Umbonia crassicornis

The species is restricted to warmer climates where temperatures stay, on average, above zero. This limits the species to the tropical regions of Northern South America to Mexico and Florida. Any range of temperature below 0 degrees is fatal resulting in a 10-90 percent decrease in the population as temperature decreases.[1] Its preferred hosts are ornamental and fruit trees of subtropical regions feeding on sap inside the plants.[2]

Female Umbonia crassicornis often deposit about 100 eggs at a time into a plant stem,[2] under the bark. However, sometimes a female will lay her eggs in the leaf petiole even though they can be shed occasionally. The female will choose her brooding site based on if she can wrap her legs around the branch (diameter about 3.8mm)[3] The mother will stay with the clutch (a group of young) until the nymphs are mature. Without an adult female present during the nymphs development, the survival rate of the clutch drops from 53% survival to 27%.[3] If a clutch's defending female dies, the clutch can be adopted by a nearby female and the two clutches will combine into one, though the survival rate decreases significantly.[3]

A clutch normally consists of all full siblings, since the female usually only mates once, and sibling are more likely to defend each other based on this close relatedness.[2] A problem with a large group of closely related young growing up together is the possibility of Inbreeding once the individuals reach reproductive maturity.[4] When this happens survival rate of the offspring drops significantly due to mutations cause my mating with a sibling.[4]

Mate selection

Because a female U. crassicornis only mates once in her life (typically), males are forced to search and compete for a female. They do this using the 'fly-call-walk' strategy [5] in which a male will fly from one plant to another, then will walk around and strut. Once he encounters a female, the male will present vibrational signals to the female and if there is no response he will fly to another tree. If the female responds to these vibrations the male will approach the female and climb onto her side to initiate mating.[5]

A group of Thorn Bugs aggregated on a tree branch

When a female plants her eggs on a branch, her clutch uses up a lot of that site's resources. Due to this, some clutches use up more resources than the host plant can provide, this can result in adults being much smaller than preferred.[7] A female having a smaller body size, though can lay fewer eggs than a larger female, will not impact her overall reproductive fitness and will still be able to produce viable offspring. Males, on the other hand, have to compete with other males to reproduce so being smaller than average will affect his ability to mate and find food.[7] These factors contribute to the altered sex ratio we see in U. crassicornis being a larger number of females in populations than males.

Once the young have matured to a point where they can leave the congregates, males will leave first averaging about 7 meters from the host plant, whereas females, who leave after the male stay closer only traveling 3 meters.[8] This separation of sexes helps discourage inbreeding and results in a population that is more stable. Though inbreeding is not ideal, we still see it in close populations and there doesn't seem to be many preventive measures in place to prevent inbreeding from taking place other than the female's preference for older males.[4]

Umbonia crassicornis generally use a substrate-borne form of communication, allowing the parent and nymphs to communicate through chemically triggered vibrations. The main reason for these vibrational signals is in response to predators like the vespid wasp (one of the nymph's main predators). It is also seen that vibrations can be used to communicating within the brood.[2]

These signals act as an anti-predator response and alert the mother to the presence of danger, the mother, however, will only respond to the brood if their signaling is synchronized, so the whole group has to work together to alert the mother otherwise she will not respond.[2] To achieve this, young must respond immediately after hearing the signal from another nymph for their response to be synchronized. In doing this, the brood is deciding whether a threat is worth risking the mother attacking and possibly being removed from the congregate. If all the young in the brood decide the threat is worth the risk, then a synchronized signal is created. If only a few nymphs call about the disturbance then the synchronization won't be achieved and the mother will not respond. If the mother still can not find the threat after the young have signaled her, she will respond 'asking for help' to find the predator.[9]

Though the nymphs are a good detector of predators, no one nymph can predict if the threat has left or not due to each individual only being able to scan a small area. The only good predictor that the predator has retreated is the mother. After they have deterred the threat the mother will send out a signal to her young to tell them they are safe.[9]

A mother Thorn bug and some of her nymphs

When a large number of offspring are attacked by a predator, the risk factor is spread unequally amongst all the young. Certain circumstances lead to sibling rivalry in a brood of thorn bugs. Whether or not the mother is present can determine if a brood will compete with one another. When the female is present, a predator is only successful in removing a nymph 25% of the time, this number jumps to 95% when the female is absent.[10]

If a predator approaches the aggregation, it is more likely to take a nymph that is on the margin on the congregate, this produces a rivalry for sibling to want to be in the center of the brood and will fight over who will be on the outside because they have a higher risk of being preyed on.[10]

Another major factor leading to competition is the closeness of the nymph to the mother. The average time it takes the mother to travel 1.5 cm (about the body length of the mother) is 7.7 seconds, this is plenty of time for a predator like a wasp to circle the brood and move away from the mother.[10] Because of this, siblings will fight for spots closest to the mother for better protection.

• Umbonia crassicornis on the University of Florida / Institute of Food and Agricultural Sciences Featured Creatures