Messenger - Vol. 4, No. 1, Page 6
Tree hopper may move Darwin's evolutionary theory to methodology stage

     Are new species still being formed? Yes, says one University of
Delaware entomologist who has meticulously documented the evolution of
a group of tree hoppers-small, sap-sucking insects. And he says he
believes it may be possible to replicate the evolutionary process
     Entomologist and ecologist Tom Wood has spent more than 28 years
scrutinizing Enchenopa binotata-a small, arboreal arthropod the size
of a grain of rice that lives on a variety of host plants from
viburnum to bittersweet. Through cold winter holidays and hot summers,
he recorded how this family of insects mingled, mated, laid eggs,
hatched and matured, usually without leaving the host plant.
     His long-term observations and related experiments have led Wood
to conclude that this population can diverge into several species,
each so expressly adapted to a specific host plant that they do not
interbreed. His results reinforce the concept of sympatric speciation,
where populations in the same geographic area can evolve into
different entities without a physical separation like a mountain range
or continent. In the case of the tree hopper, specific hosts take the
place of geographic separation, Wood says, and differences in plant
physiology induce life history changes in the insect.
     And, Wood believes, by forcing a population of tree hoppers to
shift to a novel host plant, a recognizably different entity will
     "We are creating species, or attempting it, anyway," says Wood,
who thinks his is the most convincing way to illustrate the specific
processes through which organisms change over time.
     "Speciation can proceed much faster than some people would think.
We are re-creating the conditions under which we think new species
arose in this group of insects. If we can do this, we will have a
documented case of sympatric divergence."
     According to Wood, there are about 3,500 species of tree hoppers
divided into New World and Old World faunas. All New World tree
hoppers have a sort of horn-shaped helmet called a pronotum. These
insects spend their lifetime together in some form of aggregation,
practically never leaving their host plant.
     Although Enchenopa binotata was considered one species, Wood
noticed that nymphs on different host plants were colored differently
and some had differently shaped or sized pronotums. He also observed
dissimilar habits of aggregation and feeding-some fed from bark,
others from leaf-and differences in where the females deposited their
eggs. In the early 1970s, he decided to put six cages next to each
other, each containing a different tree type and its resident tree
hoppers. He learned that tree hoppers in each cage deposited eggs at
different times of day and in different points during the summer, from
mid-July to mid-August.
     To determine preference, he gave females a choice of plant
species, and he learned they would only lay eggs on their native host.
     The next step in his long-term studies involved placing six
different host plants and their associated tree hoppers into a single
cage. "I wanted to know if mating would only be carried out on native
host plants or if mixed marriages occurred," he says.
     By painting pronotum tips different colors with model plane
enamel, Wood was able to watch "who did what with whom" for two years.
He found before mating, they all moved around, sometimes even
exhibiting the pre-copulation ritual, but not mating. "Fifty percent
of the males get together with females of the wrong plant, but when
actual mating occurred, they went back to the hometown girl," he says.
In fact, of 106 matings in one year, only six were mixed.
     By then, Wood says he felt confident that different species were
     Taking female tree hoppers from one host species, the black
locust, he forced them to lay eggs on five other host plants. Some
females died, some deposited a few eggs, and some laid eggs in the
fall rather than in late summer. "The bottom line was very few eggs
hatched, even fewer immatures lived past the second growth stage and
none survived to the adult stage. Gene flow by that mechanism was not
     Next, Wood chose to investigate whether the life history of the
egg hatch was a function of the host plant, rather than the result of
the tree hoppers' own genetics. Water was crucial to the egg hatch, he
found, because the eggs are inserted under the bark and require water
from the host plant's vascular system to develop. All six trees break
dormancy and bring in water at different times, resulting in
asynchronous egg hatch. "It was clear to me that differences in plant
physiology were inducing life history changes," he says.
     Wood decided to create a test situation that could never occur in
real life. He transferred with a camel hair brush some 6,731 immobile
nymphs the size of an aphid onto different host plants. "It appears
everyone can survive on the locust, so perhaps it is the oldest, the
ancestral host. The ability to survive on other host plants was
related to where the bugs are on the phylogenic tree"
     At this point, after 26 years of study, Wood has set out to test
if species formation can be directly observed. He has set up a long-
term experiment to see if a host shift can lead to sufficient genetic
differentiation among the tree hoppers to call them a new species.
     "If the speciation process is completed experimentally and in
such a way that it can be repeated by someone else, then Wood will be
bringing scientific methodology to the study of evolution," says
Robert T. Allen, chairperson of the Department of Entomology and
Applied Ecology at Delaware. "Darwin's theory of evolution will move
to the experimental methodology stage."
                                                        -Cornelia Weil