This site is no longer maintained and has been left for archival purposes

Text and links may be out of date


Big fleas have little fleas
upon their backs to bite 'em;
little fleas have smaller fleas,
and so
ad infinitum.

To this verse, the poet Jonathan Swift added:

Thus every poet, in his kind,
Is bit by him that comes behind.

Those couplets would win no prizes, but they neatly describe the chain of events in many natural environments. For example, in the desert we often find parasitic plants such as the mistletoes, living off the resources of their host plants. There are even some mistletoes that parasitise other mistletoes! You can find an example by going to The Parasitic Plant Connection website (not on this server).

We also find another common parasitic plant - dodder - though it often is not as conspicuous as the mistletoes. And there are several small wasps and other insects that lay their eggs in plants and induce the plant to produce a nutrient-rich gall in which the insect larvae will develop.

Below, we deal with the mistletoes of desert environments. On other pages you can find dodder, and insect galls.


The desert mistletoe is commonly seen on desert trees such as ironwood, mesquite, paloverde and acacias. It usually produces a tangled mass of narrow, branching stems - sometimes green but usually tawny brown, with small, scale-like leaves. It is an evergreen plant. It eventually attains such a size that it kills the major branches of its host - or even the whole tree.

[All images are clickable, for a larger version]

Several large tawny brown clusters of desert mistletoe (Phoradendron californicum) on a paloverde tree in the Sonoran desert

Single large cluster (about 1 metre long) of desert mistletoe on paloverde

Part of a mass of stems with fruits of desert mistletoe. The scale-like leaves are inconspicuous.

Swollen and cracked part of a branch of paloverde, where desert mistletoe branches have emerged

A different type of mistletoe, called Phoradendron flavescens, has broad, rounded and fleshy leaves. It is found at higher elevations in the desert, and is quite common elsewhere in south-western USA. It parasitises a different range of plants, such as cottonwood (in the poplar family), willows and sycamores.

The broad-leaved mistletoe, Phoradendron flavescens, parasitising a shrub in the higher elevations of the Big Bend National Monument, Texas

Yet another type of mistletoe, Phoradendron bolleanum, is a common parasite of juniper and cedar trees.

P. bolleanum on juniper in an upland region of Texas

All mistletoes produce fleshy, succulent berries which are eaten by birds to obtain the viscous, sugary flesh. In many cases the bird scrapes the seed onto a branch or other object, or the seed is voided with the faeces after a few minutes. In this way the seeds can become lodged on the bark of a host tree. The seeds germinate spontaneously if exposed to light - usually they need no other germination trigger. The structure that emerges from the seed is termed a hypocotyl - a green stem-like structure with a rudimentary root at its tip. This structure elongates and penetrates beneath the bark of a tree. Then it produces a special nutrient-absorbing structure termed a haustorium, which establishes close contact with the conducting tissues of the host and will eventually tap the host's water and nutrients.

The mistletoes have green, photosynthetic tissues, so they can obtain at least part of their sugars and other organic nutrients from photosynthesis. But they depend on the host for their supply of water and mineral nutrients, and so they inevitably extract some toll on the host's resources. Parasites of this type often are termed hemiparasites (as opposed to holoparasites which depend on the host for all of their nutrients).

An interesting feature of mistletoes in general is that they can have quite broad host ranges, but with varying degrees of host specificity. For example, the desert mistletoe (Phoradendron californicum) typically infects nitrogen-fixing trees in the family Leguminosae, such as mesquite, paloverde, ironwood and acacias. The basis of this specificity has never been satisfactorily explained.

Another interesting feature of mistletoes is that their seed dispersal is limited by the distance that birds travel (and the short retention time of the seeds in the gut). Thus, mistletoe-infected trees tend to be seen in patches. Also, it is quite common for a single tree to have multiple infections, if birds eat the mistletoe berries on that tree and scrape the seeds onto another branch of the same tree. The first image on this page shows this clearly.


This site is no longer maintained and has been left for archival purposes

Text and links may be out of date

Accessibility Statement