By Lauren Diepenbrock
Non-psyllid, soft-bodied insects are common in Florida citrus. Their ability to cause damage varies by pest, pest population size, tree age and tree variety. Soft-bodied insects include scales, mealybugs, whiteflies and aphids, all of which are small and can be hard to detect until the telltale sign of sooty mold development appears on their excrement (honeydew) or damage is apparent. While most insect management in Florida citrus revolves around psyllids, it is important to remember other pests that can be problematic in the system, even with regular management in place.
Damage from soft-bodied insects is related to the local population size. Damage can include honeydew with sooty mold colonization; yellowing leaves; premature leaf drop; blemishes to branches, leaves, and/or fruit; and death of young plants. There is a long history of these pests being kept in check by a combination of native and introduced predatory insects, mites and fungi in Florida. Populations have been found to achieve damaging levels when they occur in areas where predators are not readily present, weather events disrupt biological control or natural enemy populations are reduced from repeated applications of broad-spectrum insecticides.
Citrus trees are often home to both soft and armored scales.
Historically, Caribbean black scale (Saisseta neglecta), green scale (Coccus viridis), brown scale (Coccus hesperidum) and Florida wax scale (Ceroplastes floridensis) have been the most common problematic soft scales in Florida citrus. These species are known to prefer settling in the outer canopy of trees. They produce high amounts of honeydew, which can cover leaves and reduce the ability of those leaves to perform photosynthesis.
Snow scale (Unaspis citri), Florida red scale (Chrysomphalus aonidium), purple scale (Lepidosaphes beckii), Glover’s scale (Lepidosaphes gloveri) and chaff scale (Parlatoria pergandii) are the most important of the armored scales found in citrus groves.
Most scales can be controlled with a well-timed application of horticultural oil to the crawler stage. To check for crawler activity on branches, tape traps made of white double-sided tape can be wrapped around branches near adults. When crawlers emerge and begin moving, they will get stuck on the trap and indicate the appropriate time to perform management. Tape traps should be checked and changed weekly, or debris will build up over time and decrease the effectiveness of the traps.
To determine crawler timing for citrus snow scale, bark should be brushed clean and checked weekly for new colonization. When applying oil or insecticides for scales, coverage is key to suppressing populations. Sprays need to penetrate the canopy, not just cover exterior portions as psyllid sprays generally do.
Common whitefly pests in Florida citrus include citrus whitefly (Dialeurodes citri), cloudy-winged whitefly (Singhiella citrifolii), woolly whitefly (Aleurothrixus floccus) and citrus blackfly (Aleurothrixus woglumi). Whiteflies are only known to be active when flush is present and are generally controlled by natural enemies including parasitoids, predators and fungi. Like many soft-bodied insects, whiteflies produce honeydew, which can lead to sooty mold buildup. Rarely do populations of whiteflies in a grove get high enough to warrant management actions.
Green citrus aphid (Aphis spiraecola), cotton aphid (Aphis gossypii) and brown citrus aphid (Toxoptera citricida) are often found in low numbers in most citrus groves. Their populations are generally kept low by natural enemies. Brown citrus aphid is the primary vector for citrus tristeza virus, which is now rarely seen impacting trees as most growers no longer plant trees on sour orange rootstock. This rootstock is highly susceptible to the virus, and trees planted on sour orange are known to succumb to quick decline.
Citrus mealybug (Planococcus citri) is the most commonly found mealybug in citrus, though several native species are often found feeding in citrus trees. More recently, the lebbeck mealybug (Nipaecoccus viridis) has joined the suite of mealybug pests in Florida citrus. Mealybugs are often controlled by natural enemies. Their waxy coating, which can be hard to penetrate, and tendency to feed in concealed places on trees make them particularly challenging to manage with insecticides. Mealybugs secrete a large amount of honeydew on which sooty mold will quickly colonize.
In the spring of 2019, lebbeck mealybug was found causing fruit, leaf and stem damage in Central Florida. Unlike the common mealybugs found in citrus, this species injects a toxin as it feeds, causing fruit and leaves to become distorted. This distortion can lead to fruit drop in addition to physical blemishes on fruit.
In initial insecticide trials, application timing and coverage were key. Materials that showed promise include acetamiprid, buprofezin, chlorpyrifos and spirotetramat. All materials were applied at their labeled rate for mealybugs at 125 gallons per acre with 3 gallons of 435 oil per acre using a power blast sprayer. The power blast sprayer applied material with enough force to physically remove most mealybugs from trees. Therefore, data were taken on recolonization of plants over a period of five weeks. Laboratory trials are currently being conducted to provide more data to add to field findings.
It should be noted that in other regions of the world where lebbeck mealybug has caused problems, natural enemies have done a better job at controlling populations than insecticides. Chemical management is being used only on occasion to knock back heavy infestations.
Author’s Note: All current knowledge on non-psyllid, soft-bodied pests and their management is based on open-air groves. It is likely that this information will change as new production practices are incorporated.
Lauren Diepenbrock is an assistant professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.