Caterpillars inherit food preferences through their blood 

Many caterpillars exhibit specific food preferences that persist when they become butterflies. For example, monarch butterflies feed exclusively on milkweed, while Lime butterflies prefer lime leaves. 

Researchers at the National University of Singapore (NUS) are investigating the origins of these dietary preferences.

Caterpillar odor preferences 

A previous study by NUS researchers revealed that caterpillars develop a preference for the smell of leaves they consume outside their typical diet within a few days and pass this preference to their offspring. 

This adaptation can occur naturally when female butterflies lay eggs on non-typical plants. Caterpillars that feed on these new plants develop and inherit a preference for the new smell, potentially leading to host switching and the formation of new species with distinct dietary preferences.

“Since new food preferences develop in the brain of caterpillars, it was unclear how such preferences were inherited by their offspring,” said professor Antónia Monteiro, who led the research team.

“Offspring develop from the fusion of two cells (an egg and a sperm) produced in the gonads of each parent, which are located far from the brain. It was unclear how a smell preference was communicated to these cells,” explained lead author V. Gowri, a PhD student at NUS.

Smell preferences across generations 

To understand this adaptive behavior, the research team conducted experiments and discovered that hemolymph, the blood of the caterpillar, which bathes both the brain and reproductive organs, contains factors that promote the inheritance of new smell preferences.

A caterpillar’s blood can transport factors from the brain to the gonads, influencing smell preferences in the next generation. Alternatively, it could transport these factors from food to the embryo’s brain if included in the sperm or egg cells.

Food preferences in caterpillar blood 

To test if caterpillar blood contained such factors, newly hatched caterpillars were fed plants with a new smell or a control plant. The researchers collected blood from mature caterpillars and injected it into others that had not consumed either plant type.

The experiment showed that caterpillars receiving blood from control-fed caterpillars maintained their usual diet. In contrast, those injected with blood from caterpillars fed with the new smell plants adapted to this dietary change. Remarkably, their offspring also inherited this new preference.

“This was very surprising to us, as this experiment shows that learning a preference towards a smell can occur without the need for the smell to enter the caterpillar’s body via the antennae, as suggested in textbooks,” Monteiro said.

Mechanism for odor learning and preference 

The findings suggest a possible mechanism for caterpillars to switch food preferences over evolutionary time. The researchers aim to further explore this mechanism and identify specific factors inherited across generations.

“These results indicate that factors in the haemolymph, potentially the odor molecule itself, play an important role in odor learning and preference transmission across generations,” wrote the study authors.

“Furthermore, this mechanism of odor preference inheritance, mediated by the haemolymph, bypasses the peripheral odor-sensing mechanisms taking place in the antennae, mouthparts or legs, and may mediate food plant switching and diversification in Lepidoptera or more broadly across insects.”

Monarch butterflies and milkweed 

Monarch butterflies and milkweed have a fascinating and crucial relationship. Monarchs are pretty much dependent on milkweed for their survival. The female monarchs lay their eggs exclusively on milkweed plants because their larvae (caterpillars) eat milkweed leaves as their sole food source.

The connection goes beyond just a food source. Milkweed contains toxic compounds that caterpillars ingest, making them unpalatable to predators. 

This toxicity is carried through to their adult butterfly stage, offering them protection throughout their life cycle. The bright orange and black wings of Monarch butterflies are a warning sign to predators: “Stay away; I taste terrible!”

This dependence means that milkweed is critical to the survival of monarch butterflies. Unfortunately, with the decline of milkweed due to habitat loss and herbicide use, monarch populations have been struggling. 

Many people are now planting milkweed in their gardens to help support these incredible creatures. It’s a simple but impactful way to contribute to the conservation of monarchs.

The study is published in the journal Biology Letters.

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