Saturday, January 28, 2012

Aphis Nerii and I


Two summers ago there was an incredible demonstration that took place right outside my back door involving the biological life cycle of a tiny insect species. This interesting display even included a lesson in phenotypic plasticity. It was a fantastic course in Nature 101. Who were the teachers? The Aphis nerii. Who was the pupil? That would be me!

What in the world is Aphis nerii? It is the scientific name for a tiny sap-sucking insect, the Oleander Aphid. What in the world is phenotypic plasticity? It is the ability of an organism to change its phenotype, or body form, in response to changes in the environment (more about that later).  Yea, I became so fascinated with these tiny creatures that I watched and photographed them every day throughout the entire summer.

The picture below is how I first observed the community sucking sap from my flowering vine.



Notice the sharp "stylet" that the aphids utilize to pierce a plant stem in order to access the sap.



Safety in numbers is a crucial rule of survival for many species, especially for those micro-sized individuals. I observed the oleander aphids exhibiting a few intriguing defensive tactics utilizing “team work”. This effort of the entire community is actually quite effective and crucial in order to maintain their existence. One of these strategies is what first drew my eye to them! I was watering an ornamental flowering vine when I noticed the tiny, bright orange insects. When I leaned over to investigate, the entire colony of aphids would, in perfect harmony, “kick” their tiny black legs up into the air much like a herd of miniscule “bucking broncos”. I was very intrigued with this cooperative behavior and after a little research I learned the very good reason they do this little jig. They get a bit edgy whenever they feel that something is approaching them too closely. The aphids are especially sensitive due to the danger of an aerial attack from tiny parasitoid wasps that will attempt to oviposit eggs into their soft bodies. The synchronous kicking movements help discourage the wasps from being able to gain access to them.  Despite the efforts of the aphids, some of the lucky wasps will eventually find their opportunity to jab an egg into their aphid host. The developing wasp will then eat the aphid from the inside out, and when it dies, it will turn hard and brown like a mummy. When fully developed, the wasp will simply emerge from the back of the aphid mummy by cutting a hole in its back.


When I waved a finger above the aphids, they began their kicking and twitching.



This picture shows most of them involved in a full "kick". They do this very forcefully and quickly!



The aphids have other tricks up their little black sleeves to help ensure their survival in their volatile environment! The plants they consume (mainly milkweed and oleander) contain deadly toxins called cardiac cardenolides, which cause them to not only taste disgusting, but to also actually be poisonous to those who prey on them. It doesn’t take long for some predators, like birds or spiders, to realize their mistake after tasting one.  It has even been observed that some species of spiders quickly retreat, washing off their mouthparts after coming into contact with the noxious flesh of the aphis nerii. The toxin can actually kill some predators after consuming one of these aphids, and SHOULD the predator survive, it may display problems later on. It has been noted that the ingested poison may cause some spiders to weave disfigured webs, which results in the inability to capture food, resulting in a lower survival rate. Ladybugs (another known predator) may develop wing deformities, resulting in a decreased ability to find food or to reproduce. I guess this is an attempt for the aphid species to reduce the population of potential predators. Any insect or bird searching for a meal should heed their bright yellow/orange coloration; in fact most species of the natural world instinctively know that this color is a warning!


The arrows below indicate droplets of the poisonous cardenolides being expelled through the tube-like cornicles located on the aphids' abdomens. This happens when they are threatened or aggravated. I observed this behavior quite frequently.




As the days passed by, I noticed that their numbers were increasing very rapidly, and it was no wonder... a closer look revealed they were giving birth to live young! Due to their incredibly small size, I was only able to witness this event through my 200mm macro lens. This is where things really get bizarre...these aphids are thought to be parthetogenetic, which means the adults are all female and reproduce without males (in fact males are thought to be absolutely non-existent in the wild). The females give birth to live young, which are perfect clones of themselves, and believe it or not, the newly emerged nymph already has an exact clone inside of her, which has an exact clone inside of her! These “telescopic generations” are not only nature’s ingenious way to ensure a rapid increase in their population, but it also quickly passes on the genetics needed to survive in the particular environment they are living in.

The picture below shows the aphids beginning to give birth to live young! It was an exciting event!

The picture below show how much the daughters look like the mother.



The aphids are extremely environmentally sensitive. Interestingly, at first the young are un-winged, but they have the amazing ability to change their body form based on changing conditions within their environment.  Overcrowding, decreased temperatures, decreased food sources or the increase in presence of natural enemies are factors that will cause this transformation. For example, when a colony reproduces to the point where their food supply becomes scarce, winged aphids will start to appear. This remarkable alteration further increases their chance of survival by allowing them to relocate where they can continue to feed, reproduce and recolonize. In addition, extremely detrimental conditions can also cause changes within the mothers and they begin giving birth to daughters who will now be winged. These characteristics are adaptations that allow a species to better adapt to its environment to aid in “survival of the species”.  I witnessed this exact scenario as soon as the population exploded to the point that they were becoming overcrowded.  I was amazed at how many winged aphids began appearing all of a sudden, and even more appeared when the temperatures started to decline in the evenings.


You can see here how crowded things are becoming on the vine in the foreground. The background displays the transformation that has occurred because of that situation. They will now be able to relocate to a fresh food source. The arrow below shows the "wing buds" starting to form. 


The white objects in the picture below are the skins that were shed during molting as the aphid nymphs were growing and developing. I read that the aphids may pile the molted skins up (as pictured), forming an infrastructure to serve as added protection from predation. I've also pointed out a dead, "mummified" aphid that fell victim to a parasitoid wasp.




Below, it is evident how the body style differs between the winged aphids and the un-winged. The winged ones need to be successful at flight, so they are much lighter and thinner.




 That summer, what began as a small group of aphids, grew to immense proportions in just a couple of months! My vine literally had thousands of them covering the branches and new shoots. Then as summer ended and the temperatures dropped, so did the their populations. Most likely a large portion of the winged aphids were dispersed by the wind, passively bound for warmer regions. They can migrate great distances, and that is probably how they first ended up here, since they do not overwinter, or begin as eggs. The remaining un-winged aphids succumbed to extinction by freezing to death and dropping to the ground. I was actually sorry to see them perish.  

Last summer I looked forward to once again being able to observe their complex life cycle, but there were very few aphids visible on my vine. I guess there are a lot of atmospheric variables that affect the dispersal of insects, and last year just wasn’t as favorable as the previous year. Even though aphids are a well-established invasive species that can cause massive damage to the plants they feed on, I felt their establishment on my flowering vine was an enlightening experience and a wonderful glimpse into an incredible world where survival is the name of the game. One can learn so much about life by observing even the smallest of organisms! I feel fortunate to have been able to witness this incredible journey through the lens of my camera.  

  

2 comments:

  1. Fascinating! Very interesting information and photographs.

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  2. I appreciate your comment, Bill! They are indeed fascinating little critters! :-)

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