Insect Pests and the One-Week Rule
Many insect pests reproduce rapidly during the warm summer months and during this time there will be an interval of approximately 7-10 days between generations. In warmer weather this interval is often shorter, and can be just a few days.
As many pest eggs are usually unaffected by insecticides, this means that a repeated treatment is usually necessary at weekly intervals for a few weeks, to ensure that any newly hatched young do not develop and lay eggs themselves before the next treatment.
This is known as the ‘one week rule’.
Most common garden insecticides are toxic but break down very quickly with exposure to moisture or UV light, so even systemic insecticides may need to be used repetitively. The use of horticultural soap is usually the best course as it will not cause wider damage to predatory bugs or pollinating insects provided you are careful with the spraying.
Mealybugs are small, white, fluffy-looking insects in the scale insect family which are often found as pests of houseplants and greenhouse plants. They are one of the harder to treat insect pests because they have a fluffy waxy coating which prevents most insecticides from coming into contact with the body. There are many species of mealybug around the world with some species giving birth to live young, however the most common species tend to lay their eggs in clusters protected by white fluffy waxy, which protects the eggs from predators and pesticides.
The newly hatched young are almost invisibly small, so they are almost impossible to eradicate in a large-scale setting, but in the home or small greenhouse they can be successfully eliminated with a persistent course of systemic insecticides, several courses of topical insecticides (such as non-toxic horticultural soap), or by regularly introducing their chief predator the mealybug ladybird, Cryptolaemus montrouzieri (or ‘Crypto’). ‘Crypto’ can be introduced at any time provided there has not been a recent application of insecticide, and these voracious ladybirds will also produce larvae which are themselves formidable predators of mealybug.
Please note – the larvae disguise themselves to look just like mealybugs – the major distinction being that they can be observed eating the ‘other’ mealybugs. Mealybugs are often found on newly-acquired houseplants, especially Phalaenopsis orchids, Cacti, and Dracaena. The bugs reproduce quickly and travel to other plants, and when they infest a plant it will result in weak, stunted growth, and eventual death. Mealybugs do not tolerate cold temperatures well, but they are also surprisingly resilient to short cold spells.
Often called ‘greenfly’ Aphids are numerous in form and habits. Some are red, black or woolly in appearance. Woolly aphids are often mistaken for Mealybugs, but mealybugs are not found outdoors in colder climates, whereas Woolly aphids are, and the bodies of Mealybugs are a different, more flattened shape.
The most commonly encountered aphid is usually one of the green-coloured species, and these are common on quickly-growing plants which have soft juicy tissues as these are easy for the aphids to feed on. Aphids (and mealy bugs) are often ‘farmed’ by several species of ants, which harvest the sugary ‘honeydew’ produced by the aphids as they suck the plant juices. In return for this sweet nectar, the ants protect the aphids from predators and move them to new pastures by carrying them around.
Dealing with aphids also often involves dealing with ants. Indoors, Aphids are easily dealt with using spray treatments of toxic insecticides or non-toxic horticultural soap sprayed weekly. Outdoors, they can often be sprayed off the plants on a regular basis. A number of natural predators can also be used, including the well-known ladybird/ladybug, but also a tiny predatory wasp such as Aphidius colemanii, which lays eggs inside the soft aphid body. The aphid then turns into a papery-looking ‘mummy’, which serves as a packed lunch while the young Aphidius develops inside. After a few weeks the new Aphidius wasp will hatch out and fly off in search of new aphids to lay eggs inside.
Red Spider Mite (Tetranychus urticae)
Also known as ‘two spotted mite’, these tiny mites are often nearly invisible to begin with, and the damage caused to foliage is usually the first sign that they are there. Highly mobile, these will quickly spread to every single plant in a greenhouse before the first signs of damage are seen on the first plant.
Spider Mites favor conditions which are warm and dry, with low humidity. Under these conditions they develop and reproduce quickly and there may be an interval between generations of just a couple of days instead of the usual ‘one week rule’. There are multiple stages of development, but from egg to egg-laying adult the interval can be just a matter of days, so action needs to be quick and often preventative.
These pests are highly resistant to many insecticides and will need careful handling. The damage to foliage is usually seen as a speckled, dry, brown pattern of tiny spots on the leaf surfaces. The bugs themselves are averse to UV light, and are usually found on the underside of the leaves to start with, but as the colonies become increasingly dense they can be found in leaf axils, and plant crowns – alongside their characteristic webbing which they use as a shelter from UV light and predators, and as a bug highway to travel from point to point. As the colony becomes increasingly dense, mites will start dangling from threads of webbing so that they can swing Tarzan-style onto new plants, be caught by gusts of wind or passing animals and taken to new locales and pastures green. It was once possible to manage Red Spider Mite populations by maintaining high humidity, as native predatory mites would be able to keep them under control.
Since the common application of insecticides since the 1940’s in fields and orchards, these native predatory mite species have almost entirely disappeared, whereas the spider mites have gained immunity to many of the common insecticides. Thankfully it is possible to buy predatory mites which have been reared from recently discovered wild populations. Amblyseius californicus and Amblyseius swirskii are both long-lasting mites which can survive on other prey as well as pollen. They are also active at lower temperatures, meaning that they can be released early in the season for a protective effect. The chief predator of spider mite is Phytoseiulus persimilis, which is a quick-moving mite with a bright red body. Phytoseiulus persimilis can be less effective in smaller greenhouses or in lower volumes as they tend to eat themselves out of prey and then die out, with the spider mites returning just a few weeks later. Repeated applications of these mites are often needed.
There are many species of Thrips, with the most problematic being Frankliniella occidentalis the Western Flower Thrip. These can cause damage which looks superficially similar to spider mites, with small brown speckles on the foliage, but the damage from flower thrips is usually more clustered in linear blotches, and the Thrips themselves are often observed on the leaf surfaces. Thrips are tiny, quick-moving little bugs with a long narrow body about 2mm in length, and they are quick to jump or run off if they see you coming.
The insects are weak flyers, but can travel considerable distance on a breeze. Mostly they run around and hop from leaf to leaf, and are very fond of eating members of the mint and daisy family. The females inject their eggs into soft plant tissue (often flowers, but also leaves and stems), which causes visible damage, and the newly hatched nymphs will feed for a while on the plant tissue before dropping to the soil where it develops further. A healthy soil biome will help to prevent Thrip problems, as the nymphs that drop to the soil will often be eaten by predator bugs living in the soil, but other measures may be necessary. Thrips spread disease as well as causing damage to leaves and flowers, so it is worth taking action. The bugs are attracted to bright colours especially blue, so blue sticky traps are used to monitor and limit Thrip populations.
Caterpillars are major pests of Vegetable plots (especially Brassica crops), but can also effect many ornamentals, especially outdoors. Caterpillars are the larval stage of the insects well-known by pretty much everyone as Butterflies and Moths. Caterpillars are voracious feeders and can quickly defoliate an entire plant. Each species has its own habits and tricks of survival, so it can be tricky to deal with them if there is an infestation. There are numerous natural predators of Caterpillars including common wasps, which snatch them up and take them back to their nests to feed to their young. Additionally, there are several commercially-available predators that can be introduced for long-lasting control.
Sometimes certain Nematode species are used, but more commonly bugs like Macrolophus pygmaeus, which eat the eggs before they hatch (in addition to numerous other Pests) and the specialised tiny wasp Trichogramma brassicae, which parasitizes the eggs before they hatch. Caterpillar damage is easily recognized as it usually occurs at the edges of leaves, with large sections of foliage being eaten away, whereas Slugs and Snails will eat sections in the middle of leaves as well.
Sciara (Fungus/Compost Gnats)
These are also known as Sciarid Flies, or Compost Gnats, and are often found buzzing around moist compost. They are tiny little flies which lay their eggs on wet compost and the larvae eat decomposing organic matter. However, they larvae are not very picky about their food and will also eat living plants, often nibbling at seedlings roots and stems, causing the collapse and death of young plants. Severe infestations can also lead to damage to roots and the introduction of fungal or bacterial disease which can kill even mature plants.
Sciara are a major problem in greenhouses and also in houseplants, especially when the compost is consistently moist. The larvae are small white to transparent worm-like creatures with simple eyes that are apparent as a small black dot on one end of the tiny worm-like body. The larvae are typically about 5mm in length, although they start out smaller they usually go unnoticed until they reach this size. Sciara can be prevented somewhat with a top-dressing of mulch, which makes it hard for the flies to lay their eggs. Also, allowing the compost surface to dry out regularly will limit the rate of population growth and may even prevent them from sticking around at all. Many professional growers use the compost-dwelling mite Hypoaspis miles, which eats various species of small insects and other arthropod prey. These mites will eat the eggs and small larvae of Sciara, as well as the larval/nymph forms of Thrips when they drop to the ground.
Slugs and Snails
These tough molluscs come in a range of shapes, sizes and habits. These creatures are hated by some and adored by others (mostly people who don’t have plants). Unlike most other plant pests, Slugs and Snails are not strictly speaking herbivores, but are detritivores who play a vital part of the community of creatures responsible for decomposition.
Both Slugs and Snails eat a wide variety of food including decaying fruit, algae, animal feces, plant material (living and decomposing), paper, cardboard, mushrooms, and even other slugs and snails.
These slimy rascals will often emerge in wet and rainy conditions, and at night when they are less likely to be seen and eaten by predators.
The problem with both creatures is that they eat so quickly and so much. Within a short period of time a single Slug can destroy a row of seedlings, ravage delicate flower buds, or topple a new plant shoot entirely by eating away the base of the stem. As if to taunt the gardener, the ghastly gastropod will also leave its calling card; a gleaming trail of slime.
There are numerous solutions, depending on the specific plants that need to be protected. Keep in mind that they have their own natural predators, and it is desirable to minimise harm to these as much as possible. These predators include Hedgehogs, Frogs, Woodpeckers, Robins, Blackbirds, Toads, Nematodes, Ground Beetles – to name a few. So it really is important to ensure that any poisons you use are confined to the target, and that poison-loaded Slugs or Snails do not end up in the wider food web.
Copper – the shiny surface of metallic copper will react with the mucus of the Slug or Snail to give it an electric shock when it touches it. Copper tape is often used around the stems of certain shrubs or around the pots of potted plants to protect them.
Diatomaceous Earth – This is a mineral deposit made up of fossilised diatoms – marine algae with hard silica shells. It is usually available as a white powder which can be an irritant to the offending mollusc. Most useful on ground that stays dry – this material will often wash away in rain – which is when the rascals emerge.
Coffee Grounds- The mechanism by which these work is not entirely clear – in likelihood the slugs find that they get poor traction when trying to cross the area covered by the grounds and they steer away. Molluscs are also affected by caffeine, so it could also be an avoidance of any residual caffeine content, or even the smell. Either way, if you have a regular supply of coffee grounds these may be helpful.
Beer Traps– Long favoured by gardeners, these are simply a small jar containing beer, mostly submerged in the soil, and loosely covered to allow the target pests in, but preventing dirt, other bugs, or rain falling in. For some reason they are attracted to the smell – possibly due to the yeast which would ordinarily be a sign of fallen fruit (which Slugs and Snails love). The molluscs fall in and drown, so you will need to regularly empty the jar and refill with fresh beer.
Egg Shells (or Sea Shells) – The presence of crushed shells will cause most snails to turn tail and go the other way. Either it makes them nervous to be around an area littered by broken shells, or they find the sharp edges annoying – as with many of these traditional solutions we don’t really know how they work.
Nematodes – These are incredibly effective and sustainable, being effective on a number of other soil pests as well. The species Phasmarhabditis hermaphrodita is often employed and is especially effective in greenhouse situations where the soil remains warm. The nematodes feed on bacteria and carry a special selection of symbiotic bacteria in their bodies. When they have entered a Slug or Snail body they will release some of the bacteria into the host body, causing the host to lose its appetite and then die.
Orange Peels or Cabbage Leaves – These can be laid on the ground in the evenings to act as traps for the slugs and snails as they go about their nocturnal excursions. As dawn breaks, they will seek out refuge under cover – which you have graciously provided. In the morning you can inspect the undersides of the orange peels/cabbage leaves, and destroy the offending creatures. Don’t just throw them away over a hedge – Molluscs are territorial and have a strong homing instinct and the act of moving them to a new area will only cause them to come back within days – even over a distance of several miles.
Garlic, Rosemary and Thyme – These may possibly repel Slugs and Snails which may find the pungent aromas to be confusing to their sensitive olfactory organs. Tradition regards Garlic as a potent repellent of Slugs and Snails, as well as a myriad of other pests. In addition, garlic extracts seem to be beneficial to plant health.
Ducks and Chickens – Both of these birds will happily munch on Slugs and Snails – but will also nibble at plants and scratch up the ground. With careful design, a defensive zone can be established to help prevent the ingress of new slugs and snails to the garden, and existing pests can be tackled with other means.
Iron (Ferric) Phosphate Slug Bait – Often marketed as a more environmentally friendly alternative, these can be very effective, but care must be taken to avoid ones containing EDTA which can cause the leaching of heavy metals out of the soil and into the groundwater. These are approved for organic farming in many jurisdictions.
Metaldehyde Slug Pellets – These are the traditional ‘Slug Pellets’ used by farmers and gardeners for many years. These are now illegal in many jurisdictions. The substance is introduced into the mollusc by contact or through consumption, where it quickly transforms into acetaldehyde (the metabolite of alcohol responsible for a hangover), and damages the Metaldehyde is a toxic substance, but is not very toxic, with the lethal dose being in excess of 200mg/kg body weight for most small mammals (but only 100mg/kg for dogs). As a comparison the lethal dose of Difenacoum (the active ingredient in many rodent poisons) is only 0.5mg/kg, the lethal dose of Bendiocarb (a common Carbamate insecticide) is only 35mg/kg, and the lethal dose of Caffeine is in excess of 300mg/kg (both based on small mammals). Acute Human toxicity is rare, but theoretically possible. Cats and Dogs are particularly prone to metaldehyde toxicity due to their increased likelihood of encountering the slug pellets in large amounts, and dogs are very sensitive to the toxin. Levels of consumption much lower than the 100mg/kg threshold can still produce noticeable symptoms and will make animals very sick. When used in totally controlled environments (such as a greenhouse, on benching) it can be used safely but a great deal of care is needed. It is a very effective Slug killer.
These are tough Bugs, which as adults will commonly nibble at the edges of leaves, causing unsightly ragged foliage on prized specimen plants. But the adults are not the biggest problem, rather it is the larvae of the Vine Weevil which cause the most problems, as they burrow their way into the rootstock and will kill even mature plants over the course of the winter months. Vine weevils are a major pest of container-grown plants, and also plants in the open ground – particularly prevalent in alkaline soils, but can effect any soil type.
Insecticides are of very limited use, and will often cause more harm to beneficial insects than damage to the Vine Weevil population. Due to their tough shell, the adult bugs are resistant to most contact insecticides, and systemic insecticides are the only chemical option – but not on edible crops.
An effective solution is the application of nematodes – and several species are effective at controlling the grubs in the soil or compost. Both Steinernema kraussei and Steinernema feltiae are marketed for use against Vine Weevil and can be used watered into soils once the temperatures are regularly above 5C. In greenhouse conditions the other option is the nematode species Heterorhabditis megidis, but it is sensitive to temperatures below 12C, so is only really useful in warmer growing environments.
Sticky traps can be employed to monitor Vine Weevil activity in your garden, and is also quite useful as a control as each female can lay several hundred eggs each season.
Although there is no shortage of Pests for your garden, there are many options which can help solve the problems. In many cases, prevention is key, and a good part of that is good garden hygiene. Being careful with the application of pesticides, and encouraging Predatory species can also prevent Pest outbreaks – but the important thing is to keep an active mindset, and not wait until Pests attract your attention!