Plesioclytus relictus, Giesbert, 1993

Schnepp, Kyle E., 2019, Notes on the natural history of Plesioclytus morrisi Wappes and Skelley, 2015 and Plesioclytus relictus Giesbert, 1993 (Coleoptera: Cerambycidae), Insecta Mundi 734 (734), pp. 1-8 : 2-3

Plesioclytus relictus . With the host of P. morrisi not yet known but thought to be a species of Polygonum , a trip to Archbold Biological Station in Highlands County, Florida, in September 2018 allowed for investigation into the host of P. relictus . While looking in open patches and trail sides for Polygonum , P. polygamum was found to be conspicuous and flowering ( Fig. 11 ). Soil adjacent to and below plants of P. polygamum was removed to inspect the root ( Fig. 7 ), exposing similar “sawdust” and damage seen in plants from Georgia with P. morrisi present. After some time finding and inspecting plants, a pattern of which ones had larval workings and which ones didn’t began to emerge. Plants with the very base of the stem exposed to sandy soil, not dark soil or leaf litter, almost always had larvae or damage. Plants that were not in open sand never had damage or larvae. Some plants in areas of thick leaf litter, but with at least some sand exposed where the stem met the ground, also almost always contained larvae ( Fig. 10 ). Many Polygonum dentoceras T.M. Schust. and Reveal and Polygonum nesomii T.M. Schust. and Reveal found in open, exposed sand in similar habitats were investigated but no damage or “sawdust” was found. Multiple P. pinicola in open sand were checked, and only one was found to have an individual cerambycid larva. Although left in the root, this larva desiccated and died before pupation; it is possible this was a specimen of P. relictus .

The first batch of roots collected at Archbold in September and October 2018 were processed in the same manner as plants from Georgia but did not produce adults. Frass or “sawdust” was kicked out for approximately one week before stopping. After six months the roots were cut into and investigated with no adults or larvae found. Additional trips to Archbold in April, May, and June 2019 allowed for the niche hypothesis to be tested and more habitats to be surveyed ( Fig. 6 ). Roots of ~100 plants with active damage were again treated in the same manner as those from Georgia in hopes of rearing adults. Roots taken on April 21 and May 12 contained larvae which persisted but did not pupate at the same time as those left in the field and collected on June 8. Approximately half the roots taken on June 8 contained larvae, the other half held pupae or teneral adults ( Fig. 4 ). It is thought that lower indoor temperatures delayed development slightly, up to 2–3 weeks. A single adult male was also collected June 8 resting on a short plant in the vicinity of P. polygamum . In the original description, Giesbert (1993) had adults collected in April and July.

Although adult beetles are rarely collected, virtually every plant checked in open sand had at least one larva. In less than two hours in an area of approximately 0.25 acres (1000 square meters), 40 plants containing over 50 larvae were taken. While many smaller plants and roots contained a single larva, it was not uncommon to find two to three larvae in one root, with the most larvae found in a single root being four. Roots with larvae were quite fragile, frequently breaking even when sand around the plant was dug carefully by hand. A small shovel was helpful in exposing roots, reducing the number broken. Although most commonly found in the root, larvae are not confined to feeding in roots. There were several instances in which stems of a plant were covered by sand and were swollen and had ejected frass ( Fig. 12 ). Beetles infest the same plants year after year, as old damage was present in almost all plant specimens ( Fig. 5 ).

From June 8 through August 19, 2019, 76 adults emerged from roots taken in April, May, and June. These consisted of 37 males ( Fig. 1 ) and 39 females ( Fig. 3 ). Six specimens of a braconid wasp belonging to the genus Bracon Fabricius also emerged from the root samples in June 2019, indicating possible parasitism.

Additional records. Since the original description, specimens of P. relictus have been collected in Collier and Polk counties, Florida ( Fig. 13 ). Specimens from Collier County indicate this species is not restricted to the Lake Wales Ridge and are more dependent on a suitable microhabitat than on geologic age of the sand. Two Collier County specimens were collected in flight in July and September in open sand where Florida rosemary, Ceratiola ericoides Michx. (Ericaceae) , is abundant ( Fig. 8 ). A third specimen emerged in October 2018 from a swelling taken the previous month on Polygonum polygamum brachystachyum (Meisn.) T.M. Schust. and Reveal , occasionally treated as a full species by some authors. In addition, damage in roots of P. polygamum from Bay County, Florida found in June 2019 was consistent with old larval workings of Plesioclytus . An area of exposed, open sand containing P. polygamum , the presumed required niche, was not found. One root was located with fresh damage and frass, but no larvae or adults were found. Additional collecting efforts near this locality and others containing the host plant are encouraged in order to find if new species are present or if there are large distributional gaps, as populations of P. polygamum occur as far west as Texas and as far north as Virginia ( Fig. 14 )

Figures

Figures 9–12. Root damage and habitat of Plesioclytus. 9) Specimen of P. polygamum with larvae of P. relictus in open sand in Emanuel County, Georgia. 10) Active damage by P. relictus in root of P. polygamum. 11) Blooming specimen of P. polygamum in open sand with larvae of P. relictus. 12) Active damage in stem of P. polygamum containing P. relictus covered by sand.

Figures 5–8. Root damage and habitat of Plesioclytus. 5) Damage to roots of P. polygamum by P. relictus. 6) Habitat of P. polygamum and P. relictus in Highlands County, Florida. 7) Active damage by P. relictus in root of P. polygamum. 8) Habitat of P. polygamum and P. relictus in Collier County, Florida.

Figures 1–4. Adult, pupa, and habitat of Plesioclytus. 1) Adult male of P. relictus. 2) Habitat of P. polygamum and P. morrisi in Tattnall County, Georgia. 3) Adult female of P. relictus. 4) Partially dissected root of P. polygamum exposing pupa and adult of P. relictus.

Figures 13–14. Distribution of Plesioclytus and Polygonum. 13) Distribution records of Plesioclytus. 14) Distribution records of Polygonum polygamum.