Moiradiomus species

Vandenberg, Natalia J. & Hanson, Paul E., 2019, Overview of the lady beetle tribe Diomini (Coleoptera: Coccinellidae) and description of a new phytophagous, silk-spinning genus from Costa Rica that induces food bodies on leaves of Piper (Piperaceae), Zootaxa 4554 (1), pp. 255-285 : 278-280

Biology of Moiradiomus species

The larvae of all four species of Moiradiomus described above construct small tents on the leaves of Piper and inside these shelters they induce the production of food bodies. The larval tents contain fibers that resemble silk in that they are very thin, shiny white and are secreted by the mouthparts of the larva. When approximately 30 tents were removed from P. reticulatum and incinerated, the odor resembled that of burned hair and resulted in dry particulate ash, which is consistent with this material being silk. Although we use the term “silk”, further research is required to verify that this is indeed the material secreted by the larvae.

The tents are always situated on the underside of the leaf and the silk is usually attached to leaf veins, which serve as walls for their shelter. There was never more than one larva inside a tent. Eggs were not observed but on a few occasions very young, apparently recently eclosed larvae were observed in the initial stages of tent construction. Based on these limited observations it appears that the young larvae begin by constructing a smaller tent, but they soon expand it to its final size. Very small larvae (probably first instars) were frequently observed in full-sized tents.

Larvae of M. lachesis on P. lanosibracteum usually construct a triangular-shaped tent at the intersection of a secondary vein with the primary (central) vein, and the apical end of the tent is attached to a small tertiary vein joining the other two veins ( Fig. 40 ). There was nearly always a thin, brown, necrotic line on the periphery, where the tent was attached to the leaf veins, which is possibly a result of chewing by the larva. Most tents of M. lachesis were located in the basal half of the leaf, probably due to the absence of secondary veins originating in the apical part of the leaf. The tents are 3–8 mm in length, with an apical width of 2–5 mm, the most common size being 4–5 mm long and 3 mm wide (42% of 62 tents measured). The number of tents per leaf varied from one to eight. From a total of 88 affected leaves collected on three separate dates at Zurquí de Moravia, 65% had just one per leaf, 26% had two per leaf, 7% had three per leaf, and the remaining 2% had four to eight.

The shape and placement of the tents of M. clotho on P. holdridgeianum are similar to those of M. lachesis described above, but the tents of the other two species are slightly different, possibly due to differences in the leaf venation of their host plants. The leaves of P. reticulatum lack a prominent central vein and instead have five primary veins radiating from the base of the leaf. On this plant M. nanita constructs either a triangular tent at the extreme base of the leaf, between two primary veins, or a more quadrate-shaped tent beween a primary vein and two secondary veins ( Fig. 41 ). For reasons that remain unclear, the vast majority of tents observed at the La Selva Biological Station in northeastern Costa Rica were located in the latter position (often in high numbers), while the majority of those at Rincón de la Osa in southwestern Costa Rica were located at the base of the leaf.

The tents of M. atropos on P. friedrichsthalii are elongate, rectangular in form and are usually located between two secondary veins, sometimes at the base where these two veins meet, but often more apically ( Fig. 42 ). It appears that larval chewing along the edges of the veins (where the tent is attached) results in a thickening of the veins and the upper surface of the leaf becoming slightly convex; moreover, the two secondary veins become drawn closer together in the area containing the tent.

All larval instars appear to contribute to the construction of the tent, and the latter changes over time as the larva develops. Tents with very young larvae are white, soft, and have relatively few fibers, allowing one to see the larva inside. The fibers form a fine, net-like pattern as opposed to the linear threads produced by many spiders. Over time the tents become light brown, more parchment-like, and opaque, making it impossible to see the larva inside without cutting open the tent; however, mature quadrate tents of M. nanita are usually less opaque. In M. lachesis the absence of trichomes from the floor and walls of the chamber (i.e. the leaf blade and sides of the veins, respectively), and their presence in the tent, strongly suggest that the young larva cuts trichomes and incorporates them into the initial silken threads. In all four species the inner surface of older tents contains numerous amorphous patches that consist of membranes from collapsed food bodies, presumably after the larva has sucked out the contents. On one occasion an older larva was observed lifting a collapsed food body toward a small hole in the tent that was created with forceps in the laboratory. The gradual accumulation of collapsed food bodies and natural aging of the materials probably account for the older tents becoming more opaque.

On four occasions forceps were used to make a small hole in a young tent of M. lachesis on a recently collected leaf and after a couple hours one or two very thin, silken threads were observed across the hole. Apparently the larva attempted to repair the damage, although in none of these cases was the hole completely covered. In one case the larva was observed secreting silken fibers from its mouthparts across the hole.

Larvae were never observed outside the tents, nor were any larval exit holes ever observed. Moreover, there appears to be no need for the larvae to leave their tents since the quantities of food bodies found inside can be quite astounding, often 50–100 (of variable size), representing a combined volume greater than that of the young larva. The food bodies occur on the floor of the chamber (the leaf lamina) as well as the walls (the sides of the leaf veins). On a couple of occasions a larva was observed feeding on a food body, during which time it remained very still, except for minor movements of its legs. Larvae maintained in undisturbed tents on detached leaves in the laboratory remained alive for up to three weeks, presumably feeding on the accumulated food bodies.

The larvae of all four species are white colored, although very young larvae are sometimes light yellowish, at least in M. nanita . The pupae are obtect as in other coccinellids, but white in color. Unlike many other coccinellids the pupae are not attached to the leaf, probably because they are enclosed in the tent. There is no cocoon. Adult emergence of M. lachesis at Zurqui de Moravia (1600 m) appears to occur primarily from March through May. On the other hand, preliminary observations suggest that the life cycle of M. nanita in La Selva (100 m) is less synchronized (both young larvae and pupae were found in January). Teneral adults are yellowish, becoming darker with time, and they often remain inside the tent for some time before chewing a hole in the roof and emerging. Adult emergence holes are larger (about 1.5 mm across) and more oval shaped than the smaller more circular holes of parasitoids.

On April 20, 2010, 49 tents of M. lachesis were collected at Zurquí de Moravia and dissected in the laboratory. Of these, 32% were parasitized (as evidenced by a parasitoid larva, pupa, or emergence hole), 24% were torn open (probably by predators), 20% had live beetle larvae, 16% showed successful emergence by adult beetles, and 6% had decomposing beetle larvae. In some cases the tent was completely absent rather than merely torn open, but the previous presence of a larva was indicated by brown necrotic lines on the veins where the tent had been attached. The vast majority of parasitoids reared from M. lachesis over a period of four years were Galeopsomyia sp. ( Eulophidae : Tetrastichinae ).

Newly emerged (<24 hours) adults of M. lachesis reared in the laboratory were used to observe adult feeding habits. Three adult beetles were placed individually in glass vials closed with cotton and provided with one of the following: whitefly eggs with first instar nymphs, first instar ortheziid nymphs, or sooty mold on a citrus leaf. During periodic observations over a two-hour period the beetles did nothing but walk around the vial, occasionally attempting to fly. After 24 hours the number of potential prey items in the vials had not changed. Two adults were then provided with cut pieces of Piper umbellatum L., 1753 leaves containing food bodies (larval tents have never been observed on this species). Upon encountering a food body the rapid walking ceased immediately and the contents of the food body were consumed. Over a two hour period one of the adults consumed 14 food bodies. Although these results are based on a very limited sample size, they strongly suggest that the adult beetles, like the larvae, feed on food bodies. Preliminary observations suggest that, before they emerge from the tent, adults do not induce food body production, but rather utilize food bodies that were not consumed in the larval stage, but this requires confirmation. After emergence it is possible that the adults fly from plant to plant (including non-host species of Piper ) in search of scattered food bodies that occur naturally on Piper . Adult behavior in the field, however, is difficult to observe since they are very active and readily drop from the plant.

Figures

FIGURES 40–42. Tents of Moiradiomus spp.: 40, M. lachesis on Piper lanosibracteum; 41, M. nanita on P. reticulatum; 42, M. atropos on P. friedrichsthalii.