Last year several consultants and I noticed something strange. A number of greenhouses in Ontario had noticeable infestations of mealybug in their poinsettia crop. This was the first time many in their 25+ year careers had seen this (e.g. see Mel Sawaya’s article in Greenhouse Canada Magazine).
It’s unclear yet if phenomenon will repeat itself this year (or perhaps affect even more operations), but growers should be on the lookout for this pest. Keep reading for more information about the potential cause of these mealybug infestations, and possible solutions.
Exotic to Ontario, mealybugs (which can’t overwinter outside in Canada) generally hitchhike up from warmer climates like Florida on cuttings. Typical crops affected in Ontario are tropical plants and cut gerbera. Mealybug often spread slowly within the greenhouse, moving from tropical/foliage plants (or the dreaded “pet” plants) to potted plant crops, or can be passed from greenhouse to greenhouse on clothes, plant material and equipment.
We generally see two species here: the cirtrus mealybug (Planococcus citri) and the long-tailed mealybug (Pseduococcus longispinus) – both were reported on poinsettia last year. The Madeira mealybug is also a problem in the United States and can feed on poinsettia.
Similar to whiteflies, the “crawler” stage (or smallest nymphal stage) of mealybugs is very, very small, making mealybug difficult to detect on cuttings when they first arrive. Generally, the scout or grower doesn’t notice a problem until larger life stages are present, and the mealybug population has become sizable. First sightings in poinsettia crops didn’t happen until around mid September last year.
As pointed out by John Sanderson, the Floriculture Entomologist at Cornell University, mealybug crawlers can also be misidentified as “pinkish”-seeming whitefly pupae if a grower has never encountered mealybugs before.
Though poinsettia has, historically, been reported as a host for mealybug, sightings on this crop in Ontario were rare until last year, and occurred in greenhouses where no tropical plants or other common host plants were grown. We suspect that reduced use of neonicotinoid insecticides – an effective systemic pesticide for mealybugs – by poinsettia propagators may have led to a sudden resurgence of this pest, which were then brought to Ontario on cuttings.
Besides being an unexpected pest, mealybugs are also relatively difficult to control. Not only do we have limited pesticide options (see Table 10-2 in Pub370), but contact pesticides don’t even work that well for mealybug because they’re repelled by the mealybug’s waxy coating.
Landscape oil (a smothering agent) is an effective option for mealybug control in some crops, but can cause phytotoxicity in poinsettias – unless a lower rate is used as a cutting dip and subsequently washed off (see the label). Those growers doing poinsettia cutting dips for whitefly this year may also have unknowingly headed off mealybug problems (though the efficacy of these treatments for mealybug control needs confirmation). If you’re on the fence about adding dips to your poinsettia pest management program, this may end up being another point in its favour.
The best option at this point in the growing season may simply be to throw out any infected plants. Mealybugs don’t spread plant-to-plant very quickly, since females don’t have a flying stage (slow meandering only for these guys!). By culling infected plants as soon as mealybugs are detected, an infestation in a relatively short-term crop like poinsettia can often be eliminated. However, systemic pesticides like Beleaf (flonicamid), when applied for whitefly control, also have the added benefit of simultaneously controlling mealybug, and should not interfere with a biological control program for whitefly control.
Lastly, if you see mealybug on your poinsettia crop this year, shoot me an email (firstname.lastname@example.org) so I can get some pictures, some specimens to play with, and possibly get a better idea of which suppliers these guys are coming from…