Many growers in Southern California are reporting mines in citrus leaves. It is mostly likely citrus leafminer (CLM).
This pest was first reported in California in October 2000. In the past 5 years, it has made its way from Calexico (2000) to the Coachella Valley (2002), to Blythe (2003), Escondido (2003), and now is found throughout San Diego County (2004). Single reports have come from Ventura County (late 2004) and Orange County (2005). Most recently (August 2005) CLM was found in Temecula (Riverside County) and Mentone (San Bernardino County).
This small moth lays eggs on the newly emerging leaves of citrus. Mines are generally visible to the unaided eye. Citrus leafminer prefers leaves, whereas, citrus peelminer prefers fruit or stems. The citrus leafminer can be found on the upper and lower side of leaves; however, it prefers the lower side. One of the distinguishing characteristics of this insect pest is that it leaves a trail of frass in the mine.
Additionally, CLM tends to pupate at the edge of the leaf, causing the leaf to roll around the pupae. There are differences in susceptibility of citrus types to CLM.
Worldwide, grapefruit and pummelo seem to be most heavily damaged by citrus leafminer, while lemons, some lime varieties and mandarin oranges are least damaged. In California, CLM is most frequently found on lemon, possibly because it is the predominant host plant. Citrus leafminer will also feed on other plants such as mock orange and white sapote that are commonly used as ornamental plantings.
CLM in the Coachella Valley is active primarily in late summer through the fall. It can be found occasionally in early winter. During the remainder of the year, there is little to no activity on citrus. In San Diego County, in 2005, using pheromone traps developed by Jocelyn Millar, professor of entomology UCR, traps began catching CLM in early June. It’s anticipate that CLM in San Diego County will disappear in late fall when temperatures drop. In two years of surveys, CLM has not been found to be active in San Diego County until early summer. It’s uncertain if CLM is dormant when it is not active in citrus (winter through spring) or if it is moving to an alternate host.
For mature citrus, damage caused by CLM is mostly cosmetic. The damaged leaves can be a concern if leaves are being harvested for the market. Yield reductions caused by CLM have not been documented in California. On mature trees, even though leaf damage will be apparent, chemical control should not be necessary. Worldwide, biological control has been sufficient in reducing CLM. This is true for the Coachella Valley.
J.M. Heraty, professor of entomology at UCR, reported 9 species of parasites (Closterocerus utahensis and Cirrospilus coachellae, predominating) found to be attacking CLM in the Coachella Valley. These parasites helped to reduce the damage from CLM on mature trees. The number of parasites and level of control in San Diego County is not known, however, minimizing chemical control will increase the potential of beneficial insects, including parasitic and predatory insects, to control CLM.
Citrus leafminer is mainly a concern in young citrus trees (less than 4 years old). Young trees tend to produce numerous leaf flushes and thus CLM can build more rapidly. Severely damaged leaves can drop and thus young trees could potentially be defoliated by CLM, but this is not typical in California. So far, trackers have not observed consecutive defoliations which would be most damaging to young trees. Where control is necessary such as young trees, there are several pesticides registered for use in controlling CLM.
For more information, please contact a local pest control advisor or a local farm advisor. There are restrictions for use of pesticide products, therefore read the label carefully before applying any pesticide.
Citrus leafminer is a “B”-rated pest. This means that citrus packinghouses in areas not infested with CLM should accept fruit and bins from areas infested with CLM as long as they if they can be inspected (certified) at their origin and found apparently free from green citrus foliage or the fruit and bins are covered (tarped) to reduce the risk of foliage being blown out during transport.
Once tarped bins are in the packinghouse they must collect and destroy all green foliage associated with citrus fruit and harvest bins. Before shipping trees or fruit, check with you local ag commissioner to determine if there are compliance agreements in place for your county.