While feeding, mealybugs may inject toxic saliva into plant tissue and excrete copious amounts of honeydew that serves as an excellent growing medium for black sooty mold fungi (L). Citrus mealybugs have short waxy filaments around the margin of their oval body with a slightly longer pair of filaments at their rear (R). | L. Pundt (L), and weerapat1003, via Adobe Stock (R)
When you ask growers which greenhouse pests tend to be the most problematic, one that comes up frequently is mealybugs. Not only are mealybugs a pest, but many growers say they need more information on them so they can develop improved management strategies.
In this article, we’ve compiled tips on identification, damage assessment, and control options from research experts from across the country. Use their advice to come up with your own program.
Identification and Damage
Mealybugs extract plant fluids through their piercing-sucking mouthparts, leading to stunted growth, yellowing leaves, wilting, and the production of sticky honeydew.
According to Jaden Gimondo with Michigan State University Extension, identifying mealybugs is relatively easy due to their elliptical shape and distinctive white, waxy filaments protruding from their body. While they retain their legs in all instar stages, mealybugs seldom move except for the first nymphal instar stage or crawler, which actively searches for a place to feed. Their ability to hide in plant crevasses makes them particularly difficult to manage.
Mealybugs usually enter a greenhouse on infested plant material. An online mealybug guide from the University of Massachusetts Greenhouse Crops and Floriculture Program notes that while feeding, mealy-bugs may inject toxic saliva into plant tissue and excrete copious amounts of honeydew that serves as an excellent growing medium for black sooty mold fungi. The presence of black sooty mold fungi inhibits the ability of plants to manufacture food via photosynthesis and detracts from the plants’ aesthetic appearance.
Mealybugs feed on a wide range of plant hosts including coleus, croton, dracaena, English ivy (Hedera helix), fuchsia, gardenia, hibiscus, mandevilla, stephanotis, palms, orchids, and cacti and succulents.
The mealybug’s wide host range, high reproductive potential, tendency to hide in protected locations, ease of spread on workers, tools, and plant material, and ability to survive on greenhouse benches and in cracks and crevices without live plant material for two weeks, all make it challenging to eradicate.
Cultural Considerations
One of the most effective ways to stop the spread of mealybugs in the greenhouse, Gimondo says, is to dispose of heavily infested plant material. Setting up pest thresholds in your operation is important to determine when to cut your losses and throw away plant material. A pest threshold is the level at which a pest population in a crop reaches the point where it begins to cause economic losses. In other words, it is the point at which the cost of controlling the pest exceeds the cost of the damage it causes to the crop.
Plant material susceptible to mealybugs should be quarantined before introducing it into the greenhouse. For high-value crops where disposal is not ideal, quarantining infested plants helps ensure mealybugs do not spread to unaffected plants. Quarantine protocols will differ depending on the size of the operation and the crop type, but the basic steps include the following:
- Isolation: Place new plants in a separate area, away from the main greenhouse, to prevent the spread of mealybugs to other plants. This could be an area as large as a greenhouse or as small as a grow tent.
- Inspection: Before bringing new plants into the greenhouse, inspect them thoroughly for mealybugs, focusing on growing tips and areas where the leaf attaches to the stem.
- Prevention: Rejecting the shipment and contacting the supplier may be necessary.
- Treatment: If mealybugs are found on new plants, treat plants with an insecticide before introducing them into the greenhouse. Thorough coverage of all plant parts is important and multiple applications will be required.
- Monitoring: Regularly inspect quarantined plants for mealybugs and treat them with an insecticide as needed.
- Recordkeeping: Record when and where mealybugs are found, the severity of the infestation, and any insecticides applied to help determine the quarantine’s effectiveness.
Sanitation Tips
Proper greenhouse sanitation is crucial to mitigate the spread of mealybugs. It is important to keep greenhouses clean. Doing so will help to minimize mealybug problems before the spring growing season begins.
The first step in greenhouse sanitation is to remove all plant debris. Weeds, plant debris, and unsalable plants can serve as hosts for insects, mites, diseases, and plant viruses. Remove all weeds and plant debris and place them into a tightly sealed, covered garbage container to prevent pests and pathogens from migrating out and back onto the main crop. Remember to remove organic material and debris (media, spent plants, other organic material) daily, to increase the effectiveness of disinfectants.
Mealybugs spread easily in a greenhouse. Therefore, educating employees/workers on the importance of sanitation practices is important. Workers should be trained to identify signs of mealybug infestations on plants and the areas in the greenhouse where mealybugs are commonly located to prevent infestations from spreading. It is also important to remind workers to wash their hands frequently and disinfect not only tools and equipment, but also any containers, trays, or other items used to transport plants. By taking these measures, workers can help reduce the risk of spreading mealybugs in a greenhouse.
Since only adult male mealybugs fly, yellow sticky cards are not effective for scouting. Therefore, using a hand lens or magnifying glass to confirm the identity of mealybugs is recommended. Keeping records of mealybug infestations and locations will help in making effective management decisions.
Chemical and Biological Controls
Due to their protective waxy covering, mealybug populations can be challenging to manage with insecticides. The covering is water-resistant and reduces their exposure to insecticide residues. Most insecticides have limited activity on mealybug eggs. The nymphal stages are the most susceptible to insecticides because they have not formed the waxy covering. Insecticides need to be applied frequently, at least once per week, due to the presence of multiple generations.
Contact insecticides such as insect growth regulators, insecticidal soap, and horticultural oil effectively kill young nymphs; however, as eggs hatch throughout the growing season, repeat applications are required. Application frequency depends on the residual activity of insecticides used, varying from one to three weeks. Thorough coverage of all plant parts is essential. The use of a spreader sticker may improve coverage and penetration but may also increase the risk of phytotoxicity. Insecticidal soaps and horticultural oils effectively kill eggs, nymphs, and young adults. Systemic insecticides applied as a drench to the growing medium may effectively control or regulate root mealybug populations.
In general, predators are less efficacious against mealybugs than parasitoids. The predatory ladybird beetle, known as the mealybug destroyer, Cryptolaemus montrouzieri, may be used to manage citrus mealybug populations. Both the adults and larvae are predaceous. The mealybug destroyer is less effective in managing longtailed mealybug because adult beetle females lay their eggs among the egg sac of citrus mealybug; long-tailed mealybug females do not lay eggs since they give birth to live offspring. The mealybug destroyer is most effective from spring through fall, being less effective in winter.
