Welcome to the Citrus Leafminer Biological Control Blog

Welcome to the Citrus Leafminer Biological Control blog. We are a group of Cal Poly students who have been working on a Citrus Leafminer collection during the fall quarter of 2009. We as a group have learned a few things about leafminers along the way, and would like to share that information with citrus growers on this blog.

Citrus Leafminer Description

Figures 1 and 2. Examples of Citrus Leafminer Damage

The citrus leafminer (CLM) (Phyllocnistis citrella) is a Lepidoptera moth that has developed into one of the most destructive pests in the citrus industry. The citrus leafminer originated in Asia and is now found all over much of the citrus producing countries in the world. It was first discovered in the United States in southern Florida in 1993, and is now in every major citrus growing region in the lower 48. CLM made its first arrival in to California in 2000 in the Imperial Valley.

CLM damage is very noticeable as the larval feed and leave behind a maze of tunnels on the under and upper parts of the leaves. CLM larva are typically a light yellow color and typically leave a brown fecal trail inside the tunnels they mine. The larval have four instars that include 3 feeding instars and one non-feeding prepupa instar. The length of the larval stage is anywhere from 10-19 days. The larvae pupate by rolling the edge of a leaf over them and covering themselves with silk for protection.CLM adults are less than 2mm in length and have a wing span of about 4mm. They have off-white silvery forewings that have black and brown markings and a black spot on the end of each wing. The rear or hind wings are white with long scales protruding from the margins.

The CLM females lay eggs on new growth near the veins and midribs. Typical mating of the CLM occurs during the evening hours. This is mainly due to the adults emerging in the morning hours. The females typically lay the eggs on the underneath side of the leaves when the population densities are low. A typical female lays around 50 eggs in her lifetime, which hatch 4 to 10 days upon laying them. There can often be more than one leaf miner on a single leaf. It’s not uncommon to find 3 or more larval feeding on one leaf.

Citrus Leafminer Lifecycle

Figures 1 and 2. Citrus Leafminer larva (http://kcc-weslaco.tamu.edu/oldwww/webpages/leafminer.htm)

The Citrus Leafminer lifecycle consists of four primary stages; egg, larva, pupa, and adult moth. The adult moth has a very short lifespan of just two weeks, being most active in the mornings and spending most of its time on the underside of leaves. Soon after emerging from the pupal case, the adult female moth gives off sex pheromones to attract males. After fertilization, the female moth lays its eggs on the underside of new leaves. The newly emerged flush growth is the preferred egg-laying site. After being laid, eggs often hatch one week later and the larvae immediately begin feeding on the inside of the leaves, creating tiny mines. As the larvae grow, the mines become bigger and bigger, creating a more visible mine. The larvae will molt 4 times during a 3 week period. The larvae will come out from the mine as a prepupa rolling over the edge of the leaf to form a type of dwelling. During its time in the dwelling, the leafminer will become a pupa lasting for up to 3 weeks. The citrus leafminer lifecycle lasts approximately 7 weeks and develops best at temperatures of 70 to 85 degrees Fahrenheit but will adapt to most California climates.

Citrus Leafminer Biological Control Agents

Figure 1. Pnigalio spp. (http://kcc-weslaco.tamu.edu/oldwww/webpages/leafminer.htm)

Figure 2. Closterocerus spp. (http://kcc-weslaco.tamu.edu/oldwww/webpages/leafminer.htm)

Biological control of the citrus leafminer is one of the key tools to controlling it. In Southern California there are many generalist parasitoids that typically attack the citrus leafminer. Nine species in five genera including; Closterocerus spp., Cirrospilius spp., Pnigalio spp., Chrysocharis spp., and Sympieses spp. have been found helpful in the control of leafminer. These parasitoids generally attack the gracillariid leafminer which is native to California and lives in oaks. Euderus spp., Zagrammosoma spp.,Sympieis spp., and Chryonotomyia spp., are four genera associated with the gracillariid in the San Joaquin Valley.

As the population of citrus leafminer become larger it will be important to manage and conserve these natural enemies. Things like not spraying citrus with broad-spectrum insecticides and avoiding other practices that disrupt natural enemies will be imperative. There are a number of parasites around the world that may be implemented in California at some time to help the native natural enemies. These natural enemies include Ageniaspis and Citrostichus spp. which have been used in Florida but have a hard time spreading in high enough densities to be comparable to the native parasitoids. Their host range is much smaller than the natives, which limits the colonization during times when leafminer is not in high densities. On the central coast of California where native natural enemies have trouble controlling the leafminer because citrus have multiple flushes of new growth a year, the Ageniaspis and Citrostichus may be more effective.

Citrus Leafminer Management

Figure 1. Pheromone Trap (cals.arizona.edu/crops/citrus/photos/figure19.jpg)

Figure 2. Insecticide Toxicity (jenny.tfrec.wsu.edu/opm/opmimages/WTMf8.jpg)

Citrus leafminers can be controlled biologically by many native species of parasitoids. These include many species that are mentioned in the biological control section. Due to the biology of CLMs chemical control of the species is extremely difficult. Proper timing is crucial when applying chemicals for control, however this can be extremely difficult making chemical control only marginally effective. There has been research conducted in Florida, which concluded that CLM damage typically does not reduce yield of mature citrus trees. Mature trees are trees beyond 4 years of age.

Monitoring for CLM is very important and many tools are available to help. First many pheromone traps are available to capture male moths. This is not a good way to monitor for CLM density, but it does reveal the insect’s presence or absence from an orchard. Traps should be hung at shoulder height just inside the foliage of the citrus trees; the traps need to be put out when citrus trees are pushing new growth. Do not prune out CLM-damaged leaves because most leaves are still being sources for the tree, and pruning pushes new leaves creating new susceptible growth. Finally, nitrogen application should be provided at a stage that promotes new growth when CLM are least active. The bottom line is that CLM damage does not reduce the harvestable yield in mature citrus trees.

Additional Resource

UC IPM Website: http://www.ipm.ucdavis.edu/

Our Hardwork and Dedication

Thanks to hardworking members of our Citrus Leaf Minerator group for their intense research and attention to detail in bringing the most recent and accurate information to you.