Friday, July 04, 2014

Hidden Life on a Pear Tree

Bradford pear in bloom, attracting numerous pollinators
© 2014 Beatriz Moisset
Spring finally arrived, although a little later than usual. At the end of April, the Bradford pear trees in front of the Abington Library were in full bloom, white clouds against a sapphire sky. Hordes of little bees eagerly visited the sweet flowers.

Pollinators were not the only visitors to the trees. Surprisingly, life was also present in places you wouldn't think to look at. A cluster of bright yellow dots in a bark's crevice caught my eye just by chance. Afterwards, I began to see many similar clusters, partially hidden by the bark's irregularities. I knew them as lady beetle eggs, so I wasn't surprised to see some adult lady beetles engaged in romance or in search of potential nurseries for their babies.

Lady beetle eggs
© 2014 Beatriz Moisset
 A solitary green aphid painstakingly climbed the tree trunk, probably in search of tender shoots to feed on its juices. What was it doing so far from its traditional food? Perhaps, it had fallen to the ground and, obstinately, was engaged in the arduous voyage back to distant branch tips.

I puzzled at the lady beetles' choice of egg-laying sites. The tree trunk may offer refuge for the eggs, but not food for the newborns. The tiny larvae would have a long way to go to find nutritious aphids. Perhaps, just like the lost aphid, they could manage the perilous trek successfully.

I visited the trees a week later. They were still blooming, although slightly past their peak. I looked for the lady beetle eggs and found many of the clusters still in place. Some had darkened, a sure sign that the larvae inside would soon emerge, hungry for aphids. Others had become food for unknown predators and were gone.

An incipient aphid colony
© 2014 Beatriz Moisset
 Once again, I wondered where they would find food and looked at the new growth, the clusters of flowers which were already beginning to leaf out. Aha! Just as I suspected, the green aphids were there, small colonies of young and adults, wingless and even some winged ones. I would have never noticed them if I hadn't suspected their presence because of the lady beetles.

Aphids have a way of multiplying at great speed. One female can produce many babies in a mere week and the babies start reproducing in an equally short time. The small colonies I observed would become huge by the time the new leaves reached full size. They would be numerous enough to damage the new growth.

Aphid in search of food
© 2014 Beatriz Moisset
Would something keep the aphids in check? The mentioned lady beetles had their job cut out for them. But they would need lots of help and I was pleased to see other aphid enemies. I caught a glimpse of a parasitic wasp. These tiny wasps lay their eggs inside aphids and their larvae feed and kill their hosts in the process. Another great helper became apparent later on, on further observation. Some minute greenish grubs were thrashing about in the midst of the aphid colony. A syrphid fly had chosen this place to lay her eggs. The recently emerged blind and apparently helpless larvae were busy devouring their favorite food, juicy aphids. They grow fast on this nutritious diet and soon metamorphose into small flies, ready to lay more eggs.

Syrphid fly maggot feeding on aphids
© 2014 Beatriz Moisset
 The accidental sight of the lady beetle eggs led me to the related findings, aphids and their predators—an entire mini-ecosystem well hidden in bark cracks and flower clusters of a pear tree. A month later, I visited the trees again and found no traces of this entire food chain. The stems appeared too tough for the aphids; the winged ones had moved elsewhere. The lady beetles and other predators also took their leave in their perennial search for nourishment.

Aphids in the web of life

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© Beatriz Moisset. 2014