There are so many different insect pests that love our produce as much as we do- sometimes it can feel like a constant battle! No matter where you live in the world, there are insect pests that will ravage your vegetable garden at some point. From biting to stinging to chewing to egg laying, these tiny critters can be enough to make you tear your hair out.

Being able to recognise the signs of insect pest activity is key to its management. Looking closely at your plants every day is a good starting point- I find this is easy to do when watering plants with the hose.

Let’s have a look at some of the signs of pest activity you might see in your garden during summer.

Grasshoppers

While grasshoppers are a sign of diversity and provide a food source for birds and lizards, they can cause some serious damage to your food garden. Highly mobile & voracious feeders, they are difficult to see, so the damage can occur before you’re even aware of their presence.

Grasshoppers attack all sorts of plants, leaving large holes in leaves which are randomly placed

With a population that appears in spring then peaks during summer, grasshoppers hatch from eggs into nymphs which then undergo 5 moults before reaching adult size. Grasshoppers favour corn, lettuce and beans, but in reality will eat almost everything in your garden, even including ornamentals. Grasshoppers feed on soft leaf tissue, so will chew holes in and around the edges of leaves. The holes are usually large, and leaves are often left with ragged edges.

Grasshopper population and resultant damage can be controlled in a variety of ways. Encouraging natural predators, allowing chickens to roam, tilling the soil and eliminating weeds will all help a little. Other methods for grasshopper control include exclusion (netting your plants) and spraying (organic or chemical). Organic remedies include garlic oil spray and coating the leaves in flour. Inorganic solutions are those such as neem oil spray, chemical baits and pyrethrum spray.

Sooty Mould

Sooty mould looks like a layer of black dust all over your plants. It is not a disease in and of itself, but is a fungal growth caused by sap-sucking insects like aphids and scale.

A finger lime tree affected by sooty mould

These insects exude a sugary substance known as honeydew which the sooty mould feeds on. As a result, the more of these insects you have, the worse the sooty mould will be. AS the sooty mould fungi are decomposers which feed on the honeydew, they do not draw any sustenance from the host plant. Sooty mould does damage by covering leaves, blocking light and making photosynthesis less efficient.

Sooty mould can be treated in a variety of ways. If possible, start by wiping it off with a damp cloth. In the case of prickly trees like finger limes though, this will be very difficult if not impossible.

A better solution is to make your own white oil at home and spray the affected plants thoroughly. White oil is a multi-purpose pest treatment that is a must-have for every food garden. It is used to combat sap-sucking and chewing insects like aphids, scale, mealy bug and citrus leaf miner. It suffocates rather than poisons these insects, so they can’t develop a resistance to it. You can buy a commercial white oil preparation, but it’s much cheaper & just as effective to make your own.

Spider Mites

Spider mites are tiny little insects about the size of a pin head that are barely visible without magnification. Like leaf miner, spider mites suck the sap out of the middle of leaves, weakening plants, yellowing foliage and reducing harvests.

A common pest, they reproduce rapidly and will attack a wide variety of both edible and ornamental plants. A spider mite infestation will appear as yellowed foliage, tiny white spots on the upper surface of leaves, and fine silk webbing on the undersides of leaves.

To treat spider mites, firstly remove and bin all visibly affected foliage. Next, spray with a solution of wettable sulphur, which is an organic remedy. A simple home-made soap spray is another option, and it will work by coating the insects’ bodies and suffocating them. Effective sprays for spider mites can also be made from rosemary oil, apple cider vinegar and neem oil.

You’ll need to apply any spray 2-3 times per week before a spider mite outbreak is brought under control.

Leaf Miner

Citrus leaf miner (phyllocnistris citrella) is a very common pest to all types of citrus trees. No matter where in the world you live, this tiny, insidious pest is likely to appear on your citrus during summer. Its distribution is close to worldwide, so whether you grow limes, lemons, oranges, finger limes or hybrids, this common pest can infiltrate and damage your precious fruit trees without you even realising it. 

Citrus leaf miner looks like yellowed, curled leaves with ‘snail trails’

Leaf miners are tiny grey flies about 2 mm long, whose larvae feed on leaves. A moth of the family gracillariidae, these pests appear with the onset of warm weather and feed on the fresh new growth of citrus trees. Unlike gall wasp, citrus leaf miner attacks leaves rather than stems. Sadly, these larvae are practically invisible, which means that picking individual insects off by hand is not possible. They destroy the soft healthy tissue in the middle of the leaf by tunnelling through it, leaving a curled leaf with a visible ‘snail trail’. This renders leaves unable to capture sunlight and grow effectively, leading to a weak, stunted tree.

Once you have leaf miner in your garden, it is virtually impossible to eliminate. On the upside, if you keep it under control, they won’t kill your trees either. Exclusion and home-made remedies are the best way to combat leaf miner (even if you wanted to use them, insecticides aren’t particularly effective). We recommend a daily walk around your garden, so you can keep a close eye on pest activity. Make sure to remove and bin (not compost) any affected leaves, consider netting trees, and spray with a home-made white oil. White oil is a mixture of soap and oil that suffocates soft-bodied insects. Mixed with water and applied with a spray bottle, it is highly effective in combatting leaf miner, plus it is cheap and super-easy to make at home; here is our recipe.

Gall Wasp

This is a small wasp that lays their eggs in the softer, new branch growth of citrus trees. Gall wasps attack fresh growth and are particularly damaging to young trees, as the galls stunt their development. While gall wasp won’t generally kill trees, severe and repeated infestation in established trees can reduce vigour & fruiting, plus cause dieback. Gall wasps attack by making a tiny hole in a young branch then depositing their eggs in the soft tissue. The larvae feed on this citrus stem tissue as they grow. The tree then recognises this as damage, and grows over the entry hole, thereby creating a ‘gall’. As the eggs mature into larvae the gall swells and begins to look like a lump. After around 9 months the wasps are ready to be born, and chew their way out of the branch, causing further damage to the tree.

Gall wasp is an easy fix that doesn’t require the use of pesticides. Bear in mind that pesticides are indiscriminate, so will also kill all the bees & other pollinating insects plus any valuable predator insects. This destroys the balanced ecosystem that we as organic gardeners work hard to establish. There are two ways to combat gall wasp. Traditionally, stems with galls would be cut off and either burned or discarded. Composting or chipping these infected prunings is not recommended, because the galls can stay alive and potentially re-infect your other citrus. You can also solarise the infected branches by sealing them in a plastic bag and leaving it out in the sun for at least 4 days, or simply bin the infected prunings.

Gall wasp, before and after slicing open

These days, people are slicing through the sides of the galls and leaving the branch in place. This allows the tree to recover more quickly and continue it’s natural growth. Slicing the gall exposes the undeveloped wasp eggs to the air and kills them, as shown below. The slicing itself is very quick and easy to do. Use a vegetable peeler or stanley knife to slice each side of the gall off– it’s just like peeling a potato. Note that the slicing does not cause lasting damage to the tree, as the stem tissue will gradually repair itself.

Gall wasp tend to return to the same tree or trees year after year, so constant vigilance is key. Make sure you check your citrus trees at least weekly, taking time to inspect stems carefully. Gall wasps do have natural predators which are parasitic wasps. These wasps (Megastigmus brevivalvus and M. trisulcus) insert their eggs directly into the eggs of gall wasps where they slowly develop in the host larva, eventually killing it. Note that the use of any chemicals in the garden may also affect the Megastigmus. To read about other pests that may affect your citrus trees, check out our Summer Citrus Tree Care guide.