Workshop: Pests – A Deeper Dive

This article is based off a workshop delivered by Alex B at the Melbourne University Community Garden on 28/05/2024.

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A Brief Pondering on the Morality of Insects

To begin, I would like to list a few common insects you might find in the garden. As you read through this list, think about whether or not you’d classify them as good or bad.

    • Wasps
    • Earwigs
    • Praying Mantis
    • Snails
    • Hoverfly
    • Lacewings
    • Slugs
    • Cabbage Moths (Small, brown, triangular moths)
    • Spiders
    • Aphids
    • White Fly
    • Cabbage White  Butterflies (Colloquially known as Cabbage Moths. These guys are in fact, not Moths!)
    • Millipedes
    • Butterflies
    • Dragonflies
    • Ladybugs
    • Mosquitos
    • Cockroaches
    • Bees
    • Damselflies
    • Wasps
    • Earwigs
    • Praying Mantis
    • Snails
    • Hoverfly
    • Lacewings
    • Slugs
    • Cabbage Moths (Small, brown, triangular moths)
    • Butterflies
    • Dragonflies
    • Ladybugs
    • Mosquitos
    • Cockroaches
    • Bees
    • Damselflies

    Now ask yourself:

    • How many off this list earned the title of ‘good’?
    • How many of these insects did I know of?
    • What information did I use to classify them
     

    As for the answers to this introspective exercise, there are none. No insect is inherently good or bad by nature. This is an important perspective to have when interacting with all wildlife, especially insects, which often get the short end of the stick in regard to our treatment of them.

    The key to any garden is balance

    For this article, to make it a bit easier, I will refer to the unwanted plant-eating insects as ‘pests’ and the non-plant-eating kind as ‘beneficial insects’.

    While insects do not act on malicious or heroic intents they can still cause damage to the ecosystem. 

    Too many predatory insects in a garden will cause a shortage of prey and many will leave for greener pastures or die from starvation. 

    Too few, and your garden becomes overrun by their would-be prey, which in large numbers, can cause severe damage to your plants, if they don’t just kill them outright.

    Here are a few of the more negatively perceived insects we find in the garden.

    InsectSummaryProsCons
    WaspsThere are hundreds of different wasp species in Australia, lots of them native! Wasps serve many functions in the garden and are quite useful to have around!

    Some species are pollinators, with certain species of native orchid reliant on them for pollination.

    Others are insectivorous or parasitoids, helping to keep the more pesky insect populations down.

    Introduced species of Wasps can outcompete natives and are generally quite aggressive and territorial to outsiders near a nest.

    Solo wasps that you might encounter will generally leave you alone if you leave them alone.

    EarwigsA common insect to see in the garden, there are an estimated 85 identified earwig species in Australia however, there is belief that the number of unidentified species is the same.

    They can eat decaying plant matter, helping with decomposition in your garden.

    Some even eat other insects which helps keep populations under control.

    They are voracious eaters of any and all plant matter, but they really love young tender leaves of seedlings and new growth.
    Lacewings

    Lacewings are found all over Australia, particularly in urban areas as adults are attracted to lights.

    Adult lacewings come in many shapes and colours and differ greatly from their larvae which can look like little fuzzy disks to suit their tree-dwelling lifestyles or they are ground dwellers with grublike bodies and strong jaws.

    Eggs can be laid in sand or on vegetation, common species have little white eggs on thin stalks, commonly laid on leaves.

    Some lacewing families specialise in eating Aphids and Scale!

    Others help keep spider and wasp populations controlled.

    A few species feed on decaying plant material, nectar and live plant material.

    Not many, they might bite if you startle them, but their jaws aren’t really adept at grasping skin.
    Cockroaches

    Cockroaches get a bad rap, some introduced species deserve it, but Australia has a wide array of native roaches (450)! Only 5 of which are classified as ‘pests’.

    Most Australian species are wingless (phew) and some come in bright colour patterns.

    Cockroaches found outdoors are usually not the pesky kind and feed off pollen, bark, decaying wood and leaf litter.

    They are also an important food source for predators in the garden like reptiles, mammals and birds.

    Some are stinky when frightened as a defence mechanism. Some might wander into the house for respite and water during heat or low rainfall.

    Learn how to ID roaches classified as pests and put the ‘good’ ones back out into the garden when they get lost.

    Diversity = Balance!

    So how can we achieve balance and what can we do when things are out of whack?

    The easiest way to do this is also one of the better long-term solutions to maintaining balance. Plants.

    Planting lots of edible, productive plants is quite tempting, just imagine the big harvests and abundance of food you could have! However, producing crops rarely put up a good fight against pests and aren’t too attractive to beneficial insects.

    This is where diversity is key, planting repellant and decoy plants as well as encouraging natural predators into your garden assists in forming a robust ecosystem.

    Repellant/Masking Plants

    Just like humans, insects can be affected by various scents and chemicals produced by plants. 

    Personally, I find myself repulsed by the scent of Mediterranean Spurge, which smells like wet cabbage farts in my opinion, and to my dismay appears to be quite the popular landscaping plant in public areas. 

    Lots of plants can have this effect on insects, but below are some of the more common species found in gardens.

    The Dreaded Mediterranean Spurge (yucky)!
    Plant TypeExamplesImpacts
    HerbsMarjoram & OreganoPumpkin beetles, White Cabbage Butterfly (when planted near other crops like broccoli.)
    BasilFlies, Mosquitos, carrot and white flies, powdery mildew. Though you do need to grow a lot for it to really make a difference, pesto anyone?
    PeppermintAphids, squash bugs, flies.
    Borage Tomato Hornworm & Cabbage Worms.
    Thyme Cabbage Moths & White Flies.
    AlliumsChivesAphids, Japanese Beetles & Carrot Fly.
    GarlicRoot Maggots, Peach-tree Borer & Cabbage Looper.
    OrnamentalsTansy (technically a herb but is very bitter and not really used in cooking.)Ants, Beetles, Flies, Cutworms, White Flies & Tomato Hornworm.

    Decoy Plants

    If you ensure that decoy plants are a more attractive and accessible crop for pests in your garden, they will get eaten, but that’s their job. Bonus points for planting edible decoy plants, there’s plenty to choose from.

    “Some of you may die, but that is a sacrifice I am willing to make” (Lord Farquad. Shrek, 2001).

    Some easy-to-grow decoy plants include:

    PlantUsesDescriptionTargets/AttractsNotes
    NasturtiumsThe leaves and flowers are edible and come in a range of colours and patterns.Fast-growing, sprawling and prolific. Low maintenance, can’t climb but looks lovely in a hanging basket. Relatively hardy, they grow well in Melbourne and other similar climates (not frost tolerant.)

    Whitefly, White Cabbage Butterfly, Earwigs, Slugs, Snails, Thrips.

    If you’d rather not have a bunch of Nasturtium seedlings in your garden it’s best to prune back the main plant before the seeds start drying out and going brown or dead-head old flowers as they appear.

    Sunflowers

    Huge variety of colours and sizes. Seeds are of course edible. Immature sunflower heads can be grilled and eaten when the seeds are still soft, usually, this is done with the larger varieties.

    Great for warmer climates, plant anytime. In cooler climates, they have a limited window for planting. Don’t need much to grow but will likely be a bit more hardy and produce nicer flowers with enriched soil or given fertilizer while growing.

    Bees, Birds, Cabbage moths, Possums (not a bug but still a bit pesky), pollinators, Snails and Slugs.

    Depending on how tall your flowers grow, supports might need to be added to prevent the stems from snapping in strong winds.
    If you intend to harvest the seeds you might want to use mesh bags after the petals have died if you wish to eat them and not share with birds and possums.

    Mustard

    Many varieties for many uses, some as salad greens, for seeds or as green manure.

    Fast-growing and can be planted anytime in most climates. However, be careful of bolting in warmer temperatures.

    Aphids, cabbage moths, pollinators.

    Most mustard species can be used as green manure. When added to soil it protects against soil pathogens and pests, and reduces weeds. Make sure you harvest the plants before seeds are set on the plant otherwise, you might just end up with a garden full of mustard.

    Cherry Tomatoes

    Large range of varieties to suit different uses and climates.

    Fast-growing and often great for smaller gardens. Some varieties are quite prolific whereas others are larger.

    Aphids, Birds, Cutworms, White Fly, Possums**.

    **Honestly one of the best decoys for possums in your garden, a prolific cherry tomato near a fence or other possum pathway (remember they don’t like going on the ground) will prevent them from going near your precious berries or other delicious fruits. You might even get to eat some of the sacrificial tomatoes depending on how prolific it is!

    Some decoy plants are more than just food…

    Cabbage moths (the brown ones)  happen to find Land Cress a rather attractive place to lay their eggs, as opposed the the lovely leaves of the brassica family (Broccoli, cabbage, etc.) that they usually make their nursery. Unfortunately for them, Land Cress is rather toxic to cabbage moth grubs, it is for this reason that Land Cress is known as a dead-end trap crop as it breaks the lifecycle of this insect. 

    Unfortunately, there aren’t too many known dead-end trap crops, but there is active research continuing to look into trap crops for commercial uses to reduce pesticide usage. 

    A few simple tips/rules when planting decoys:

    1. Be mindful of what you plant and when they flower, shoot and fruit. Decoys only really work when they’re at their most appetising at the same time as the crops they need to protect are growing! If your Nasturtiums won’t be fully grown and flowering at the same time as your lovely leafy green seedlings are going in, then they won’t do their job, will they? 
    2. Keep them at a distance, you want them close but not too close, but also not too far. Think of each plant as a house, we want detached suburban houses, not attached inner city terraces or rural farmhouses. 
    3. Ideally, your decoy plants should make up 20% of your garden, too many and you won’t have enough room to plant productive crops, and it may even increase the population of pests in your garden! Too few and there won’t be enough to go around, your nice leafy greens will certainly look like prime real estate…
    4. Keeping your decoy plants in smaller pots enables you to move them around to where they’re needed, especially in small gardens, as well as keep them contained and prevent them from competing with other crops.

    Insect Attracting Plants

    When planting to attract insects and other garden friends, one must ask themselves, what do you want in your garden? If you are highly allergic to bee stings it’s probably best not to plant copious amounts of flowers that attract stinging bees. Many other pollinators exist including a variety of native stingless bees and other less-known insects who often specialise in pollinating certain flowers. There are also lots of edible attracting plants, some of which might already be in your garden!
    When planting flowers, shape and size matter! You want to have as much floral diversity as possible when possible, plan for seasonality and aim for all-year-round floral coverage.

    Here are some notable examples that attract a wide range of insects to your garden.

    PlantInfoAttracts
    DandelionAn edible ‘weed’, easy to grow and very nutritious.
    Young leaves can be used as a slightly bitter salad green, however, I prefer young and middle-aged leaves stir-fried in a bit of oil and garlic! The root can also be dried out and ground down into a caffeine-free coffee alternative!
    They are also some of the only flowering plants during and after winter in colder climates and thus provide an important food source to pollinators in early spring.
    Bees (even the stingless kind), Butterflies, Lacewings, Ladybugs, Hoverflies.
    Umbel (Umbrella) Shaped Flowers – Apiaceae
    (Angelica, Dill, Fennel, Parsley, etc.)
    All are edible and quite prolific, producing plenty of seeds, be careful with plants self-seeding. Some are annual and others are perennial.Ladybugs, Hoverflies, Native Wasps, Parasitic Wasps and Lacewings.
    Also attractive to smaller ‘prey’ insects.
    MarigoldsWhile flowers are edible, they are more common as an ornamental plant.

    Ladybugs, Bees, Hoverflies, Parasitic Wasps, Pirate Bugs and Damsel Bugs.

    NativesWhile many flowering native plants might not give much in the way of a plentiful harvest they do provide numerous benefits for a huge range of native and non-native wildlife!Bees, Butterflies, Hoverflies, Native Bees, Native Wasps, Moths, Bats, Birds and Possums.

    Do Plants Have BFFs?

    Now that we know all about how different plants impact insects, we can put that knowledge to good use!

    Companion planting is all about utilising one plant’s strength to benefit another, being mindful of what you plant and when, ensuring all plants get enough sunlight, shade and support while growing. There are plenty of reasons to plant specific plants together:

    • Deterring Pests
    • Attracting beneficial insects or pollinators
    • Shade
    • Natural Supports
    • Soil nutrition
    • Efficient usage of space
     

    Here’s a few good examples of companion plants:

    PairingWhy it Works
    Parsnips/Carrots & LeeksLeeks are shallow rooted and are an allium so they prevent pests from eating the parsnip which needs lots of space to tunnel its roots down.
    Basil & Tomatoes (name a more iconic duo!)Basil is a good repellant plant especially for pests that prefer tomatoes.
    Beans, Corn and Squash (pumpkin, zucchini or similar)The traditional ‘Three-Sisters’ planting method was used by the Native Americans for thousands of years to provide the most food from the smallest space. The beans use the corn as a support while also enriching the soil with nitrogen and the squash shades the soil and prevents other plants from growing nearby.
    Cucumbers and Dill (pickles anyone?)Cucumbers benefit from the pollinators as well as the aphid eating predators that the dill attracts.
    Cabbage/Brussel Sprouts (Brassicas) & NasturtiumNasturtiums are not only a good repellent plant but they are also a good decoy plant for cabbage moths and cabbage white butterflies which do find cabbages rather tasty.

    Now do keep in mind that you can’t just go about planting willy-nilly! Not all plants are going to get along with each other and some might just get outright hostile at the thought of sharing space with others.

    A great example of this is in Sunflowers, an unaware gardener may think that the tall strong sunflower stem would be the perfect trellis for their beans or peas this year. Unfortunately for them, Sunflowers are ‘Allelopathic’, meaning they ‘poison’ the soil around themselves to prevent other plants from encroaching on their space. It’s probably best to keep them in pots than in a bed with all your other plants.

    Allelopathy is when plants contain chemicals that prevent the growth or germination of other species. These chemicals can be stored in different parts of the plant and can affect certain plants more than others. Other allelopathic plants include:

    • Eucalypts: their fallen leaves contain chemicals that are transferred to the surrounding soil.
    • Black walnut: chemicals are contained in the roots and nuts.
    • Mango: Fallen leaves as well.
    • Brassicas: leftover plant matter in the soil may stunt the growth of future seedlings.
    • Cucumbers: roots release the chemical into the soil. Doesn’t play well with lettuce despite both being a key salad ingredient.
    • Fennel: all parts contain chemicals that stunt the germination and root growth of surrounding plants.
    • Rosemary/Thyme/Oregano: Some of the tastiest and most fragrant herbs in the garden, unfortunately, they too don’t like to share space with others. This effect is exacerbated in rosemary by dry conditions.

    Workshop Pests – Citrus Gall Wasp and Woolly Aphids

    Here is a more detailed overview of what we talked about and had a go at treating at the working bee.

    Citrus Gall Wasp:

    Citrus Gall Wasp (CGW) is a very, very, small insect that lays its eggs in citrus trees causing the area to swell forming a ‘gall’ around the developing larvae. CGW is actually a native insect, however it is not native to areas like Victoria, South Australia and Western Australia, even if it has spread to these areas. The trouble with this is that the common predator of CGW, two species of predatory wasps, failed to get the memo about this migration and as such they have no established presence in Victoria. This means that CGW are practically endemic to citrus trees in Melbourne, though they do tend to prefer Lemon, Grapefruit and the occasional Orange over Limes and their native prey, Finger Limes.
    When Treating CGW infected trees do be mindful of promoting excess growth during breeding and egg-laying seasons (September – December). Excessive pruning and fertiliser in winter will often cause an explosion of tender new growth in spring, the perfect nursery for CGW larvae. A note for when dealing with infected branches when pruning, best practice is to burn the branches with unhatched Larvae in fully formed galls as they will continue to develop and hatch before the branch is dead.

    When to prune and fertilise seems to be quite the conundrum for Melbournian gardeners as detailed by Duncan over at Leaf, Fruit and Root who went through a detailed experiment for treating CGW which is well worth a read.

    Our treatment method in the uni garden was a bit more oldschool than Duncans methods, we used a sharp blade to slowly peel back one side of a developing gall until the larvae or eggs are exposed. This method is best for trees that have a few galls otherwise you’d be doing a lot of damage to an already weakened tree.

    We were a bit early with our attack as mid-winter is preferred, however I felt that it was a good experience for everyone to have a go at a gall. We will remember to keep at it over winter and hopefully reduce the population of CGW in the Melbourne Uni garden.

    Woolly Apple Aphids:

    Thanks to everyone who was willing to get their hands a little dirty for our two apple trees. There was quite the visible reduction in fuzzy white bodies all over the branches and with them gone you could really see the damage to the tree (hopefully this eases any guilt anyone might harbour).

    Our treatment method was the good ol’ squash ‘em and leave the bodies as a warning. Most species of Aphids when injured or killed, produce a chemical that warns other Aphids of dangerous areas where predators might be. This means that squishing them is quite effective at reducing the population.

    Before
    After

    As I explained at the workshop, Wooly Apple Aphids (WAA) are an interesting species that has specialised in targeting apple trees, they give birth to live young or nymphs, and adults produce a white furry coating to protect them from predators.

    When treating most pests naturally it’s important to know the lifecycle of the insects you are looking to treat. WAA usually inhabit two trees during their lifetime, one to feed the newly emerged nymphs and another to breed, nymphs often retreat into cracks into the bark or into the roots at the base of the tree to go through a rigorous growth period where they will moult four times before emerging as an adult.

    It’s during this time, in winter, that chemical treatments are recommended to be used on large infestations. Our plan of attack for our WAA after squishing the adults was to look into utilising Diatomaceous Earth or planting Aphid repelling species near the two trees in upcoming working bees.

    Additional Reading: