PROC. ENTOMOL. SOC. WASH. 99(1), 1997, pp. 42-49 NESTING BEHAVIOR OF KROMBEINICTUS NORDENAE LECLERCQ, A SPHECID WASP WITH VEGETARIAN LARVAE (HYMENOPTERA: SPHECIDAE: CRABRONINAE) Karl V. Krombein and Beth B. Norden Department of Entomology, National Museum of Natural History, Smithsonian Insti-tution, MRC 165, Washington, DC 20560, U.S.A. Abstract. — Nesting behavior of the recently described Sri Lankan wasp, Krombeinictus nordenae Leclercq, is discussed. Females nest in the hollow internodes of the leguminous myrmecophyte, Humboldtia laurifolia Vahl. The biology of this stem-nesting crabronine is unique among Sphecidae in several aspects. An adult female exhibits remarkable ma-ternal care, rearing one larva at a time, and feeding it progressively. Progressive provi-sioning has not been noted previously for any Crabroninae. Nests lack cell partitions and mature larvae are transported to the basal regions of their stems for cocoon spinning. The cocoon is also unlike that of any other known crabronine species, exhibiting adaptations to internode morphology and allowing movement of adults within the nest cavity. Finally, K. nordenae is remarkably different from all other known Sphecidae in feeding pollen rather than paralyzed arthropod prey to its larvae. Key Words: Sphecidae, Crabroninae, Humboldtia, Sri Lanka, internode, myrmecophyte, cocoon, pollen Sri Lanka has been called the land of ser-endipity. A recent example is the amazing nesting behavior of a Ceylonese wasp, a newly described genus and species, Krom-beinictus nordenae Leclercq (1996). This pretty little wasp (Fig. 1), 5-6 mm long, has creamy to pale yellow markings on its black head and thorax, and a mostly light red ab-domen bearing narrow, transverse, brown to black stripes on some of the dorsal seg-ments. Krombeinictus belongs to the Sphe-cidae, normally a family of predaceous, mostly solitary wasps. Materials and Methods Our first encounter with this species was when we received a single male of K. nor-denae from a colleague. Prof. Fred R. Rick-son. It was among a few wasps that emerged from a dozen internodes of the myrmecophyte, Humboldtia laurifolia Vahl, that he collected in the Sinharaja Forest Re-serve in Sri Lanka in 1992. We visited Sri Lanka in 1993, and spent five days (18-20 Jul and 2-3 Aug) in the rainforest near Gilimale, Ratnapura District, 06°46'N, 80°26'E. We hoped to make be-havioral observations on Krombeinictus during this brief period, but intermittent rains of the delayed monsoon season pre-cluded nesting activity by the wasps. How-ever, we censused about a thousand inter-nodes from H. laurifolia, and placed several hundred unopened stems directly into al-cohol for subsequent study. These inter-nodes were split open carefully in the lab-oratory to avoid damaging associated or-ganisms, and notes were made on their con-tents.