New Citrus Pest, Not as Serious as First Thought

A surprising number of citrus trees are found in local landscapes. The biggest challenge is of course, protecting the plants through brief winter freezes. Though growing citrus along the northern gulf coast is risky business, many local gardeners continue to defy the odds and enjoy home-grown fruits.

Well established, healthy citrus trees do not normally require an intensive pest control program. Scale insect infestations sometimes occur, but these can be brought under control with relatively non-toxic products.

A new pest has begun to cause some excitement among home citrus growers. The citrus leaf miner (CLM), Phyllocnistis citrella, was first found in south Dade County in 1993. Within four months, infestations were found in 11 counties, and presently it can be found in every Florida county.

Identifying an infestation by the citrus leaf miner is easy. Look for small, light colored, meandering trails throughout the leaves. These areas later turn brown. Severe infestations can result in leaf distortion or browning of many leaves.

Biology of CLM: The adult, which is a small moth, lays a dome-shaped transparent egg singly on the underside of the leaf. Upper leaf surfaces are sometimes used for egg laying, especially when populations are high. Immediately after hatching, the larva bores through the leaf epidermis and begins drinking cell sap by lancing leaf cells with its sharp mandibles.

The larva continues its wandering path between the upper and lower leaf surface, leaving a trail of liquid frass. This damaged area dries, turns white and then brown. Sometimes only one mine is present, but 2 or 3 mines is most common. Up to 9 mines per leaf have been found in Florida.

Pupation occurs within the mine in a special pupal cell with the leaf margin slightly curled over it. The small, silvery-brown adults emerge about dawn and are active in the morning, although eggs are laid at night. The total generation time varies from 13 to 52 days, depending upon temperature.

Control of CLM: The leaf miner is an indirect pest, affecting photosynthesis by reducing leaf surface area. Fortunately, since the citrus leaf miner entered Florida, native species of tiny parasitic wasps have been found to prey on it.

Because of help from this beneficial insect, damage from CLM has not been as serious as previously feared.

Only small trees need to be protected from CLM. Trees under 6 feet tall suffer most because the smaller leaf canopy is more susceptible to excessive reduction of photosynthesis.

Small trees should be monitored for the first signs of CLM. Typically, the first flush of spring growth is not affected but the summer and fall flushes of new growth may experience moderate to severe damage. All flushes should be checked for tunneling in the new foliage.

When mines are first found on new foliage spray immediately with a solution of ultra-fine spray oil. Make a follow-up spray application 7 to 10 days later. Each time that a new flush of growth appears it should be checked and sprayed, if needed. Trees over 6 feet tall do not need to be protected from CLM.

The spray pattern must provide good distribution of the material to both surfaces of flush leaves only. Canopy penetration is not necessary and would increase unwanted contact with non-target organisms such as beneficial insects.

Note: Thanks to the Brevard County Extension Office for information used in this article.

Question of the Week: I would like to grow a live oak tree from an acorn. How is it done?

Answer: The live oak belongs to the white oak group, meaning that the acorns begin the germination process soon after they fall and without any special treatment. Pick up several from beneath a selected tree and plant them about an inch deep in pots or in the ground. Roots emerge from the live oak acorn during the fall and winter, followed by shoot growth in the spring.