Crape myrtle, Lagerstroemia indica L., (Lythraceae) is one of the most important ornamental plants in the southeastern U.S. Its beautiful summer flowers, fall foliage color, unique bark and growth habits and general utility make it an excellent landscape plant (Egolf & Andrick 1978). Crape myrtle is an exotic species introduced from southeast Asia to Africa, Australia, Europe, North and South America. In the U.S., crape myrtle is attacked by only a few foliar pests of economic significance: a host-specific powdery mildew, Erysiphe lagerstroemiae E. West, the Japanese beetle, Popilia japonica Newman in the upper Southeast and Northeast, a flea beetle, Altica sp., and the host-specific crapemyrtle aphid (CMA), Tinocallis kahawaluokalani (Kirkaldy). Other insects commonly observed in Florida on crape myrtle include planthoppers and leafhoppers, most notably the glassy-winged sharpshooter, Homalodisca coagulata (Say), and occasionally other aphid species, although these insects are of little consequence as pests of crape myrtle.
CMA commonly occurs with L. indica and has been reported from North America (Richards 1967, Smith & Parron 1978), Hawaii (Zimmerman 1948), Thailand (Banziger & Hengsawad 1985), China, Taiwan, Japan (Zimmerman 1948, Richards 1967) and India (Agarwala et al. 1989). Additionally, the senior author has observed CMA in Cameroon, Malaysia, Mexico and the Philippines (Mizell, unpublished). While CMA is ostensibly host-specific to L. indica in North America, Agarwala et al. (1989) reported CMA as infesting Lawsonia alba L. another member of the Lythraceae in India.
CMA reproduces rapidly (Alverson & Allen 1991) and populations can reach very high numbers (>100/leaf on susceptible cultivars, Mizell & Knox 1993). CMA produces copious amounts of honeydew which serve as a substrate for sooty mold. Sooty mold turns the leaves and bark black and unsightly. Therefore, CMA is controlled with pesticides in the nursery and landscape despite recommendations to conserve the aphids for their value to enhance biological control in neighboring crops (Mizell & Schiffhauer 1987, Mizell & Knox 1995). CMA can be very important in sustaining generalist predator populations as many beneficial arthropods also feed on the aphid’s honeydew (Mizell & Schiffhauer 1987).
Although many native and introduced generalist predators prey upon CMA in the U.S., they are unable to control it with one exception. The exotic multicolored Asian ladybird, Harmonia axyridis Pallas, has colonized Florida and most of the U.S. (Chapin & Brou 1991, Tangley 1999) and is capable of eliminating CMA populations on individual crape myrtle plants (Mizell, unpublished). Although Alverson & Allen (1992) reported emergence of a parasite, Lysiphlebus testaceipes (Cresson), from CMA mummies in a greenhouse, no other parasites have been reported from CMA in the U.S. or the world.
We completed several collecting trips in Southeast Asia, the native habitat of L. indica, and searched for CMA natural enemies in other areas we visited from 1988-2000. Our objectives were to locate candidate parasites, import them to the U.S. in quarantine, evaluate and release them in the field. Collection sites varied from urban street plantings, parks and temple gardens to rural roadsides and farms. We found large populations of CMA everywhere crape myrtle occurred along with numerous species of Chrysopidae, Coccinellidae, Miridae and other generalist predators. However, no parasites were ever found. Because it is unusual for a foliar-feeding aphid to be free of hymenopterous parasites, we present here the areas searched to date (Table 1).
Why CMA is not parasitized by Hymenoptera is open to speculation. Several scenarios are possible: (1) Parasites may occur but have not been found; (2) The aphid may be able to defend itself by encapsulation or with chemical toxins; and (3) CMA may not have parasites.
L. indica, other species of the genus Lagerstroemia, L. parviflora Roxburgh, L. speciosa (L.) and the plant family Lythraceae have been reported to contain alkaloids in the leaves, stems and seeds. Six alkaloids were isolated from L. indica seed pods. Trace amounts were also found in the stems and leaves. Three of the L. indica alkaloids, lagerstroemine, lagerine and dihydroverticillatine were unique while three others, decamine, decinine and docodine, were also found in Decodon verticillatus (L.) (Lythraceae) (Ferris et al. 1971a,b, Barik & Kundu 1988, Jehan et al. 1990). Moreover, we have observed adults of some species of Chrysopidae and Coccinellidae feeding on CMA but have not found immatures of these species feeding on CMA (Mizell, unpublished 1999, Mizell & Schiffhauer 1987). Therefore, a speculation is that CMA could use L. indica chemicals for defense as has been reported for other aphids (Jones & Klocke 1987, Malcolm 1990).
Another possibility is encapsulation of parasite eggs or larvae as an aphid physiological response to foreign objects which is well known (Hagen & van den Bosch 1968, Carver & Sullivan 1988, Tardieux & Rabasse 1988).
We believe that the single observation of CMA parasitism by Alverson & Allen (1992) may be best explained as an opportunistic and unusual association. L. testaceipes has a broad host range and attacks many species of aphids on plants from many families. The oleander aphid, Aphis nerii Boyer de Fonscolombe, feeds on oleander, Nerium oleanderL. Oleander also contains alkaloids. However, A. nerii is successfully parasitized by Lysiphlebus testaceipes (Cresson) (Stary et al. 1988b). Moreover, L. testaceipes is capable of expanding its aphid host range rapidly (Stary et al. 1988a). However, we have not observed it attacking CMA in the field.
Extensive searches for parasitic Hymenoptera of the crape myrtle aphid, Tinocallis kahawaluokalani (Kirkaldy), on several continents have failed. A parasite-free condition is an unusual occurrence among the foliar-feeding Aphididae. Such an extensive search has been conducted for only a small percentage of aphid species of economic importance, thus, the frequency of occurrence and the underlying determinants of the phenomenon remain to be determined.
The first and third authors were hosted by Merle Shepard in the Philippines, Banpot Napompeth in Thailand and Lim Tong Kwee in Malaysia. We thank them very much for their time and efforts toward this project. We thank Ray Yokomi and Harold Browning for organizing the travel to the People’s Republic of China. We thank Susan Halbert for helpful discussions on the general issues. We thank all of the other many professionals and new friends too numerous to mention that we made in all of the countries that facilitated every aspect of our travel. The China portion of this study was supported in part by a Southern Region IPM grant No. 92-34103-6926 to R. Mizell and F. Bennett. The Malaysia, Thailand and Philippines portion was partially funded by USDA, ARS. We thank Stephanie Bloem, Ray Yokomi, J. Howard Frank and two anonymous reviewers for helpful comments on an earlier draft of the manuscript. This is Florida Agricultural Experiment Station publication R08527.