The life cycle of seed beetles will inform and entertain all ages says Michael Dockery
When adult seed beetles emerge from the bean in which they have developed, they live for two to four weeks. Amazingly, they neither eat nor drink during their adult lifetime. Though their natural habitat is a field crop of beans (such as adzuki, mung or black-eyed, available in health food stores) in a tropical or sub-tropical country, this is just one fascinating aspect of their behaviour which can be seen in the classroom.
Seed beetles (Callosobruchus maculatus) begin life as eggs, laid on a bean.
The egg hatches four to six days later and the larva eats its way into the bean until it is about 18 days old, when it pupates. About one week later, the adult emerges and searches for a mate. The beetles engage in multiple mating: in fact, males appear to do nothing other than mate, recover and look for more mates. So, male seed beetles do not climb cranes or balance precariously on palace ledges on duty for Fathers 4 Justice. Males die well before their offspring are adults. They rely on the parental skill of their mates to identify good egg-laying sites to ensure that their offspring survive, mate and make a genetic contribution to future generations.
Like males, females are also eager to mate, after which they lay their eggs on beans. Females subsequently mate again and lay more eggs, repeating this pattern of behaviour until they die.
Seed beetles are remarkably easy to keep in schools. They can be housed in old coffee or jam jars with some muslin (or plastic netting) over the top, held in place by an elastic band. And they require only a few minutes'
maintenance once every five to six weeks, so no worries about the summer break.
Two maintenance tips
1 Establish new cultures of beetles in clean jars with fresh beans. This minimises the build up of beanbeetle debris to which some people may be allergic.
2 A barrier of Fluon around the top 5cm of the jar makes the beetles lose their footing so they cannot reach the top and get under any folds in the muslin. Fluon is available from Blades Biological, check their website www.blades-bio.co.uk
KS2 Children don't realise we can ask beetles questions. For example, do females prefer to lay eggs on mung or adzuki beans? By carefully observing their behaviour we can answer this and let children discover that females are adapted to search for beans, though some beans are preferred to others.
Put nine mung and six adzuki beans (roughly comparable masses) in a Petri dish to give females a choice of where to lay their eggs. Put six to 10 mated females in the dish and leave for a day. Count the eggs (small white dots) on each bean. The usual preference is mung then adzuki. Why? Is the time from egg-laying to adult emergence the same for each type of bean? (Often 3-5 days quicker on mung beans.)
KS3 If you offer females both clean (egg-free) beans and egg-laden beans, do they prefer to lay on clean beans? Put several females, each in its own Petri dish, in with, say, five clean adzuki beans and five with a known number of eggs on them. Count the eggs the following day. Is there a difference? What are the adaptive advantages of laying on clean beans?
Or Seed beetles walk quickly: how quickly? Can students design an investigation to measure their speed? It is best to confine the beetles, otherwise they won't walk in a straight line, so an empty thermometer tube works well. Draw a 10 cm line on a sheet of graph paper and Blu-Tack it to the table. Put one beetle in the tube, insert the stoppers, and time how long it takes to walk 10cm: repeat for 10 males and 10 females. Compare performance when the tube is horizontal and vertical.
KS4 Do females lay eggs on artificial beans (such as Blu-Tack beans or glassplastic beads)? Would this be adaptive behaviour? How do females recognise an appropriate surface? Females always inspect a bean surface before laying an egg. Why? As females live only a short time, is the selection of suitable egg-laying sites likely to be under genetic control rather than learned?
Or: Woodlice seek dark places during daylight - what about seed beetles? Pupils can determine if they are positively or negatively phototactic (move towards or away from high light intensity) by putting one beetle at a time in the small circle and recording where they cross the perimeter of the larger circle (see diagram for shape). Do beetles walk towards or away from the light? Usually seed beetles are positively phototactic. What is the adaptive advantage of walking towards the area of highest light intensity?
ASA2 Females lay their eggs on a bean surface. What happens if the seed coating (testa) is removed? Do females still lay on it? Put mated females in a Petri dish with 10 clean adzuki beans and 10 with the testa removed.
Do females lay as frequently on each type?
Or Females may lay up to 100 eggs during their lifetime so get some egg-laden beans and ask students to count the number of eggs on each. The size of beans means that only two to four adults usually emerge, so why are there often more than four eggs on a bean? Do the offspring of the first female to lay benefit? How? Students should be able to devise an experiment to test this.
Michael Dockery is education officer at the Association for the Study of Animal Behaviour, Manchester Metropolitan University.
* To obtain eggladen beans and background information Email: m.dockery@ mmu.ac.uk