University of Arizona a dot Cooperative Extension

Yuma County Farm Notes

Biological Control in Arizona Citrus

David L. Kerns, Associate Specialist, Entomology

Eight years ago Arizona citrus growers primarily had to deal with one major pest, citrus thrips, Scirtothrips citri, (Detour signpicture) and the occasional opportunist such as mites and mealybugs. Little attention was given towards preserving natural enemies; for the most part it wasn't of paramount importance nor was it really possible. The insecticides we had available then did not lend themselves to beneficial insect preservation. However, this has changed in recent years with the development of new insecticides, and with the influx of new pests; biological control has taken a forefront in Arizona's citrus pest management system.

The woolly whitefly, Aleurothrixus floccosus, (Detour signpicture) appeared in Yuma in 1998. Within two years of its arrival it was evident that biological control would play an important role in the management of this pest. A parasitoid, Eretmocerous comperei or E. dozieri (exact species not certain), appeared in large numbers during the late summer of 2001. This parasitoid was responsible for reducing some extremely high populations of woolly whitefly to almost non-existent levels in some groves. These small wasps appear as small yellow "gnats" with three distinctive red dots (ocelli) on the tops of their heads. They can be easily seen with the naked eye crawling among whitefly colonies. Parasitized whitefly nymphs appear darkened and somewhat swollen compared to non-parasitized ones. Evidence of parasitoid activity is also evident by examining the eclosed pupae of the whiteflies. Those exited by woolly whiteflies will have a vertical split on the anterior dorsum of the empty exuvae (a 10X or better hand lenses will be necessary to view). An eclosed pupae exited by a parasitoid will have a round hole on the anterior dorsum of the exuvae. When parasitoids or parasitoid activity is present, insecticide applications for whiteflies are usually not necessary. Additionally, care should be taken to avoid using insecticides that will disrupt biological control. Those insecticides include: narrow-range petroleum oils, Esteem, Applaud, and Provado. In addition to parasitoids, there are a number of predators that have been observed preying on woolly whitefly. These include various lacewings, but most notably predaceous mites. These mites include Tydeus sp. and the Yuma spider mite, Eotetranychus yumensis. The Yuma spider mite is unusual in that it also pest of citrus, feeding primarily on the underside of leaves, and on the fruit when populations are high. Because of the "Dr. Jekyll and Mr. Hyde" nature of the Yuma spider mite, their populations should not be controlled until they begin to infest the fruit. Damage to the leaves of mature trees is inconsequential.

When the citrus leafminer, Phyllocnistis citrella, appeared in Florida in 1993, it spread through that state, Louisiana, and Texas in less than one year causing a great deal of damage. When this pest appeared in Yuma in the spring or 1999, we were very concerned. Although the citrus leafminer can be easily found during the fall and winter in Yuma, it has not spread to central Arizona, nor has it reached economically damaging levels. The primary reason the citrus leafminer has not reached "severe" pest status in Arizona is the presence of biological control agents. Parasitoids that prey on the endemic pest, the citrus peelminer, Marmara gulgosa, a close relative of the leafminer, crossed over and attacked the citrus leafminer. In addition to these parasitoids, the previous mentioned mites have been observed preying on citrus leafminers as well. In a study during the fall and winter of 2002, 66% of citrus leafminer larvae were killed via predation and parasitism.
Not only has biological control been important for control of newly introduced insect pests, but it has proven important for traditional pests as well, i.e. the citrus peelminer (mentioned above). Other good examples are the citrus mealybug , Planococcus citri (Detour signpicture) and the cottony-cushion scale, Icerya purchasi (Detour signpicture). Although an occasional pest, the citrus mealybug can be an extremely damaging and difficult to control. Because it infests the fruit and is often deep within the tree canopy, insecticide applications are often not very effective due to poor coverage. Applications of non-selective insecticides for mealybugs will sometimes aggravate the problem due to the destruction of natural enemies. There are a number of predators and parasitoids that will prey on mealybugs. Among these is a particularly effective parasitoid, Anagraphus sp. This small wasp is very common during late summer and is the primary biological control agent responsible for the apparent "disappearance" of citrus mealybugs in the spring in groves where heavy infestations occurred the previous summer. Since chemical control of citrus mealy is not particularly effective, it is important that Anagraphus sp not be disrupted with the use of non-selective insecticides such as Lorsban or Supracide. The only insecticide that appears to have good activity on citrus mealybug without harming the parasitoids is Applaud. For best control, Applaud should be used before the mealybugs move to the fruit in large numbers. Additionally, Applaud should be sprayed at or just after egg hatch for maximum efficacy.

Another difficult to control pest is the cottony-cushion scale. This pest occurs throughout Arizona, but infrequently reaches damaging levels. In almost all instances of the occurrence of cottony-cushion scale, the vedalia beetle, Rodolia cardinalis (Detour signpicture), appears and effectively controls this pest. Thus, the key to managing cottony-cushion scale is to encourage the occurrence of the vedalia beetle by using selective insecticides. If insecticides are required for cottony-cushion scale, the product of choice is Applaud since Lorsban, Supricide, and Esteem have all been shown to adversely affect vedalia beetle.

A lot of the insect problems we encounter in the summer can have their origins traced to what we did for thrips control in the spring. When whiteflies, scales, or mealybugs are present in the spring, insecticides choice for thrips control has important long-term implications. If products such as Carzol, Dimethoate, Danitol, or Baythroid are heavily used, the natural enemy complex can be devastated leaving the secondary pests with little or no natural control. Unfortunately we do not have a large number of selective insecticides to use for thrips control. Presently, Success is the only product available that fits this niche although there are several experimental insecticides that have promise. Predaceous mites, (Tydeus and Yuma spider mite) can effective suppress citrus thrips populations. In 2003, in experimental plots where these mites were not killed by insecticides, only one application of Success was required for season-long thrips management, whereas where the mites were eliminated, two to three insecticide applications were required.

As the citrus pest complex continues to change in Arizona, it is evident that biological control has and will become to be more heavily relied upon. We should be cognizant of the implications non-selective insecticide use has, not only on the target pest, but also on the potential impact on natural enemies and subsequent relationship to outbreaks of secondary pest such as whiteflies and mealybugs.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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Information provided by:
Barry Bequette, Extension Agent, Urban Horticulture
Barry Tickes, Extension Agent, Yuma County
Mohammed Zerkoune, Extension Agent, Agriculture
University of Arizona, Tucson, Arizona.
Material written September 2003.

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