The curious case of the toxic slugs

If you’re in anyway tuned into the world of environmental news, you will have most certainly heard of neonicotinoids. Neonicotinoids (or neonics for short) are a class of chemicals which have become increasingly popular for controlling pest insects in agricultural systems. Neonics are highly toxic to insects, but usefully have low toxicity to birds and mammals.

While neonics have been widely adopted by farmers, they have less than a sterling reputation among many groups. Neonics work through a mode of action known as systemic delivery. This is achieved by coating the seed with the pesticide treatment. This active ingredient is then incorporated into the plant tissue, and is delivered directly to an insect feeding on any part of the plant. By applying neonics to seeds, riskier methods of pest control – like widespread spray programs – can be avoided during later stages of production. Due to their efficacy and relatively targeted delivery, neonics have become an important tool for suppressing insect pests.

The largest problem with the use of neonics is that they are applied  before their use is known to be required, a process known as ‘prophylactic application’. This process is analogous to taking aspirin to relieve a headache, you could foreseeably have later in the day. Over-reliance on a substance can eventually lead to reduced efficacy through pest resistance, and when little pest pressure is present – can contribute to the loss of beneficial invertebrates through non-target effects. When these two factors are considered together – potential damage caused by pest species is further exacerbated.

A new open access study published in the Journal of Applied Ecology has demonstrated a novel pathway through which neonics can impact the natural environment. This pathway involves a herbivore unaffected by the insecticide, which through feeding on treated plants become toxic prey for its predators.

No-till soybeans were the study system used within this paper. In the particular region studied, the dominant pest problems are caused by slugs. These slugs feed on the soybean seedlings, causing mortality and reducing yields. While the neonic treated plants are highly toxic to insect herbivores, slugs are unaffected by the presence of neonics. However, these slugs are kept in check by a diverse group of organisms including: ground beetles and farmland birds. We call these predators ‘natural enemies’. One of these natural enemies is Chlaenius tricolor – a wonderfully charismatic ground beetle. In the study region, these beetles are common within arable fields,  are highly mobile, and have a particular fondness for eating slugs.

In a series of laboratory experiments, the team confirmed that slugs were unaffected by consuming soy plants treated with thiamethoxam (a type of neonic commonly used to treat soybeans). However, when beetles were offered slugs which had fed on treated plants – more than 60% of individuals died or were seriously impaired.

The team then scaled this up to a field level, and found a similar pattern. In field plots grown from neonic treated seeds they found significantly higher densities of slugs, and significantly lower captures of natural enemies. Amazingly, the team even found five percent higher yields from non-treated seed (although pressure from insect pests was very low).

NEONIC_CROP

The rapid movement of a pesticide through this food-chain is an important reminder of how interconnected our food production systems are. It beautifully demonstrates sustainable agricultural production demands healthy ecosystems. While some would use the findings of this study as evidence for the condemnation of neonics and other insecticides within agricultural ecosystems, I wouldn’t agree. Insecticides remain an important tool within agricultural production. To me, the findings from these experiments serve to demonstrate just how cautious we must be with our application of pesticides.

The curious case of the toxic slugs brings to light just how complex, and sensitive our environment is. We need to think about the impacts of our actions on a multitude of scales. Whoever would have guessed that a predaceous beetle would be negatively impacted by chowing down on slugs made toxic by feeding on a neonic-treated soybean plant?

Reference:
Douglas, M. R., Rohr, J. R., Tooker, J. F. (2015), EDITOR’S CHOICE: Neonicotinoid insecticide travels through a soil food chain, disrupting biological control of non-target pests and decreasing soya bean yield. Journal of Applied Ecology, 52: 250–260. doi: 10.1111/1365-2664.12372

How plants respond to ‘bad vibes’

Post_3The term ‘stand your ground’ has particular significance for the plant kingdom. When faced with hardship (eg: drought, flooding, extreme heat, freezing temperatures, or herbivory) plants do not have the option of retreating. Instead plants stick it out, finding more subtle ways of fighting back.

A new open access study from a team at the University of Missouri-Columbia and published in the journal Oecologia has uncovered a fascinating new discovery: plants are able to detect the vibrations made by their enemies, and respond by producing higher levels of chemical defences within their leaves. These chemical responses of the plant can reduce herbivory in many ways including: making plant tissues unpalatable, or even negatively impacting growth of the herbivore.

In this study, the team used two familiar species. The first is rockcress (Arabidopsis thaliana)  a plant in the mustard family familiar to most geneticists. The second was the larval form of the cabbage butterfly (Pieris rapae) – a species often too familiar to veggie gardeners, and producers of brassica crops.

The researchers placed a caterpillar onto a leaf of the rockcress. By measuring the miniscule movement of the plant leaf as a caterpillar fed, the team was able to capture the vibrations caused by the chewing mandibles. This isolated sound was then used within further experiments to test the impact of sound in triggering host-plant defence.

 

The recordings were then played back to two naïve plants, through the use of a tiny motor called an actuator. Plants that received vibrations based on caterpillar mandibles were known as ‘chewing treatment‘, while two other naïve plants were subjected to a still actuator control ‘silent treatment‘. Larvae were added to the plants and allowed to feed until 30% of the leaf had been consumed. Then, the researchers sampled different types leaves: the leaf which had been directly vibrated + chewed, a leaf within the rosette of similar size and age, and a younger leaf in the rosette centre.

The team then analysed the concentration of a class of chemicals within the leaf that is known to deter herbivores: glucosinolates (think of the pungent, peppery taste of raw cabbage). The team found that leaves subjected to the ‘chewing treatment’  had roughly 20% higher concentrations of glucosinoates than leaves sampled from the ‘silent treatment’. This was found both locally (leaves directly chewed), and systematically (un-chewed, but systemically vibrated rosette leaves of the same age).

The team then looked to test whether plants could differentiate ‘bad vibrations’ (caused by herbivory of the caterpillar) against ‘benign vibrations’ caused by wind, or the song of a leaf-hopper. This time the researchers looked at a different group of chemicals: polyphenol anthocyanins – the same chemicals that give a blueberry it’s rich purple-blue colour. These chemicals are also secondary defence chemicals used to reduce herbivore vigour, and render leaves less palatable by herbivores.

Using the same experimental design the researchers found that plants exposed to vibrations simulating caterpillar feeding resulted in significantly higher levels of polyphenol anthocyanins than plants subjected to benign vibration treatments. This effect was not observed unless plants were subsequently exposed to actual feeding. This suggests vibrations had a priming effect, rather than directly affecting the level of a plants response.

This study is a perfect demonstration of just how successful plants have become at defending themselves against their enemies. It is even possible that plants might ‘eavesdrop’ on the vibrations of a neighbouring plant, similar to the way plants can respond to the chemical cues of their neighbours/competitors. We can now add ‘sensing vibration’ to the growing toolbox that plants use to defend themselves against their enemies. Please stay tuned for more about the wonderful world of plant defence. It only gets stranger.

Reference:
Appel, H. M., & Cocroft, R. B. (2014). Plants respond to leaf vibrations caused by insect herbivore chewing. Oecologia, 175(4), 1257–66. doi:10.1007/s00442-014-2995-6

Plants trick moths by faking injury

Plants are the ultimate masters of defence. As they are unable to run away if being attacked, they have to find different ways to fight against their enemies. Many plants produce foul chemicals to ward off herbivores, while others produce structural defences such as thorns or dense hairs. Other plants recruit natural enemies of their herbivores through the use of a chemical signal called a synomone.

As we continue to learn more about the secret lives of plants, we discover more strange and wonderful strategies plants utilise to stay healthy. In the case of the rainforest plant species Caladium steudneriifolium a novel method of self defence has been discovered: pretending to be damaged.

You’re probably familiar with plants in the genus Caladium. Their tolerance to a bit of neglect, large leaves, and glossy sheen make them one of the world’s most popular house plants. Interestingly, another quality that makes these plants such a desirable addition to your home is essentially the same mechanism that allows the plants to deter predators: variegation.

Plants that are variegated have more than one type of genetic make-up within their tissues. This leads to differential leaf colour, causing white, or yellow  zones on some part of the plant. These unusual features can often be quite beautiful or interesting, and correspondingly plant breeders often propagate variegated varieties en mass. This gives us the interesting variegated varieties found throughout gardens worldwide.

In wild ecosystems, retention of genes causing variegation is a slightly different situation. Non-green zones on the leaf do not contain chlorophyll, and as  chlorophyll is required for plants to photosynthesise there is an associated fitness cost of having variegated leaves. However, in the wild – the species C. steudneriifolium exists in both variegated (1/3 of all plants) and uniformly green (2/3) types.

In 2009, a team lead by Sigrid Liede-Schumann noticed that variegation on the leaves strongly resembled damage caused by leaf mining insects. They noticed leaf miners were present in both morphs, and set out to test whether presence of a leaf miner was influenced by whether leaves were variegated or not. They expected that moths would preferentially select healthy leaves free of competition for their larvae to develop, selecting entirely green leaves over variegated leaves.

To test this, the team tagged 800 brand-new leaves near a car park in South Eastern Ecuador. These leaves were divided into four treatments shown in Figure 1 below. The team used either white correction fluid, or a transparent correction fluid thinner (shown in blue for clarity) to mark simulated variegations on the leaves. The team came back 3-months later and counted the number of leaves infested by leaf mining larvae within each treatment.

Post_2b
Figure 1: Experimental manipulation of C. steudneriifolium leaves

The team found infestation rates were 4-12 times higher in uniformly green leaves in comparison to variegated leaves. This was true regardless of presence of clear painted defoliation, suggesting mining moths use visual cues to select their host plant.

Alongside the direct cost of herbivory, when leaf mining insects emerge from the C. steudneriifolium leaf after completing development a wound in the plant is formed. Plant pathogens can opportunistically use this wound to infect  the host. By tricking moths into believing it is already in use, the moth passes up the ‘used’ leaf and seeks out a ‘fresh’ healthy host – which in reality, is more likely to be in use by the undetected larva of another moth than a leaf of the variegated plant. The variegated plant can then breathe a sigh of relief, enjoying it’s comparatively higher quality of life made possible by deception. A pretty clever strategy, even for a plant.

Reference:
Soltau, U., Dötterl, S., & Liede-Schumann, S. (2008). Leaf variegation in Caladium steudneriifolium (Araceae): a case of mimicry? Evolutionary Ecology, 23(4), 503–512. doi:10.1007/s10682-008-9248-2