How to combat aphids on marijuana plants

What are aphids?

Aphids
A couple of aphids on a leave

Aphids are parasites that form a sub-family within the insect order Hemiptera. Like many other insects, they may represent a serious pest in many plant species, besides being an insect that can also carry several viruses and diseases. Easy to recognise as small insects of different colors (yellow, black or green) with a size of 1 to 3 mm. The body is ovoid, with 3 parts difficult to distinguish (head, thorax and abdomen). They may lack wings or have two pairs of small membranous and transparent wings. In the final part of the abdomen they have two small appendages that secrete repellents for their natural predators.

They are usually found on the underside of the leaves and stems, forming large colonies of tens (or hundreds) of individuals. They can develop symbiotic relationship with other insects like ants or bees. Aphids secrete a sugary substance from the anus - similar to molasses - that is used by ants, which in turn protect aphids from predators. Something similar happens with bees, hich incorporate this substance to their honey.

Aphid propagation

Aphids may need just one plant to complete their life cycle (monoecious cycle) or two different plants (dioecious cycle). They can also reproduce by laying eggs (sexual reproduction, involving females and males, which are smaller) or asexually (parthenogenesis). The reproduction cycle is curious; as several generations of aphids pass, and depending on environmental factors, there will be wide disparities in the offspring, with a very high degree of polymorphism. The metamorphosis suffered by the nymphs until reaching the adult stage is minimal, so larvae and adults keep a close resemblance except, of course, in regard with size.

Aphids
Aphids are normally found on the underside of the leaves

Their breeding period is short, so they can quickly become a serious problem. Outdoors, eggs usually remain dormant during all winter and until weather conditions for hatching are optimal. However, it will continuously propagate in indoor grow rooms, which can have serious consequences if the pest is not treated on time. Aphids usually stay on the same plant that have colonised until the conditions are no longer favorable, at this time females are able to produce a winged offspring that migrate to another plant.

Symptoms and damages of an infestation of aphids

Aphids feed using a sucking mouthpart that pierces the plant tissue and sucks the sap of the plant. As we mentioned before, and besides the direct damage caused to the plant by their sucking action, we must also add the risk to get infected by a plant virus or another disease. By not tearing the plant tissue to feed, it is very difficult to see any sign of the plague, for they leave no marks as other insects do (spider mites, thrips, etc).

Aphids
Aphids are also found on stems

The first symptom that we'll observe is the actual presence of aphids on the plants, since they are easily visible to the naked eye. We can also spot how some leaves turn yellow and dry out, also the appearance of droppings (molasses) that can bring different diseases and fungi like sooty mold (Fumaginas sp.), which we can treat with copper oxychloride. Unlike other insects, aphids feed on the tender shoots of plants, which may look sticky. The presence of ants (Asus niger) can also be a symptom of aphids on the plant, as they feed on the honeydew secreted by aphids and take care of them, acting as a shepherd with his flock.

Thus, aphids (as other insects like whiteflies) may represent a threat that can ruin a crop for its ease of reproduction and displacement, besides their ability to carry and transmit diseases and viruses. Aphids like warm climates and environmental dryness, being spring-summer the period of natural reproduction. Using too much fertilizer is also believed to increase the risk of aphids.

Prevention and control of aphids

As always, we dwell on the importance of prevention to deal with the various pests that can affect our plants, especially if we use grow tents, which are usually ideal as a propagation habitat. Keeping the garden clean, pulling out weeds or decaying plant debris is a good way to start. We can try the beneficial association of plants, growing honeysuckle, lupine, foxglove, nettles, marigolds or wormwood - among many others - near plants which are sensitive to aphids such as roses, hydrangeas, tulips or the different marijuana varieties. Natural insecticides like potassium soap, neem oil or pyrethrins sprayed regularly on the plants always help to prevent infestations of insects, even fungi in some cases. If we choose to use natural predators, the parasite Aphidius Colemani (a small 3-4 mm wasp) feeds primarily on aphids.

If you already have a small population of aphids settled on your plants use natural and organic remedies (as we always recommend) better than chemical insecticides, which are often much more harmful to the environment. While aphids have many natural predators - like the aforementioned Aphidius Colemani, the Adalia Bipunctata or the Chrysopa Carnea), it is relatively difficult to control an infestation by simply using these predators; if they don't work, try using other treatments such as spraying rotenone, nettle flour or garlic, or a home made tobacco-based insecticide (nicotine is a potent poison used in many insecticides). If the pest is found only in some tender shoots, we can remove the affected parts and continue with the usual prevention treatment for this pest.

Aphids and ants
This ant is protecting its aphids from a ladybug

In case we need to use chemicals, we must always look first for our Nature, trying to use chemicals as environmentally friendly as possible. Also, try to choose the one that is not  effective against the natural predators of the pest you want to treat. In this case we will use a systemic insecticide (most systemic chemical insecticides against aphids on the market usually have Dimethoate as active ingredient), which will remain on the plant for a few weeks and poison any insect that feeds on it. From Alchimia we try to promote eco-friendly cultivation techniques, so always try to avoid using chemicals and leave them as a last, desperate measure.

As always, keep in mind that prevention is always the better option. The best way to avoid pests is following a series of simple steps that will impede their appearance, which will save you a lot of work and allow you to enjoy a healthy and natural harvest.

Happy growing!

Comments in “How to combat aphids on marijuana plants” (5)

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Doc Brown 2020-01-24
Is Spinosad effective for killing aphids?

Tim Alchimia 2020-01-29
Hey Doc, thanks for your question. No, I'm afraid that Spinosad isn't particularly effective against sap-sucking insect pests like aphids, it only works well at controlling foliage-munching pests. You'll have better success with insecticidal soap, neem oil, karanja oil or pyrethrin such as EcoProt Kenpyr. Natural predators such as ladybirds, hoverfly larvae and lacewings are also a great option for the organic gardener. I hope that helps, best wishes and happy growing!

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Magickmoon 2020-01-08
I've read but not tried that you'll need to have some predatory insects around before any aphids have arrived in order for them to colonize the area that you want to be free of them! Quote "Beneficial insects are commonly used as biological control for pest insects in an Integrated Pest Management program. An IPM program has four chief components: the use of traps to monitor and capture pest insects in their adult stage, management and cultural techniques to improve the growing conditions, the use of beneficial insects to kill pest insects in their early developmental stages and the use of insecticides if necessary. For maximum effectiveness, beneficial insects should be released when pest densities are low. Keep in mind that beneficials are not pesticides and do not provide a miracle cure. Just as it takes time for pest problems to develop, it also takes time before beneficials can resolve them. Proactivity and patience pays off! Correctly identifying the target pest(s) is important before choosing a predator or parasite for control!" Unquote Hope this helps

Tim Alchimia 2020-01-09
Hi Magickmoon, big thanks for your comment! You're right to a certain extent, it's definitely a good idea to introduce the predator insects as soon as you see the first signs of pests, but if you release them before any pests have arrived, it might mean that the predators would either starve or go elsewhere in search of prey. At the same time, it's no good waiting till there's an infestation to release the beneficial insects, as they will struggle to cope with the exponential reproduction of pests like aphids under optimal conditions. Timing of the application is crucial for maximum efficacy, as you say, it's best when pest densities are low. If a grower wanted to be extra-safe, and they've previously had aphid problems then it may be worth introducing small numbers of predator insects as a purely preventive measure, especially if the conditions are warm and ideal for aphid reproduction. Thanks again for your input, we appreciate it. All the best and happy growing!

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Stephen A Wheeler 2019-11-01
I had a medium aphid infestation indoors my last grow. Kept them away from flowers by ladybugs and regular defoliation. My question. How do I disinfect the grow space (100 square feet) before my next grow cycle? Any help appreciated. I figure I'm going to use a 10% bleach solution, contacting all room surfaces. Steve

Tim Alchimia 2019-11-01
Hi Steve, thanks for your question. It seems that aphids are becoming more of a serious problem recently, I've seen some real horror stories of hemp aphid infestations in California. I think you're on the right track with the bleach solution, that's going to do a great job getting rid of not just the aphids, but fungal spores, bacterias and viruses too. If it were a smaller space I'd recommend using the insecticide bombs we sell. I use them after every cycle and they are very effective, although only for up to 20 square metres/60 cubic metres. There's also research suggesting that raising CO2 levels for a few days will also kill any insect pests, although the exact levels are unclear. Organic insecticides like Pyrethrins are another option if the correct safety practices are followed, of course. I hope that helps, all the best for your next grow cycle, happy growing!

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jafarth 2019-09-27
Is there nothing on how to get rid off them?

Tim Alchimia 2019-09-30
H jafarth, yes, the information you need is in the fourth section on "Prevention and control of aphids". You can either mechanically remove them (by hand or using water pressure spray) or try and organic pesticides like Neem Oil, Potassium Soap or Pyrethrin. The best approach is to alternate between these three treatments, to prevent the aphids from building up resistance to any ingredients. If you decide to use biological control, ie. predator insects like ladybirds etc, then don't use any insecticides otherwise, you'll kill the predator as well as the pest! I hope that helps, all the best and happy growing!

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Mike 2018-11-11
Is neem oil to eliminate aphids safe to use when buds are present on Cannabis?

Tim Alchimia 2018-11-12
Hi Mike, thanks for your question. In short, no, it's not a good idea to spray budding plants with neem oil at all, as it can combine with the natural plant oils in the trichomes and affect flavour. At a stretch you could spray during the first 2 weeks of flower, but even then I wouldn't really recommend it. A less aggressive, non-oil based option, and one that oughtn't affect the flavour too much would be to use insecticidal soap, but again, I'd exercise caution and not spray past the first few weeks of flower. Other options that don't involve spraying the flowers would be to use yellow sticky traps which will help to control the aphid population by trapping the flying, egg-laying adults. You could also look into biological controls, like introducing Ladybugs, Lacewings or Aphidius wasps to your grow space, where they can feed on the aphids. I hope that's helped you out, all the best and happy growing!

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