The most harmful of these soft segmented pests are the Corn Borer and Hemp Borer. They eat the marrow within the stalk creating stem cankers, which weaken the plant. A heavily infested cannabis plant can collapse. A mere five to twelve larvae can literally destroy one plant. Equally damaging are that the holes bored into the stems by Corn Borers leave an open door for other sap sucking bugs, resulting in the life literally being drained from the marijuana plants. Hemp Borers attack the flowering tops. This is one activity you want to nip in the bud right away!
Oftentimes, stem boring caterpillars are not detected until the damage has already been done because they wreak their havoc from the inside of the plant. Examine the stems for holes with surrounding brown trails. Chances are you have a problem if you see visible holes on your cannabis. The only effective treatment for stem borers is to cut them out of the plant. If the plant does not 'come back', make a clean cut at the base of the trail, completely removing it to ensure no further damage.
All caterpillars cause damage to cannabis plants; they love marijuana! The caterpillars of which we can all readily recognize eat the leaves and flowers of all plants; pot is no exception. The damage they cause is readily visible as indicated by the appearance of chew marks left on the leaves. Introducing parasitic wasps will allow nature to take its course in eradicating caterpillars. Chances are they will appear on their own once the presence of caterpillars is sensed. Mother Nature has an innate sense of when to send in the troops. Parasitic wasps lay eggs on the caterpillars' bodies. When the eggs hatch, the creepy crawly becomes a gourmet delight for the upcoming wasps.'
Another control, compliments of Mother Nature, is the praying mantis. This amazing stick like bug may not thrive in all areas, but if you have them in yours, help is on the way. Praying mantis' eat caterpillars and will not leave the plant until it has cleaned its plate, so to speak.
You can further control the caterpillar population by applying a neem oil spray. It is best to spray after the sun goes down or on a cloudy overcast day, as the sun renders the concoction ineffective. Spray the tops of the plants and at the base to deter caterpillars from climbing up from the ground. It is not necessary to saturate your plants, but coat them well. If any caterpillars are not killed on the spot, they will die as they travel through the plant and into the path of the oil.
To make your own neem oil spray, pour 1 to 1 ½ teaspoons liquid dish washing soap to a gallon of warm water. Add one ounce of neem oil to the container and shake well. Pour mixture into a spray bottle. Do not store any unused portions, as it loses its potency after four to eight hours. Apply once a week until there is no longer evidence of the caterpillars in your garden. Neem oil can be purchased online or at any garden center.
Of course, you can always hand pick caterpillars off your plants. Just make sure you destroy them or they will be crawling right back onto your precious plants.