Camel Crickets and the Dust Seeds of Parasitic Plants

A common way for plants to disperse their seeds is to entice animals to eat their seed-bearing fruits – a strategy known as endozoochory. Undigested seeds have the potential to travel long distances in the belly of an animal, and when they are finally deposited, a bit of fertilizer joins them. Discussions surrounding this method of seed dispersal usually have birds and mammals playing the starring roles – vertebrates, in other words. But what about invertebrates like insects? Do they have a role to play in transporting seeds within themselves?

Certain insects are absolutely important in the dispersal of seeds, particularly ants. But ants aren’t known to eat fruits and then poop out seeds. Instead they carry seeds to new locations, and some of these seeds go on to grow into new plants. In certain cases there is an elaisome attached to the seed, which is a nutritious treat that ants are particularly interested in eating. Elaisomes or arils have also been known to attract other insects like wasps and crickets, which may then become agents of seed dispersal. But endozoochory in insects, at first, seems unlikely. How would seeds survive not being crushed by an insect’s mandibles or otherwise destroyed in the digestion process?

camel crickets eating fruits of parasitic plants (via New Phytologist)

While observing parasitic plants in Japan, Kenji Suetsugu wanted to know how their seeds were dispersed. Many parasitic plants rely on wind dispersal, thus their seeds are minuscule, dust-like, and often winged. However, the seeds of the plants Suetsugu was observing, while tiny, were housed in fleshy fruits that don’t split open when ripe (i.e. indehiscent). This isn’t particularly unusual as other species of parasitic plants are known to have similar fruits, and Suetsugu was aware of studies that found rodents to be potential seed disperers for one species, birds to be dispersers of another, and even one instance of beetle endozoochory in a parasitic plant with fleshy, indehiscent fruit. With this in mind, he set out to identify the seed dispersers in his study.

Suetsugu observed three achlorophyllous, holoparisitic plants – Yoania amagiensis, Monotropastrum humile, and Phacellanthus tubiflorus. While their lifestyles are similar, they are not at all closely related and represent three different families (Orchidaceae,  Ericaceae, and Orobanchaceae respectively). All of these plants grow very low to the ground in deep shade below the canopy of trees. Air movement is at a minimum at their level, so seed dispersal by wind is not likely to be very effective. Using remote cameras, Suetsugu captured dozens of hours of footage and found camel crickets and ground beetles to be the main consumers of the fruits, with camel crickets being “the most voracious of the invertebrates.” This lead to the next question – did the feces of the fruit-eating camel crickets and ground beetles contain viable seeds?

Monotropastrum humile via wikimedia commons

After collecting a number of fecal pellets from the insects, Suetsugu determined that the seeds of all three species were “not robust enough to withstand mastication by the mandibles of the ground beetles.” On the other hand, the seeds passed through the camel crickets unscathed. A seed viability test confirmed that they were viable. Camel crickets were dispersing intact seeds of all three parasitic plants via their poop. The minuscule size of the seeds as well as their tough seed coat (compared to wind dispersed seeds of similar species) allowed for safe passage through the digestive system of this common ground insect.

In a later study, Suetsugu observed another mycoheterotrophic orchid, Yoania japonica, and also found camel crickets to be a common consumer of its fleshy, indehiscent fruits. Viable seeds were again found in the insect’s frass and were observed germinating in their natural habitat. Seutsugu noted that all of the fruits in his studies consumed by camel crickets are white or translucent, easily accessible to ground dwelling insects, and give off a fermented scent to which insects like camel crickets are known to be attracted. Camel crickets also spend their time foraging in areas suitable for the growth of these plants. All of this suggests co-evolutionary adaptations that have led to camel cricket-mediated seed dispersal.

Yoania japonica via wikimedia commons

Insect endozoochory may be an uncommon phenomenon, but perhaps it’s not as rare as we once presumed. As mentioned above, an instance of endozoochory by a beetle has been reported, as has one by a species of cockroach. Certainly the most well known example involves the wetas of New Zealand, which are large, flightless insects in the same order as grasshoppers and crickets and sometimes referred to as “invertebrate mice.” New Zealand lacks native ground-dwelling mammals, and wetas appear to have taken on the seed dispersal role that such mammals often play.

Where seeds are small enough and seed coats tough enough, insects have the potential to be agents of seed dispersal via ingestion. Further investigation will reveal additional instances where this is the case. Of course, effective seed dispersal means seeds must ultimately find themselves in locations suitable for germination in numbers that maintain healthy populations, which for the dust seeds of parasitic plants is quite specific since they require a host organism to root into. Thus, effective seed dispersal in these scenarios is also worth a more detailed look.

Further Reading:

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For more stories of seed dispersal check out the first issue of my new zine, Dispersal Stories.

How a Plant Could Just Kill a Man, part two

Plants falling on people was a major theme in the Caustic Soda podcast Killer Plants episode, which is why part one of this two part series was devoted entirely to the subject. Yet, in the process of discussing death by falling branches and fruits, the hosts also mentioned at least three other highly dangerous and potentially deadly plants: ongaonga, gympie gympie, and the little apple of death. Those plants are featured here.

The nettle family, Urticaceae, includes a number of species that are best admired from a distance. Several genera (out of around 53 total) in this family are equipped with stinging hairs – sharp protrusions on leaves and stems that contain a variety of toxic compounds. Contact with these plants is ill-advised. Reactions vary from mild to extreme depending on the extent of the contact and the species in question. Two of the plant species mentioned by the hosts of Caustic Soda are members of this family – ongaonga (Urtica ferox) and gympie gympie (Dendrocnide moroides) – both of which are on the extreme side of the scale.

Urtica ferox is a New Zealand endemic that is commonly found in coastal and lowland areas as well as forest edges and shrublands. It is a shrub that reaches up to three meters tall and often occurs in dense thickets. The margins and midribs of its leaves are adorned with stiff hairs that are just a few millimeters long and poised to inject toxic compounds including histamine and acetylcholine upon contact. The “sting” is painful and can cause a variety of reactions including itching, inflammation, difficulty breathing, paralysis, blurred vision, and convulsions. Symptoms can last for several days, and neurological disorders occur in extreme cases.

Ongaonga has been blamed for killing several animals, including dogs and horses, but is charged with only one human death. In 1961, two hikers ventured into a patch of the stinging nettles. Shortly after contact they had trouble walking, breathing, and seeing. One of the men died a few hours later; the other recovered.

Ongaonga (Urtica ferox) - photo credit: www.eol.org

Ongaonga (Urtica ferox) – photo credit: www.eol.org

Several species in the nettle family can be found in Australia, one of which is particularly dangerous. Dendrocnide moroides, commonly known as stinging tree or suicide plant, is an early successional species, colonizing disturbed sites and sunlit gaps in the rainforest canopy. It grows to about three meters tall and has large heart-shaped leaves with sawtooth margins. All aboveground parts of the plant are covered in silicon hairs that are packed with a highly potent neurotoxin. The hairs detach easily from the plant and embed themselves in the skin of its victims. The “sting” is extremely painful and can last anywhere from days to months, possibly even returning from time to time years after contact. A rash, swelling, and itching sensation accompany the intense pain.

Following an encounter with the stinging tree, the “stingers” should be removed from the skin with a hair removal strip or some other sticky material, taking care not to break off the embedded tips. The affected area can be treated with diluted hydrochloric acid (1:10 by volume) to reduce the pain. Live plants are not the only ones to be wary of, as even old herbarium specimens have been said to sting those that handle them. Touching the plant isn’t even necessary, as the hairs easily dislodge from the plant in the wind and can be breathed in. One researcher reports developing a severe allergic reaction to the plant after working around it for several years and was advised by a doctor to abandon her research.

The spurge family, Euphorbiaceae, has many toxic plants among its ranks, including a species that Guiness World Records has awarded the world’s most dangerous tree. Commonly known as manchineel or beach apple, Hippomane mancinella demands respect, as a highly toxic latex sap is found throughout the entire plant. Just standing near it can result in painful blistering of the skin. Manchineel occurs along shorelines and in coastal woodlands and swamps in Central America and the West Indies, including southern Florida and the Florida Keys. It is a deciduous tree that grows to about fifteen meters tall, has thick grey bark, and glossy, elliptical leaves. Its fruits look like yellow-green crabapples and are sweet smelling and initially sweet tasting, that is until the burning and swelling starts followed by severe gastroenteritis.

Manchineel tree a.k.a. little apple of death (Hippomane mancinella) - photo credit: www.eol.org

Manchineel tree a.k.a. little apple of death (Hippomane mancinella) – photo credit: www.eol.org

Interaction with manchineel is inadvisable. The thick, milky sap seeps out of leaves, branches, bark, and fruits and causes intense blistering of the skin and temporary blindness if it gets near the eyes. During rainstorms, the sap becomes incorporated in raindrops and can drip or splash onto unwitting bystanders. Smoke from burning trees can also irritate the skin and eyes, and inhalation of the sawdust can result in bronchitis, laryngitis, and other respiratory issues. Modern history does not include reports of human fatalities resulting from eating the little apples of death, but descriptions offered by those who have consumed it confirm that it is an incredibly unpleasant experience.

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