about new research into beneficial species in grain crops, Cesar Australia are holding an online webinar on the 21nd of September.
Presentations will include new information on parasitoids and the impact of pesticides on beneficial insects. There will also be the opportunity to ask our researchers questions.
You can register here
Acknowledgements
Thank you to Garry McDonald (University of Melbourne), Peter Ridland (University of Melbourne) and Paul Umina (Cesar Australia) for contributions to this article.
Cover image: Photo by Andrew Weeks, Cesar Australia
