Decorative houseplants such as coleus are a pleasant reminder of spring and warmer days to come. Someone donated a rather unhappy looking coleus for this week's bug tale. This poor plant, tiny leaves, withered branches, flocked with small white tufts of cottony wax on the stem. The wax covers the eggs of a common pest called the citrus mealybug. Mealybugs are relatives of aphids, adelgids, and whiteflies visited in other bugs of the week. These remarkable insects suck plant sap through very long, straw-like mouthparts beneath their body that snake through the outer surface of the plant leaf or stem in search of nutrients such as sugar and protein. Once the mouthparts hit the mother load a tiny pump is activated in the mealybug's head. The pump sucks the plant sap into the bug's gullet and finally the gut where nutrients are extracted. As plant sap is processed, a sticky, sugar rich liquid called honeydew is excreted. Honeydew is a favorite food of ants and wasps outdoors. Ants that gain entry to your home will readily feed on the honeydew produced by mealybugs on your houseplants. Honeydew on plants outside can serve as a substrate for the growth of a fungus called sooty mold. Sooty mold does not directly harm the plant but its luxuriant growth will turn a green plant black and make it unsightly.
Mealybugs cluster in the angles of branches and around buds and stems.
The mother mealybug will produce as many as 600 eggs during the course of her lifetime. Eggs are deposited in a white fluffy mass of wax called an ovisac. The tiny eggs hatch into nymphs called crawlers that fan out across the leaves and stems in search of places to feed. The nymphs molt several times before reaching adulthood and generate volumes of honeydew in the process. Mealybugs are not very fussy about which plants they eat. In addition to coleus these little suckers attack African violets, jade plants, cacti, schefflera, palms, and avocados to name a few. What should you do if you discover mealybugs on your houseplants? Many options are available but for some real excitement I choose bug wars. It's a bug eat bug world and there are several predators that would like nothing better than to find a coleus full of mealybugs for dinner.
This lacewing larva has sharp sickle shaped jaws: the perfect utensils for enjoying a mealybug feast.
I have had excellent success reducing mealybugs by releasing the mealybug destroyer, Cryptolaemus montrouzieri. Adults and larvae of this little beauty eat mealybugs by the scores. Not having any mealybug destroyers on hand, this week I choose green lacewing larvae instead. We met the green lacewing in bug of the week last year as the lion in the garden. Within seconds after their release these alligator-like terrors discovered the mealybugs and began to feast. Both of these predators and several other natural enemies of mealybugs can be purchased commercially. I have attempted to remove mealybugs by hand with tiny pins and other crude weapons. While the carnage is great and sometimes rewarding, due to their tiny size and fondness for hiding in hard to reach places such as leaf axils and buds, it is difficult to get them all. Garden centers and hardware stores have an array of insecticides labeled for use against mealybugs. If you choose the path of pesticides, be sure to read the label completely including the cautionary remarks and follow the directions precisely. Depending on the severity of the infestation and the relationship with an afflicted plant, there is one more option the compost. Sometimes the expense, time, and frustration of battling the mealybugs are simply not worth the price. It may make more sense to discard the embattled plant and find a new one. Before you purchase any houseplant, be certain to inspect it carefully for signs of honeydew and little tufts of white on the leaves and branches.
References:
For more information on mealybugs and natural enemies that you can buy, please visit the following web sites.
http://www.ipm.ucdavis.edu/PMG/r107300511.html
http://edis.ifas.ufl.edu/CH057
http://www.cdpr.ca.gov/docs/ipminov/bensup.pdf
http://ohioline.osu.edu/hyg-fact/2000/2122.html