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  • Mitchell Stern

How to Get Rid of Aphids

Updated: Jan 12

Aphids are a nasty little pest. In addition to draining their host plant of its juices, they are extremely mobile and can be harboring molds, diseases, viruses, and fungi that further denigrates your growing environment.


At least six species of aphids attack cannabis plants, with the majority of damage taking place in warm, wet weather. Damage increases in hot dry weather and in the presence of strong wind.



In this article we're going to discuss:

  • Signs & symptoms of aphid infestations

  • How to get rid of aphids

  • Garden cleanliness & pest prevention

Table of Contents

Aphid Taxonomy

Early Signs of Aphids

How to Get Rid of Aphids

Aphids on Plants

Bibliography



Aphid Taxonomy

Aphids, also known as "plant lice," are soft-bodied, pear-shaped pests with long legs and antennae. A pair of tubelike cornicles or siphons project backward from the abdomen, almost like the tailpipe of a car.


Adult aphids are typically measure around 2mm and the rear end tapers to a pointed, tail-like caudum between the cornicles.


Aphids vary in color from black to green and some species of aphids are known to have paired bumps between the antennae on the head called "antennal tubercles."


The majority of adult male aphids do not have wings and are called apterae. Adult males that do have wings are called alatae, and their wings are much longer than their bodies.


These winged adults are called spring migrants and fly off to secondary hosts, such as cannabis.



Aphids attack cannabis plants by colonizing the stems and the undersides of the plant's leaves. They puncture the stems, branches, and leaves to suck the sap with straw-like mouths.


Aphids excrete a concentrated sugar solution called "honeydew" that attracts ants who heard and "farm" the aphids, thus protecting them from predators.


An aphid excreting "honeydew"
An aphid excreting "honeydew"

Differential Diagnosis

Early signs of aphids, wilting and yellowing, can easily be confused with spider mite or whitefly infestations.


That's why it's important to make a positive identification with a 100x microscope before deciding on a course of action.


Turn over the leaf and examine the underside to make a determination.



Early Signs of Aphids

Aphids congregate on the undersides of leaflets and cause yellowing and wilting. Some species of aphids prefer older leaves while others thrive on newer growth. Other species prefer flowering tops.


Aphids excrete a sugary substance known as "honeydew" that attracts ants who thrive on the honeydew and will protect the aphids against predators in exchange.


However, this honeydew also promotes sooty fungus growth which leads to necrosis of the leafs.


Honeydew on a leaf
Honeydew on a leaf

Heavy infestations are characterised by leaf curl, wilting, stunting of shoot growth, delayed fruit & flower production, and a general decline in plant vigor.


Unlike other common pests, female aphids are born fertile and can reproduce asexually with new generations cycling every 7-14 days.


Their ability to reproduce so quickly makes them seem to appear overnight. Without proper control, aphids can overrun an entire garden in just days.


We recommend deploying yellow sticky traps to get an early indicator of aphids.



How to Get Rid of Aphids

There are a number of different ways to control aphids. The following control methods are presented in order from most recommended to least recommended.


Prevention

Cleanliness is the best defense against an aphid infestation. Aphids are airborne for part of their life cycle so good air filtration is key to preventing an outbreak.


Indoor gardeners should abide by all generally-accepted cleanliness practices, including:

  • Wearing decontaminated coveralls over clothing and covering hair when working around or near plants.

  • Dawning clean shoe covers when entering a growroom

  • Filtering any air that enters the grow space

Greenhouse growers are advised to maintain a three meter wide 'weed-free zone' around their greenhouse at all times.


All indoor and greenhouse growers should decontaminate walls, floors, ceilings, and equipment; with a lime-sulphur spray, fish oil soap, or solution of household bleach (sodium hypochlorite) diluted to 5%.


Biological Control

It's a good idea to practice biological control before aphid populations explode. If aphid populations are able to increase unchecked, the late addition of predators will be ineffective.


In addition, any biological control of aphids must be effective against ants as well.


Biological Control of Ants

Ants can't swim so having any type of 'moat' around your plant can be an effective barrier against ants. This is easy in hydroponic gardening but difficult to achieve with soil containers.


Ants are also repelled by a variety of natural herbs, including:

  • Cinnamon

  • Cloves

  • Bay Leaves

You can brew a tea using these ingredients or simply grind them up and pour them on the ground.


In addition, boric acid, diatomaceous earth, pyrethrum, carbon dioxide, and insecticidal soap are all effective non-toxic biological controls as well.


Biological Control of Aphids

Parasitic wasps, such as Aphidius matricariae and Aphidius ervi, will lay their eggs inside of aphids and can dramatically reduce aphid populations; bringing infestations under control within two weeks.


The following predators will actually feed on aphids:

  • Lady beetle (Hippodamia convergens)

  • Green lacewing (Chrysopa rufilabris)

  • Predatory fly (Aphidoletes aphidimyza)

Fungi are also excellent microbial biological controls of aphids due to the fact that they work on contract and do not need the aphids to ingest them.


Fungi that are effective against aphids include:

  • Verticillium lecanii

  • Metarhizium anisopliae

  • Entomophthora exitalis

  • Erynia neoaphidis

  • Beauveria bassiana

  • Beauveria globulifera

Consider companion planting with coriander, anise, wormwood, or mint. Aphids are also repelled by nasturtiums, marigolds, chives, cloves, onions, garlic, capsaicin and cinnamon.


Regardless of the method you choose, biological control must be achieved before the flowering cycle begins.


Chemical Control

Aphids can be killed with insecticidal soap, diatomaceous earth, and clay microparticles. Soaps can be used up to a week before harvest without any discernible detrimental effects.


Nicotine works well against aphids too. Cinnamaldehyde will kill aphids but it will also kill beneficial insects.


Two synthetic growth hormones, buprofezin and kinoprene, each kill aphids effectively, especially when sprayed in small droplets.


Aphids on Plants





Bibliography

  • J.M. McPartland, R.C. Clarke, and D.P. Watson (2000) 'Hemp Diseases and Pests', CABI Publishing, Chapter 4: Insects and Mites, pp. 31-39

  • J.C. Stitch (2008) 'Marijuana Garden Saver', Edited by Ed Rosenthal, Quick Trading Company, Chapter 2: Pests, pp. 36-43

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