mating

Why is this male carrion beetle “biting” one of the female’s antennae?

/ Ted C. MacRae / 6 Comments
American carrion beetles (Necrophila americana) aggregating at sap flow on the trunk of an oak (Quercus sp.) tree.

American carrion beetles (Necrophila americana) aggregation at sap flow on trunk of oak (Quercus sp.) tree.

Earlier this spring I came upon an interesting aggregation of insects at a sap flow at the base of the trunk of a large oak (Quercus sp.) tree. Sap flows are famous for the diversity of insects that are attracted to them (e.g., see my previous post, Party on a pin oak), although the mix of species present can vary from sap flow to sap flow. In this case, the majority of insects present were American carrion beetles (Necrophila americana)¹ (order Coleoptera, family Silphidae), a species encountered much more often on animal carcasses (in fact, the genus name literally translates to “attracted to corpses“) but also occasionally attracted to sap flows (Evans 2014). This is not surprising to me, as I have seen adults regularly in the fermenting bait traps (Champlain & Knull 1932) that I have set out over the years (although I have been unable to find any reference to such attraction in the literature). I had never seen such an aggregation of these beetles before or even yet had the chance to photograph them (although I have photographed its Ceti Eel-like larva), so I paused to setup the camera and take a few photographs.

¹ Not to be confused with the federally endangered American burying beetle (Nicrophorus americanus).

Necrophila americana mating pair.

Necrophila americana mating pair.

Among the many single adults present was a mating pair, which I selected as my subjects. As I was photographing the pair, I noticed the male had a firm grasp of one of the female’s antennae within his mandibles. As I watched them through the lens, I saw the male suddenly release his hold of the female’s antenna, move backward on top of her, and begin using his own antennae to stroke her pronotum (sadly I was unable to snap a photograph at that time). As suddenly as he had released it, the male moved forward and grabbed hold of the female’s antenna once again. It seemed unlikely to me that this represented an act of aggression, but instead must be an important part of their courtship behavior. The female, for her part, did not seem to be bothered too much by the grasping and continued to slowly lumber about around the sap flow as the male went through his routine under my voyeuristic watch.

The male has a firm grasp of the female's antenna.

The male has a firm grasp of the female’s antenna.

Intrigued by this behavior, I searched for other photos of mating/coupled carrion beetles—easy to do considering the many pages of photographs of this species at BugGuide. While the great majority of those photos are of individual beetles, I found this photo and this one of coupled pairs, each also clearly showing the male firmly grasping one of the female’s antennae with his mandibles. Neither photo makes mention of the antennal grasping, but a little further searching did turn up this YouTube video of coupled American carrion beetles, again clearly showing the male grasping of the female’s antenna and even leading the videographer to comment, “Disturbingly, it even appears that this male is threatening to lop off the female’s left antenna if she refuses to mate!” Of course, retribution seems not to be a common behavior among insects, and in looking into this further I found a short note by Anderson (1989) in which the behavior is recorded not only for N. americana but also another silphid, Oiceoptoma noveboracense. Apparently mating actually occurred during the time the male had released his hold of the female’s antenna and was stroking her pronotum with his antennae. He further noted that the antennal grasping behavior continues until eggs and larvae are present at a carcass, at which time it is no longer observed. This suggests that the behavior represents an especially proactive form of “mate guarding” by which males actively ensure their paternity of the offspring of the particular female with which they were mating.

REFERENCES:

Anderson, R. S. 1989. Potential phylogenetic utility of mating behavior in some carrion beetles (Coleoptera: Silphidae: Silphinae). The Coleopterists Bulletin 43(1):18 [pdf].

Champlain, A. B. & J. N. Knull. 1932. Fermenting bait traps for trapping Elateridae and Cerambycidae (Coleop.). Entomological News 43(10):253–257.

Evans, A. V. 2014. Beetles of Eastern North America. Princeton University Press, Princeton, New Jersey, 560 pp. [Google Books].

© Ted C. MacRae 2015

T.G.I.Flyday: Soybean nodule fly

/ Ted C. MacRae / 5 Comments

I’ve been walking the rows of soybean fields for many years now, and while it might seem that I would have very quickly seen all there was to see in terms of insects associated with the crop, this is not the case. The major players are almost always present—lepidopteran caterpillars such as velvetbean caterpillar (Anticarsia gemmatalis) and soybean looper (Chrysodeixis includens), and stink bugs such as southern green stink bug (Nezara viridula), red-banded stink bug (Piezodorus guildinii) and brown stink bugs (Euschistus spp.). However, numerous other insects can be found at one time or another—some of great importance from the perspective of the farmer producer but others with very little impact on the crop. During a tour of soybean fields in Mississippi this past September, I saw a large number of “signal flies”¹ (family Platystomatidae) on the foliage of the soybean plants that I presumed to represent the soybean nodule fly, Rivellia quadrifasciata.²

¹ I originally learned these to be “picture-winged flies”—a name now more commonly used to refer to members of the family Ulidiidae—which I learned as “Otitidae”!

² This species can be separated with certainty from the closely related and largely sympatric species R. colei only by examination of male genitalia (Namba 1956). Rivellia quadrifasciata is more common and widespread than R. colei and is the species cited in literature in association with soybean.

Rivellia quadrifasciata (soybean nodule fly) | Stoneville, Mississipi

Rivellia quadrifasciata (soybean nodule fly) | Stoneville, Mississipi

Rivellia quadrifasciata is widely distributed in the eastern U.S. where it originally fed probably on tick trefoil, Desmodium spp. (Foote et al. 1987), but has since adapted to soybean, Glycines max (Eastman & Wuensche 1977), and black locust, Robinia pseudoacacia (McMichael et al. 1990). Despite its relatively recent adaptation to soybean as a favored host plant, the species does not appear to cause much economic damage to the crop. The small, white, maggot-like larvae live in the soil and feed on the Rhizobium nodules of the roots that are used by the plant for nitrogen-fixation. Soybean, of course, is famous for its compensatory abilities and can withstand considerable nodule injury without yield impact, and as a result losses from this insect are considered minor (Heatherly & Hodges 1998).

Signal flies wave their wings constantly.

The wings of signal flies are almost always in constant motion.

Of more interest from a natural history perspective, these flies—like other members of the Platystomatidae—are almost always seen with their wings in a constant “waving” motion as they walk about on the host leaves. This seems clearly an intraspecific “signaling” behavior (and the source of the family’s common name), with the pattern of markings on the wings and the particular sequence of movements of the wings combining to provide species-specific signals for mate recruitment. Some Asian members of the family are famous for the remarkably elongated eye stalks of the males, which aid in intraspecific male-to-male combat behaviors that provide selection pressure for even more elongate eye stalks. Sadly, our North American species exhibit no such modifications of the head, but their strangely tubular mouthparts do give them the appearance of wearing a “gas mask.”

gas mask

The strangely tubular mouthparts give adults the appearance of wearing a “gas mask.”

Information on the biology of adult platystomatids is limited, but a wide range of adult foods, e.g. nectar, honeydew, plant sap, bird droppings, and carrion, have been reported for this species, and R. quadrifasciata males have been observed to feed females globules of liquid during mating.

REFERENCES:

Eastman, C. E. & A. L. Wuensche. 1977. A new insect damaging nodule of soybeans: Rivellia quadrifasciata (Macquarl). Journal of the Georgia Entomological Society 12:190–199.

Foote, B. A., B. D. Bowker & B. A. McMichael. 1987. Host plants for North American species of Rivellia (Diptera, Platystomatidae). Entomological News 98:135–139 [Biodiversity Heritage].

Heatherly & Hodges. 1998. Soybean Production in the Midsouth. CRC Press LLC, Boca Raton, Florida, 416 pp. [Google Books].

McMichael,  B. A., B. A. Foote & B. D. Bowker, B. D. 1990. Biology of Rivellia melliginis (Diptera: Platystomatidae), a consumer of the nitrogen-fixing root nodules of black locust (Leguminosae). Annals of the Entomological Society of America 83(5):967–974 [abstract].

Namba, R. 1956. A revision of the flies of the genus Rivellia (Otitidae, Diptera) of America north of Mexico. Proceedings of the U.S. National Museum 106:21–84 [Biodiversity Heritage].

Copyright Ted C. MacRae 2013

Midget male meloid mates with mega mama

/ Ted C. MacRae / 6 Comments
Pyrota bilineata on flowers of Chrysothamnus viscidflorus | San Juan Co., Utah

Pyrota bilineata on flowers of Chrysothamnus viscidflorus | San Juan Co., Utah

While looking for longhorned beetles in the genus Crossidius on flowers of yellow rabbitbrush (Chrysothamnus viscidiflorus) in southern Utah, I encountered one particular plant with numerous blister beetles (family Meloidae) on its blossoms. The orange color, two black pronotal spots, and distinctive black and white longitudinal elytral stripes leave no doubt as to its identity—Pyrota bilineata, but for good measure I sent a photo to my field mate for the trip, Jeff Huether, who confirmed its identity. I had seen singletons of this species at a few previous localities during the trip, so I was intrigued by the large numbers of individuals congregated on this single plant. As I looked at them, I saw one individual that appeared to have something stuck to the tip of its abdomen. I peered closer to get a better look and, to my surprise, discovered that it was actually a male in the act of mating. The male was tiny, only one-third the size of the female, representing about as extreme a size difference in mating insects as I’ve ever seen.

Pyrota bilineata on flowers of Chrysothamnus viscidflorus | San Juan Co., Utah

A tiny male mates with the ginormous female.

Many species of blister beetles exhibit tremendous size variability, and a unique aspect of some species’ mating behavior is the cantharidin-packed spermatophore produced by males and transferred to females during mating. (Cantharidin is a toxic defensive compound that serves as a very effective deterrent to predation.) The spermatophores are energetically “expensive” to produce and are transferred to females during relatively short-lived mating aggregations. Mating in some species may take up to 24–48 hours, thus reducing the opportunities for multiple matings, and as a result males of long-mated species end up investing rather heavily in a limited number of females compared to males that mate more often. These features lead to size assortative mating (Alcock & Hanley 1987), with males showing a preference for larger females (that are presumably more fecund) and females preferring larger males to maximize the amount of cantharidin that they receive or to ensure receipt of a spermatophore large enough to fertilize their full complement of eggs. Medium-sized individuals, likewise, would choose the largest of the remaining individuals, leaving the smallest individuals to mate among themselves. Alcock & Hanley (1987) also note, however, that not all species of blister beetles exhibit size assortative mating, even though they form large mating aggregations and individuals vary greatly in size. I have not seen any reference to size assortative mating in Pyrota bilineata; however, this example seems to suggest that the behavior is not practiced by this species. This could be due to shorter mating times (leading to more opportunities for mating) or a range of variation in body size that is not sufficient to consistently favor the behavior.

REFERENCE:

Alcock, J. & N. F. Hadley. 1987. Assortative Mating by Size: A Comparison of Two Meloid Beetles (Coleoptera: Meloidae). Journal of the Kansas Entomological Society 60(1):41–50 [preview].

Copyright © Ted C. MacRae 2013