Australasian Science: Australia's authority on science since 1938

Cockroaches Quit Sugar

By Magdeline Lum

Cockroaches have learnt to avoid sugar, rendering many baits ineffective.

The sugar-free diet is one of the latest fad diets to catch on in the human world. Now cockroaches have joined in, with US research that cockroaches will eat anything besides sugar – confirming what professional pest controllers have known for years.

Cockroaches have learned to detect and avoid the type of glucose used in commercial baits and traps. This was first noticed in the early 1990s after the use of glucose became common. It was not certain whether cockroaches learned to avoid the traps or evolved to dislike sugar. Now Prof Coby Shal of North Carolina State University has found that this aversion to glucose is inherited.

Cockroaches use their small taste hairs to sample food sources first. Glucose-laden food and baits taste bitter instead of sweet to the cockroaches with a genetic inheritance. When they detect glucose, they avoid the food source.

This gets complicated because it is unclear what proportion of cockroaches has the genetic inheritance. In 19 populations of cockroaches in the study sampled from the US, Russia, Puerto Rico and South Korea, seven had cockroaches that were glucose-averse.

Further tests were carried out where cockroaches had to choose between fructose-based jam and one that contained glucose. Video footage showed that cockroaches avoided the glucose-based jam. Cockroaches also preferred peanut butter over jam that contained glucose. They had learned to associate the taste of glucose with toxic bait.

So what now in the ever-growing arsenal against the household pest? A change is needed whereby traps no longer use glucose.

While there may be benefit in eliminating the cockroach from our homes, it must be remembered that there are around 5000 species of roaches. Other types of roaches that we do not see in our homes can be quite useful ecologically as they pollinate plants or are a key food group for insectivores.

The Guppy’s Grappling Genital Claws

Guppies are small freshwater fish found in aquariums throughout the world. They are colourful and are incredibly easy to breed. Unlike many fish that breed by releasing sperm and eggs into the water, male guppies fertilise females by delivering sperm internally.

Male guppies do not have a penis. They do, however, have a modified fin that is a penetrating organ known as the gonopodium. The tip of the gonopodium has a set of claws or hooks. Some even have spines. The differences between the features are sometimes the only method of distinguishing between guppy species.

Scientists have not been certain of the function of the claws in mating. Were they used to help male guppies hold onto unreceptive female guppies during mating? Or did the claws hold sperm at the tips to allow implantation of sperm in the females?

PhD candidate Ms Lucia Kwan of the University of Toronto decided to find out. Kwan used a scalpel to slice off the claws of several male guppies while leaving the rest of the gonopodium intact. They were then individually released into a tank with females to observe their behaviour in comparison with intact male guppies.

The results were clear. Intact male guppies transferred three times more sperm into unreceptive female guppies than male guppies that had been declawed. The same amount of sperm was delivered into female guppies that had willingly mated with intact and declawed male guppies.

The claws help male guppies latch onto females who resist mating. If the claws helped in the transfer of sperm, the declawed male guppies would not have been as successful as the intact male guppies when mating with receptive females. Mating would have been diminished in all instances.