Here We Go Again: Another Invasive Pest in Florida Citrus
By David Coyle, Ph.D.
The mealybug species Nipaecoccus viridis, originally from Asia and first found in Florida back in 2009, is a new invasive pest of Florida citrus. It was first found feeding on a non-citrus plant, but since then nearly 100 additional records of this insect species have been recorded on over 40 host plants in south Florida. It resembles cottony cushion scale (which is somewhat common in Florida citrus groves) but differences do exist in appearance and damage. Last week, a new report in the open-access Journal of Integrated Pest Management details the first confirmed identification of Nipaecoccus viridis in citrus in Florida (and the United States).
I spoke with one of the study’s coauthors (and a colleague of mine), Lauren Diepenbrock, Ph.D., assistant professor of entomology and extension specialist at the University of Florida’s Citrus Research and Education Center in Lake Alfred, Florida (Twitter: @UFCitrusBugs; email: email@example.com), about what this pest means for the citrus industry in Florida. (Note: Nipaecoccus viridis is known by more than one common name; while listed as the “hibiscus mealybug” in the Entomological Society of America Common Names List, in our Q&A below we refer to it as the “lebbeck mealybug,” a name it takes from another host plant it can be found on in its native region.)
Coyle: How damaging or potentially impactful could Nipaecoccus viridis be to the Florida citrus and horticulture industry?
Diepenbrock: Honestly, we don’t know yet—this is something we’re trying to figure out. In an era of losses from citrus greening, will this pest make the economic situation for Florida citrus growers worse? It’s possible, especially if lebbeck mealybug is not caught early in the infestation period.
Citrus greening complicates things because we already have quite a bit of fruit drop in Florida citrus associated with citrus greening, and fruit drop is one of the impacts documented in studies from other regions. We did a cursory evaluation of fruit drop in the fields with lebbeck mealybug infestations and fruit that would be harvested and didn’t see much of a difference in drop with the control, but this was not sufficiently replicated or controlled to say that there is or is not a definitive difference. This will be challenging to determine because the work we’ve done so far is at growers’ fields, and we are not going to ask them to potentially have additional losses for us to collect data.
Right now, we’re just trying to get the pest under control to help protect an industry that has taken some huge hits in the recent past. Some of the biggest impacts I’ve seen were on newly planted trees. Observations were made after this publication was initially submitted, but this spring I was contacted by a grower who had lost about 75 percent of his recently planted young trees to this pest. Initially, it looked like he would only lose about 30 percent of the field, but, over time, even after eradicating the mealybugs, trees continued to decline. The last time I was in his field, it looked like hundreds of twigs sticking out of the ground.
For horticulture as a whole, that remains to be seen. My colleagues at the Florida Department of Agriculture and Consumer Services Division of Plant Industry have recorded N. viridis on at least 40 different host species in Florida since it was first found in the state in 2009. We know that there are at least 140 plant genera in 53 families documented worldwide as hosts, which means this mealybug has the potential to cause a lot of damage across the horticulture industry if not caught before populations reach damaging levels.
What made you first notice a difference between this insect and the cottony cushion scale?
The only reason I even associated this mealybug with cottony cushion scale was from a grower telling me a crop consultant identified it as such. Before going to the site with “cottony cushion scale,” which I was excited to see in person despite the grower’s concern [Dave’s note: As entomologists, this is common—we get excited to see damaging pest species, out of scientific curiosity, though that stands in stark contrast to the land owners and land managers with whom we work.], I had already seen the damage from this mealybug from samples sent in by a different grower and had just gotten the ID back from the taxonomic expert, Zee Ahmed. From the pictures of cottony cushion scale I’ve seen before, it was obvious to me this was not the same bug just based on visual examination. The damage and infestation patterns are also different, so that was inconsistent with being cottony cushion scale.
What can growers do to prevent this pest? What if they already have it?
This is an area of active research. Ideally, lebbeck mealybug can be managed with biological control, which is the situation in most other citrus-growing regions where it is established. But it is complicated in Florida, where we are also trying to control Asian citrus psyllid (Diaphorina citri; ACP for short), which vectors the bacteria associated with citrus greening. Finding ways to manage the mealybug that are compatible with ACP management is part of the challenge.
One thing we quickly learned is that spray coverage is extremely important. Psyllid sprays are done with a speed sprayer and intended to target the new flush, which is on the exterior of the plant, so a quick dusting is what most citrus trees receive to knock back ACP. However, this mealybug infests all the nooks and crannies of the plant, so last summer, we asked the growers we were working with to slow down their sprayers and increase the rate of the adjuvant that they were using. This helped in gaining control, so much so that in one site with a heavy lebbek mealybug infestation we were able to gain control with one application of a broad-spectrum insecticide. No additional management was necessary after this; the population was knocked back sufficiently that what was left was kept in check by predatory insects that moved into the system.
We’ve been evaluating other insecticides and there are some very promising options including some biological control options, which is great. Once we are able to return to normal work, the lab component of that evaluation can be completed. We have learned that lebbeck mealybug is barely phased by pyrethroids (similar to scale insects) so I’ve been steering growers away from that group of insecticides.
What research is needed to help growers?
We need to complete the lab screening of insecticides, do additional field screening of these materials, identify predators in the system and ways to enhance them, develop methods to prevent entry into exclusion bags, which are being used to protect newly planted citrus, develop scouting-based management, and figure out how these bugs are moving site to site and even within a site.
Exclusion bags are put on young citrus trees to prevent pest infestations. Some exclusion bags are quite effective at keeping psyllids off (which is great in terms of disease prevention) until the trees are big enough to have a chance to make it to production. Unfortunately, these bags are also great for any pests that manage to gain entry. They are not fully sealed at the bottom, so things can potentially walk through the fabric crevices and up to the tree. We see ants doing this and it is possible they are moving stuff into the trees to farm, but we also find infested bags with no ants, and these mealybugs can walk pretty far on their own. (Thanks to the technician who failed to put them away properly, we accidentally learned that they can traverse several meters in a matter of hours.) And there is also potential that they are being moved on the wind, so that isn’t good.
Some might wonder if the Florida citrus industry will ever catch a break. Fresh on the heels of the Asian citrus psyllid and citrus greening, another invasive pest, the lebbeck mealybug, is threatening those very same crops. If there is a silver lining, it’s the fact that there are people working hard to help those citrus growers by providing critical information about citrus pests already here and the new arrivals.
Proper field identification and diagnostics is an important first step in the effective management of crop pests, and scouting is a critical component of an effective integrated pest management system. This work provides both to growers, extension agents, and land managers dealing with citrus and will help folks in other citrus-growing regions be prepared should the lebbeck mealybug establish in those fields.
Journal of Integrated Pest Management
David Coyle, Ph.D., is an assistant professor and extension specialist in the Department of Forestry and Environmental Conservation at Clemson University. Twitter: @drdavecoyle. Email: firstname.lastname@example.org.