Aphids of any type are bad news for your garden, but root aphids can be the worst. Burrowing underground, they are difficult to find and equally difficult to eradicate. Hydrogen peroxide is an alternative to commercial insecticides that can kill the invaders — provided you apply it properly.
Root aphids are closely related to their above-ground counterparts, and are primarily distinguished by their preferred environment. While ordinary aphids will frequent the stems and leaves of your garden plants, root aphids burrow in the root system, sucking sap from the host. Root aphids can cause stunting, wilted leaves and discoloration, but are most readily identified by the white residue they leave behind that becomes visible during transplanting.
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Hydrogen peroxide is the usual name given to H2O2, a water molecule with an extra oxygen atom. A transparent liquid only slightly thicker than water, hydrogen peroxide has many uses, primarily as a bleach at high levels of concentration and as an antimicrobial agent when diluted. These properties make it a useful weapon against aphids, allowing you to disinfect your plants and kill the pesky subterranean interlopers at the same time.
Any plants known to be infected with root aphids should be separated from any surrounding healthy plants immediately. Root aphids can easily spread from one host to another, so transplanting and subsequent quarantine are your best bets. Use a 35-percent hydrogen peroxide solution to cleanse both the new pots and the old ones, killing any aphids left behind and suppressing those that stuck around for the ride. Root aphids are known to spread through contact with water, so sterilize your watering can and any nearby gutters or tanks with peroxide as well.
Although not as effective on root aphids as their above-ground relatives due to the difficulty in reaching them, hydrogen peroxide can function as an insecticide, killing the aphid population in your plants' soil. Use a 3-percent solution to avoid harming the plants, and soak the infested soil with it. Use a spray bottle to apply the peroxide in cases when the aphids are visible, such as during repotting.