Yellow or purple? Colour choices of insects at the beach

When you think of the beach, what comes to mind? Do you think about basking in the sun, feeling the granules of golden sand underneath your toes, taking a refreshing dip in the blue, salty water with the waves crashing around you?

To think of the plant life which thrives within the beach environment, and the little creatures that call the beach home, would perhaps be a little further down your list. Most people are unaware of the important role both plants and insects hold within a beach environment.

The sand dunes at New Brighton beach in Christchurch, are home to a number of exotic plant species, including marram grass, tree lupin, purple ragwort, and ice plants. These plants, and their flowers, provide great habitat for some sand dune insects and spiders. The bright colours of the flowers are one of many signals plants use to attract insects and pollinators. Although exotic plants can sometimes compete with native plants, helping to attract pollinators to the area helps benefit the nearby native plants too.

New Brighton Beach, Christchurch
CC0-BY Bernard Spragg (Flickr)

At New Brighton beach, 33 insect and spider species have been observed visiting the flowers of the abundant ice plants, dotted across the sand dunes in both yellow (Carpobrotus edulis) and purple (Carpobrotus chilensis) varieties. Lincoln University researchers Simon Hodge and John Marris visited New Brighton beach to determine if flower colour preferences existed within species attracted to ice plants (Hodge et al., 2017). Are insect and spider species more attracted to yellow flowered ice plants, or purple flowered ice plants, or was there no preference at all?

Purple flowered ice plant (Carpobrotus chilensis)
CC-BY-NC Jon Sullivan (Flickr)

For one month, Hodge and Marris recorded which insect and spider species interacted with which coloured ice plant flowers at New Brighton beach. To figure out interactions, insects and spiders were collected from flowers using a battery operated aspirator. Bumble bees (Bombus terrestris) were recorded through on site observations, but were not collected.

Of the 3600 flowers Hodge and Marris inspected, 3360 (93.3%) were yellow, and only 240 were purple. This showed the yellow flowered plants are 14 times more common than purple flowered ice plants within that section of the beach. Although this difference in flower colours was true for the location, the imbalance is not ideal for research. Normally more even sample sizes are preferred for research studies. Despite this, the researchers continued with their investigations and came across some interesting findings.

A buff tailed bumble bee (Bombus terrestris) taking a road less travelled on a yellow flowered ice plant (Carpobrotus edulis)
CC-BY-NC-ND Stefan Marks (Flickr)

Nearly 500 individual insects from over 30 different species visited the ice plant flowers over the month long period. Compared to those with purple flowers, yellow flowered ice plants were more than twice as likely to have insects on them. By far, the most common species identified was a pollen beetle, which made up almost 60% of recorded individuals. The pollen beetle also showed the strongest colour preference with the beetles enjoying the yellow flowers 14 times more often than plants with purple flowers.

Bumble bees were quite commonly found, along with long horned grasshoppers, making up 15% and 8% of recorded individuals respectively. Bumble bees were twice as likely to choose purple flowers over yellow. It seems this could have been predicted as previous studies also acknowledge bumble bees prefer blue and violet coloured flowers.

The grasshoppers seemed to prefer the vibrant glow of the yellow flowers. As there were a lot more yellow flowers in the beach area generally, researchers were not sure if this was a true preference or simply a fluke. They would need to observe a larger number of grasshoppers to be sure.

So, colours do matter. Insect and spider species love the bright colours that flowers offer, and just like people, varying insect species have their own preferences too.

This article was prepared by postgraduate student Mel Barnett as part of the ECOL 608 Research Methods in Ecology course.

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