Life Cycle of Bean Beetles, Callosobruchus maculatus (Coleoptera: Chrysomelidae)
By Christopher W. Beck, Professor of Pedagogy, Emory University,
and Lawrence S. Blumer, Professor of Biology, Morehouse College
(excerpted from A Handbook on Bean Beetles, Callosobruchus maculatus. published online at www.beanbeetles.org)
Bean beetles, Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae), are agricultural pest insects of Africa and Asia that presently range throughout the tropical and subtropical world. This species also is known as the southern cowpea weevil. The larvae of this species feed and develop exclusively on the seed of legumes (Fabaceae) hence the name bean beetle. The adults do not require food or water and spend their limited lifespan (1–2 weeks) mating and laying eggs on beans.
Once inseminated, adult females will lay (oviposit) single fertilized eggs on the external surface of a dry bean seed (such as mung beans, black eye peas, or adzuki beans). Individual eggs (0.75 mm long) are oval or spindle shaped, clear, shiny, and firmly glued to the bean surface. The larva that hatches from the egg burrows from the egg through the seed coat and into the bean endosperm without moving outside the protection of the egg. Once the larva burrows into the bean, the remaining egg (shell) becomes opaque white or mottled as it fills with frass (feces) from the larva. The larva burrows and feeds on the bean endosperm and embryo, undergoes a series of molts, and burrows to a position just underneath the seed coat prior to pupation. Although the seed coat of the bean is still intact, a round 1- to 2-mm window will be apparent at the location where the beetle is pupating. Pupation is the complete metamorphosis of the larval maggot to a winged adult. The adult that results from pupation chews through the seed coat and emerges from the bean. The adults are fully mature 24 to 36 hours after emergence. Males seek females to inseminate and females store viable sperm in their spermatheca (a structure in the female reproductive tract for storing sperm). Neither male nor female adults require food or water during their short adult lifetime (10–14 days).
The elapsed time from newly laid eggs to the emergence of adult beetles varies between bean beetle strains and environmental conditions. Previous studies indicate that temperature and relative humidity are the most important variables influencing generation times (egg to adult) when beetles are raised on preferred host beans. Within a limited range, increasing temperature will decrease the generation time. In our laboratory, we have observed generation times as short as 3–4 weeks in a 30° C incubator (12:12 day:night light cycle) and ambient humidity (averaging 30% RH and ranging from 20%–40% RH). Cultures raised in a 25° C incubator (12:12 day:night light cycle) and ambient humidity (averaging 50% RH and ranging from 40%–60% RH) had generation times of 5–6 weeks. Cultures maintained on a laboratory bench at room temperature (22° C) with indirect outdoor window lighting and ambient humidity (averaging 50% RH and ranging from 40%–60% RH) had a generation time of a full 7 weeks. Reliably obtaining newly emerged adults for a specific date (a scheduled laboratory class meeting) requires that you grow cultures for a few months under your laboratory conditions so you can predict emergence times. In low humidity locations, or during the winter in North America, when RH is typically low, the emergence rates of beetles may be improved by increasing the RH of an incubator or culture container. Simply placing a tray of water in a temperature-controlled incubator may be all that is necessary to bring RH to the 40%–60% range and improve emergence success rates.
Generation time also depends on the host species of bean you choose to use. We have found longer generation times in adzuki beans compared to either mung or black-eyed peas. At 30° C, it takes 7 weeks for emergence from adzuki beans, compared to 3–4 weeks from mung beans. Additional information on the life history, culturing, and handling of bean beetles is available at www.beanbeetles.org at the Laboratory Methods link.