Mealybugs Suck
Mealybugs, or insects of the family Pseudococcidae, suck. By that I mean they're horrible, they suck, and they also literally suck the sap out of plants, and with enough of them they will kill just about any vulnerable plant. A tell tale sign of a mealybug infestation are white clumps of what looks like cottony fluff, usually found around the the joints of stems or at the tips of growing shoots. You will also likely see the small cottony critters themselves. They look sort of like a trilobite, or perhaps pill bug, that got covered in cotton fluff. Now, yes, not all mealy bugs share this "cottony" appearance, but most do.
Okay, but they're killing my plants, how do I get rid of them?
There are five ways of getting rid of mealybugs, horticultural oil, soap or chemical sprays, entomopathogenic Fungi (insect killing fungi), Temperature control, Predatory insects (insect killing insects), and physical removal. We've just about tried all of them and will get into each method below.
Sprays
Spraying mealybugs is a fairly tried and true method. Most of the advice you get will recommend using a neem oil spray or an insecticidal soap. Both work through similar methods, the "cotton" found on the mealybugs and their egg sacks is actually a protective coating of wax, The neem oil and insecticidal soap both act to dissolve the wax, exposing the insects' bodies to the surrounding air and fatally drying, or desiccating them. It also helps clog the spiracles (holes in insects skin, through which they breath), suffocating them.
That said, there are some differences to be considered between insecticidal soap and neem oil. Neem oil is very effective at killing the mealybugs it comes into contact with, however it smells very unpleasant (personally I've gotten used to its smell) and will leave a thin oily residue over everything for quite a while. It is also has to be reapplied about once every week, mix 2 table spoons (30 mL) per gallon or about 8 mL per liter of water. It is important to mix the neem oil into the water, for best results use very warm water, add the neem oil first and the water second, and shake the sprayer vigorously before use to mix. Neem oil is not very effective against aphids though, for that use insecticidal soap.
Insecticidal soap, does, as the name suggests contain soap, it too is extremely effective, against mealybugs (and aphids as well, unlike neem oil). The soaps doesn't have much of a smell, and leave a easily removable, water soluble residue (I've found its much easier and more pleasant to clean up and work with). Concentrations will depend on the soap you're using.
Both sprays act to dissolve waxes, plants produce a wax called cutin on their surfaces to protect from U.V. radiation, the sprays can sometimes leave them vulnerable to sunburn (yes sunburn on a plant), so its best to keep them out of sunlight for at least 24 hours if you do spray. If spraying outdoors or in a greenhouse, spray in at twilight to minimize the plants exposure to sunlight.
Entomopathogenic Fungi
These are fungi that infect, parasitize and kill insects. Different entomopathogenic fungi infect different insects. The species of fungi used to kill mealybugs and other soft bodied insects are entirely harmless to humans, pets, plants and indeed many hard bodied insects such as bees. Their ability to kill mealybugs without harming other insects, such as bees is pretty useful. Likewise they have the ability to spread and infect more pests exponentially, something that can't be said for chemicals. Beauveria bassiana is one of the more commonly used and somewhat effective entomopathogenic fungi used.
While fungi have a great deal of promise as potential future pesticides, their current effectiveness outside of a laboratory setting is somewhat questionable and highly dependent on things like humidity and temperature. I would advise using other control methods for mealybugs, outside of perhaps a very specific set of circumstances, at least for the time being.
Temperature Control
Realistically temperature control is probably among the most effective methods of controlling mealybugs, if not the most effective method, and yet the least recommended (probably something to do with it being free and not actually requiring you to buy anything *ahem* *cough cough*). it involves chilling (NOT FREEZING) the effected plants for a period of time long enough for all the mealybugs to die. Longwood gardens, yes the 80,948 square foot, massive, world famous conservatory...that Longwood Gardens (just google it, guarantee you'll be impressed)...uses temperature control as a big part of their arsenal to keep their plants from being overrun by mealybugs and other pests. Most large conservatories and growers use temperature control as a primary method to easily kill mealybugs. Although not as immediate as a spray it works well.
The only three drawbacks of this method are the time required to kill the mealybugs is significantly longer, it will not work for plants that cannot withstand even moderate cold temperatures, and it may be impossible for larger plants for those living in warmer climates where temperatures may not get low enough.
Longwood gardens, has reported that chilling their plants down to 36°F (2.22°C) for 36 hours works to effectively kill all mealybugs.
Papaya mealybug, Paracoccus marginatus was unable to reproduce at temperatures sustained below 14.5°C (58°F) in males and 13.9°C (57°F) in females.
I allow my greenhouses to get down to 40°F in winter and prevent them from becoming too warm during that time to control mealybugs.
Predatory Insects
Certain insects are very good at killing both the mealybugs and their eggs. Two insects in particular, common green lacewings, Chrysoperla carnea, and Mealybug destroyers (its in the name) Cryptolaemus montrouzieri, are very useful for killing mealybugs and aphids. Cryptolaemus montrouzieri, is particularly good at killing mealybugs and frequently destroys their egg sacks as well. It is a small ladybug-like beetle. Unfortunately both predatory insects, much like their prey are dependent on warm temperatures to be effective.
Physical Removal
Just like it sounds. Sometimes, if the mealybug infestation is small enough you can just wipe them off with an alcohol soaked paper towel, or cotton swab, or simply spray them off outside with a high pressure hose.
Keeping them gone
Regularly inspecting for mealybugs and their tell-tale white cottony clumps is absolutely essential to make sure they remain gone. Always assume that a mealybug infestation could return, full force, at any time. The insects may be present as small nymphs, called crawlers, which can hatch in remaining eggs after each treatment. Thus it is okay to continue spraying for a few weeks even after you can no longer see signs of the insects, to take care of those that have recently hatched.
Always keep plants separate and ensure that none are touching. Except for the male mealybugs, the insects aren't terribly mobile, but they can migrate directly from one plant to another if they are given the opportunity. Mature male mealybugs can fly, they look somewhat like gnats. I use a "bug zapper" which attracts and kills them as a precaution, thus disrupting the reproductive part of their lifecycle.
The Bottom line
Each method of mealybug treatment is specific to the individual situation. But I've found that a combination of physical removal, with alcohol soaked paper towels, sustained low temperatures between 38°F - 55°F and weekly spraying with insecticidal soap is the easiest, most convenient, cleanest way to control the pernicious critters.
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