Large milkweed bug

Identification

Adults can range from 10–18 mm in length and have a red/orange and black X-shaped pattern on their wings underneath the triangle that is typical to hemipterans. This feature makes the bug easily seen, acting as a aposomatic warning to predators of distastefulness. O. fasciatus exhibits müllerian mimicry and is noxious to predators. The ventral side of the fourth abdominal segment bears a black band in the male and two black spots in the female. Juveniles are born mostly red with black antennae and a few black spots, throughout growth the black spots are developed as well as wing pads. Eggs of this insect are bright orange and easily detectable.

Life cycle

This large milkweed bug is a hemimetabolous insect, meaning it grows in stages called instars and goes through incomplete metamorphisis exhibiting small changes throughout development such as coloration changes, development of wings and genitalia. O. fasciatus begins as an egg and experiences 4 nymphal stages over 28–30 days before moulting to adulthood. Females become sexually receptive within a few days of adulthood. Geographic location has a large effect on egg production rate and clutch size, although the intrinsic increase in reproduction depends on what the individual is acclimated to. For example, two close populations (60 km apart) residing on a sharp incline have differing optimal reproduction temperatures where the cooler (higher) adapted populations is at 23 °C and the warmer (lower) population is adapted to 27 °C. Highest clutch size occurs in Puerto Rico, Florida and Texas populations at 30-50 eggs per clutch. Lowest clutch size was found in California at 25-30 eggs per clutch. Iowa and Maryland (northern populations) exhibited a clutch size in between the two extremes from 25-35 eggs per clutch.

In favourable conditions (tropics) reproduction occurs continuously all year round, in less-favourable conditions (temperate zones) reproduction occurs during the warmer months. Reproducing when migrants arrive introduces gene flow between northern and southern populations, this provides an advantage to the mid-range populations (variable climates) because females then can ensure genetic variability.The milkweed bug can produce anywhere from 1-3 generations per year depending on climate and geographic location. O. fasciatus exhibits strong selection for survival and will halt reproduction as a trade-off if conditions are not ideal.

Diapause occurs on short days and on cold days in temperate regions and occasionally occurs during dry season in tropical regions. Most populations of the milkweed bug overwinter; usually after migration to their overwintering sites due to environmental triggers such as temperature and photoperiod. However, photoperiod only predicts overwintering in areas where day length effects the maturation of milkweeds. Therefore, no overwintering occurs in tropical regions as it does not supply an adaptive advantage.

Migration

O. fasciatus can be separated into migrators and non-migrators. Palmer and Dingle showed that northern populations such as the one in Iowa show the greatest tendency for long distance flight and are highly migratory. Oppositely, southern populations such as those in Puerto Rico show the lowest tendency and are sedentary. South populations grow seasonally as migrants first appear in late spring and mid-summer. A migratory syndrome has been described in the northern population, meaning that traits such as wing length, fecundity, developmental time and flight duration are all genetically correlated. Groeters and Dingle suggested that selection is specific to the populations environment due to the small correlations between life history strategies across geographic ranges. A trade-off between migration and life history traits may be the causation of such a wide geological distribution. Attisano suggested that genetic factors as well as environmental cues trigger migration in some individuals. Since long duration flights decrease with decreasing latitude, temperature is a strong factor influencing the migration. Also, this movement correlates with flowering of milkweeds which provides further evidence that environmental triggers relate to migration. Larger females are thought to allocate resources to migration simply because they have more to spare. Smaller individuals are thought to deploy alternative mechanisms; one being the reabsorption of oocytes for energy. The fact that these insects return to northern environments after migration could be the influence of a genetic predisposition or selected for due to crowding and increased intraspecific competition for resources in the southern areas

Tropical populations migrate shorter distances than temperate populations because spatial variation of their choice host is much greater so it is advantageous to seek new plant congregations rather than tolerate the depletion of resources.

Diet

O. fasciatus is an intermittent feeding herbivore specialist that relies mainly on the common milkweed seed, Asclepias syrica but can also feed on other species in the Asclepiadaceae family. Some other species it has been shown to feed on when milkweed is low are A. nivea, Sarcostemma clausa and Calotropis procera, and Nerium oleander. The southern population’s primary host is Asclepias curassivica, a tropical milkweed. When given sunflower seeds in a lab setting the insect absorbed 90% of its lipids, 50% of its protein and 20% of its carbohydrates, making it an efficient feeder. Toxic compounds in the milkweed are also sequestered, giving this insect its toxicity.

Adults wander during the daytime in search of food since milkweeds live in patches that can vary in size and distance apart from one another. When a seed is found, they inject saliva into it through a long rostrum, this pre-digests the seed and allows O. fasciatus to suck it up through their anterior pump and pharynx. Often there are multiple adults feeding on one seed, suggesting there is a signal released by feeding individuals that indicate a good food source. It has been shown that adults are more likely to find a food source when there is already another adult feeding on it, further supporting the signaling concept. Occasionally, females are seen feeding on shedding exoskeletons from moulting individuals. Rarely, cannibalization is observed in laboratory settings

Juvenile O. fasciatus require the seed of milkweed plants for development and growth. Adults can survive on other types of seed such as: sunflower, watermelon, almond and cashew, as shown in lab populations. Nymphs live in large groups on the plant of about 20 individuals. Juveniles have an advantageous discontinuous 3 part gut that acts similarly to a crop, ventriculus and ileum however they are lacking definitive parts. During the final instar of development oil accumulates to allow for more efficient absorption of nutrients as well as aid in osmoregulation.

Laboratory use

This insect is often used as a model organism and reared for laboratory experiments due to the insect being easy to rear and handle, short developmental time, few instars and a high fecundity. The phylogenetic placement of O. fasciatus is ideal to use as an outgroup to make comparisons to more derived holometabolous insects, acting as a valuable organism for the study of evolutionary patterns.