Scale insects, including mealybugs, are all members of the Homoptera order. Scale insects come in several forms, including: soft scales (Coccidae), armored scales (Diaspididae) and mealybugs (Pseudococcidae). These insects are notorious for attacking all types of ornamental plants and when not controlled, can even travel across the country on plant material.
Any time growers accept new plant material, it’s critical to consider that there is the potential for introducing pests. It makes no difference the time of year (when plants are in greenhouses) or where the operation is located, as most of these pests are capable of surviving for several weeks, even without a host. Thus, if the plants survive the trip, it’s highly likely the mealybugs will too.
What are the common signs of an infestation?
Most growers admit that mealybugs are difficult to detect and even more difficult to control. Once the young crawler stage becomes established on the plant, they may no longer move, or move so slowly that they appear dead – or may even be confused as part of the plant. Mealybugs are often located in hard to see areas likes roots, lower stems below the canopy, the underside of leaves or branches. They can then remain hiding in the nooks and crannies. Thus, light mealybug infestations can easily be overlooked, allowing the population to build to damaging levels before detection is finally made.
Mealybugs feed by inserting mouthparts into plant tissue and sucking out sap. There may be no symptoms when just a few individuals or a small population are present, but larger populations can cause stunting and chlorosis. Leaves may develop yellow spots, branches often die back and sometimes plants completely defoliate.
What are some of the different types of mealybugs?
Most mealybugs have a protective covering composed of waxy secretions that form a dense mat of wax filaments or fuzz. This makes them difficult to control with pesticides that work on contact. Common examples include the longtailed mealybug (Pseudococcus longispinus), which has characteristically long waxy filaments that protrude from the end of the abdomen, and the obscure mealybug (Pseudococcus viburni), which also has waxy filaments, but they are much shorter in comparison to the longtailed mealybug.
Most common is the citrus mealybug (Planococcus citri), which lacks any waxy filaments and has a gray stripe that extends the length of the body. Citrus mealybug is reported to lay up to 500 eggs contained in irregular cottony masses.
Another type of mealybug are Rhizoecus species, which are among the most difficult to control because they attack and feed on roots. These mealybugs typically appear powdery and white, as the nymphs are covered by a white waxy material. High populations of these mealybugs feeding on roots cause plants to turn yellow and become stunted.
Plants infested with root mealybugs that go unnoticed can serve as a source for further spread to healthy plants, as these mealybugs easily move with water by leaching through drain holes in pots and then crawling into adjacent potted plants, where they become further established on the roots.
Other mealybugs more recently introduced into ornamental production within the US include the Mexican mealybug (Phenacoccus gossypii), the Madeira mealybug (Phenacoccus madeirensis) and the pink hibiscus mealybug (Maconellicoccus hirsutus).
What plants are most susceptible to mealybug damage?
A cluster of mealybugs feeding on stem tissue and damaging new growth of Wedelia.
This emphasizes the importance of early and routine scouting of plants for mealybug. So far in 2018, there have been a lot of reports of mealybug activity that appears most prevalent on vegetatively propagated plants. Some of the most common hosts where I have seen infestations include coleus, plectranthus ‘mona lavender,’ mint, rosemary, ruellia, sedum, crassula, portulacaria, crotons and other tropical and subtropical plants including orchids, palms, hibiscus, mandevilla, gardenia and ferns.
What are some best practices for controlling a mealybug infestation?
Highly susceptible plants should be monitored closely. When it comes to effective control, early detection followed by isolation of infested plants is crucial. Evidence of mealybugs requires immediate treatment with an effective insecticide to minimize further damage and eliminate the potential for spread to healthy plants.
For optimum control, insecticides should be applied preventively. This is especially true for systemic products where the active ingredient needs time to move within the plant to active feeding sites. It’s always important to positively identify all insect pests, including mealybug, especially when using biological controls as there are some mealybug parasitoids that are species specific.
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Did you know?
Did you know that some soft scales and mealybugs are capable of transmitting plant viruses? The first documented case was on cacao (the tree chocolate comes from) infected with Cacao swollen-shoot virus (CSSV). The primary mealybug vector for this virus happens to be our well known enemy Planococcus citri (the citrus mealybug). Citrus mealybug is among the most common and widespread mealybugs attacking nearly every flowering plant grown in greenhouse production. Mealybug transmitted viruses have only been reported on food crops such as grapevine, pineapple, sugarcane and cherries, but let’s not kid ourselves as time will only tell when one of these nasty diseases attacks ornamentals.
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