Oviposition behaviour and host-seed selection in bruchid beetles (genus Callosobruchus): A review

Introduction

Global production of grain legumes exceeded 1.3 billion metric tonnes in 2023, yet an estimated 20% to 40% of this harvest is lost during storage in tropical regions a figure that translates to hundreds of millions of US dollars in economic damage each year [1] . The genus Callosobruchus, commonly known as bruchid beetles or pulse beetles, is the most important group of insects responsible for these losses. And the starting point of every bruchid infestation is a single event: a female beetle choosing where to lay her eggs. Oviposition the process by which a gravid female selects a seed, assesses its suitability, and deposits one or more eggs on its surface determines which seeds will be colonised, how quickly a population builds, and how much grain is ultimately destroyed [2] . Unlike many other storedproduct pests that feed externally as adults, bruchid larvae bore into the seed immediately after hatching and complete their entire larval development inside the cotyledon, making postoviposition control extremely difficult [3] . For that reason, disrupting the oviposition decision is widely regarded as the most promising stage at which to intervene. Females do not lay eggs randomly. Research conducted over the past three decades has established that oviposition site selection in Callosobruchus is a multi-step sensory process that involves long-range olfactory detection of host volatiles, followed by short-range tactile and chemosensory evaluation of the seed surface [4] . Seed size, seed coat texture, colour, moisture content, chemical composition, and the presence of previously deposited conspecific eggs all influence whether a female accepts or rejects a given seed [5] . But the relative importance of these factors varies across species and across ecological contexts, creating a complex picture that has resisted easy generalisation. Five species within the genus Callosobruchus account for most of the damage to stored legumes worldwide: C. maculatus (the cowpea weevil), C. chinensis (the azuki bean weevil), C. analis, C. rhodesianus, and C. subinnotatus (the Bambara groundnut bruchid) [6] . Each of these species shows a somewhat different hierarchy of oviposition preferences, reflecting adaptations to their primary host legumes and the ecological conditions of their centres of origin. C. maculatus, for example, is strongly attracted to smooth, pale-coated seeds and responds vigorously to cowpea volatiles, while C. chinensis appears to weight seed coat texture more heavily than volatile signals [7] . Despite this growing body of evidence, no recent review has attempted to synthesise findings on oviposition behaviour and host-seed selection across multiple Callosobruchus species in a single comparative framework. Earlier reviews focused primarily on C. maculatus alone or treated oviposition as one component of broader biology reviews rather than as a standalone behavioural question [8] . This gap matters because understanding the shared and species-specific elements of oviposition behaviour is essential for designing control strategies that work against more than one species particularly in mixedspecies storage systems common across Sub-Saharan Africa and South and Southeast Asia. This review addresses that gap by synthesising peer-reviewed evidence from 2010 to 2025 on the oviposition behaviour and host-seed selection mechanisms of the five major Callosobruchus species. It examines: (a) the sequential hierarchy of sensory cues used during oviposition site selection, (b) the influence of seed physical and chemical traits on egg-laying preference, (c) interspecific variation in selection strategies, and (d) environmental modifiers such as temperature, humidity, and seasonal timing. The overall aim is to provide a consolidated reference that can inform targeted management strategies to disrupt bruchid oviposition at the earliest possible stage [9] .

Material and Methods

Material

A structured literature search was carried out to locate peerreviewed articles, book chapters, and technical reports on the oviposition behaviour and host-seed selection mechanisms of bruchid beetles within the genus Callosobruchus. Five electronic databases were searched: Scopus, Web of Science, PubMed, CAB Abstracts, and Google Scholar. The search covered the period from January 2010 to October 2025. Search strings were constructed using Boolean operators: ("Callosobruchus" OR "bruchid" OR "pulse beetle" OR "cowpea weevil") AND ("oviposition" OR "egg laying" OR "egg deposition" OR "host preference" OR "host selection" OR "seed selection" OR "oviposition cue" OR "seed coat" OR "volatile"). Additional targeted searches used species-specific terms (e.g., "C. maculatus oviposition", "C. chinensis host selection"). Reference lists of key review articles and relevant book chapters were manually checked for additional sources not captured by the database searches.

Methods

Articles were eligible for inclusion if they: (a) were published in English between 2010 and 2025, (b) reported original experimental data on any aspect of oviposition behaviour or host-seed selection in at least one Callosobruchus species, (c) identified the beetle species and the host legume(s) tested, and (d) appeared in a peer-reviewed journal or a recognised international technical publication series. Exclusion criteria removed conference abstracts without full data, opinion or commentary pieces, and articles that reported only mortality or feeding damage without any oviposition data. The initial search yielded 412 records. After de-duplication (94 removed), 318 titles and abstracts were screened, and 127 fulltext articles were assessed for eligibility. A total of 56 articles met all inclusion criteria and were retained for qualitative synthesis. From each article, the following data were extracted: Callosobruchus species investigated, host legume species offered, experimental design (choice or no-choice), seed traits measured (size, coat texture, colour, volatile profile, hardness, moisture), oviposition metrics (total eggs, eggs per seed, oviposition rate), environmental conditions (temperature, humidity, photoperiod), and key behavioural findings. Where multiple experiments were reported within a single publication, each was treated as a separate observation. Data were synthesised narratively and organised around the sensory hierarchy model of oviposition site selection. Quantitative data were tabulated to enable cross-species comparisons of trait preference rankings and oviposition intensity [10] .

Seasonal variations and temporal patterns

Environmental temperature and relative humidity exert a strong modulating effect on both oviposition rate and clutch size in Callosobruchus species. Across 23 of the reviewed articles that reported temperature-dependent oviposition data, peak egglaying activity for C. maculatus was recorded between 28°C and 32°C, with a sharp decline below 22°C and above 36°C. Relative humidity optima fell between 65% and 75% for most species, though C. rhodesianus tolerated drier conditions more readily than the other four species [11] . Seasonal timing also affects oviposition behaviour through its influence on host seed availability and quality. In tropical regions with distinct wet and dry seasons, freshly harvested legume seeds enter storage at the onset of the dry season, coinciding with rising ambient temperatures that favour rapid bruchid reproduction. This convergence of fresh seed availability and optimal thermal conditions creates a window of peak vulnerability during the first two to four months of storage. In temperate and subtropical zones, where storage temperatures drop below the developmental threshold during winter, oviposition activity is restricted to warmer months, and population growth follows a more seasonal trajectory [13] .

Results

Figure 1 presents the pooled distribution of oviposition events across six host legume species when offered simultaneously to C. maculatus females in choice-test experiments. Cowpea (V. unguiculata) received the largest share of eggs (34.7%), followed by mung bean (22.1%) and chickpea (17.3%). Lentil, black-eyed pea, and adzuki bean attracted progressively fewer eggs, suggesting a descending gradient of host acceptability linked to seed coat characteristics and volatile emission profiles [3] . Table 1 summarises the multi-variable matrix of oviposition-influencing seed traits and their relative importance rankings across the five Callosobruchus species examined, drawn from 56 reviewed publications. The highest-ranked trait for C. maculatus was chemical volatile profile (mean rank score 9.2 out of 10), followed by prior egg load (8.7) and seed size (8.3). C. chinensis, in contrast, ranked seed coat texture highest (8.4), with chemical volatiles ranked second (7.8) [5] . This divergence in sensory hierarchy between species has important implications for developing species-targeted oviposition deterrents. andC. subinnotatus stood apart with its unusually strong colour-based discrimination [17] . Taken as a whole, the reviewed evidence points to a conserved but species-tunable decision hierarchy in which long-range olfactory cues drive initial host location, short-range tactile and chemical cues govern seed acceptance or rejection, and egg-load assessment acts as a final filter to avoid intraspecific competition. The consistency of this hierarchy across species suggests a shared ancestral programme, while the interspecific variation in trait weighting reflects adaptive specialisation to different host legumes and competitive environments [18] .

Biodiversity assessment

The genus Callosobruchus contains over 20 described species, but only five account for the overwhelming majority of storage damage: C. maculatus, C. chinensis, C. analis, C. rhodesianus, and C. subinnotatus. In West Africa, C. maculatus is the dominant species on cowpea, often occurring in nearmonospecific populations. But across East and Southern Africa, and especially in South and Southeast Asia, two or more Callosobruchus species frequently co-occur in the same storage system, creating mixed-species assemblages with overlapping but not identical host preferences [14] .

When two species share a storage environment, competitive displacement can shift oviposition patterns. C. maculatus and C. chinensis, the two most widespread species globally, differ in their egg-laying strategies when in competition: C. maculatus tends to deposit eggs on the seed surface, while C. chinensis females often cement eggs more firmly and cover them with a protective secretion, giving their larvae a slight developmental advantage on heavily contested seeds [15] . In three-species systems documented in parts of Kenya, Tanzania, and Ethiopia niche partitioning by seed type and seed surface micro-habitat has been observed, with each species concentrating oviposition on the seed traits that best match its sensory preferences [16] . This diversity of strategies underscores the need for management approaches that account for species-specific oviposition behaviour rather than assuming a single control measure will equally suppress all members of the genus.

Discussion

The main findings of this review can be condensed to three core observations. First, oviposition site selection in Callosobruchus operates through a hierarchical sequence of sensory assessments that is conserved across all five major species. Second, the relative weighting of specific seed traits within that hierarchy varies meaningfully between species, with C. maculatus relying most heavily on volatile cues and prior egg load, while C. chinensis prioritises seed coat texture and C. subinnotatus shows an unusual dependence on colour-based discrimination. Third, environmental variables particularly temperature and humidity modulate both the rate and the selectivity of oviposition behaviour. These findings align broadly with the conceptual model proposed by earlier researchers who described bruchid oviposition as a multi-stage acceptance/rejection sequence governed by progressively finer-scale sensory inputs [2] . The volatile-driven host location stage confirmed by the present review is consistent with electroantennographic and olfactometer work that identified specific terpenoids and shortchain aldehydes as key attractants for C. maculatus females [5] .

Similarly, the importance of seed coat texture for C. chinensis is supported by scanning electron microscopy work showing that this species deposits eggs preferentially in micro-depressions on the seed surface, a behaviour less prominent in C. maculatus [7] . However, there are points of disagreement in the literature. Several researchers reported that seed size was the single most important predictor of C. maculatus oviposition, ranking above volatiles [6] . The present review's pooled analysis, drawing on a larger number of publications, placed seed size third behind volatiles and prior egg load. This discrepancy may reflect differences in experimental design particularly whether seeds were presented in choice or no-choice settings and in the range of seed sizes tested. In no-choice experiments, where no alternative is available, the influence of volatile cues is effectively removed from the decision, which can inflate the apparent importance of physical traits like size and colour. The interspecific variation in trait weighting raises a practical challenge. Most storage management recommendations from hermetic bags to botanical repellents have been developed and tested primarily against C. maculatus. But if C. chinensis processes seed traits differently, a surface modification strategy that deters C. maculatus may not be equally effective against C. chinensis [15] . This is particularly relevant in South and Southeast Asia, where C. chinensis is the dominant storage pest, and in East Africa, where mixed-species assemblages are common. Future management research would benefit from multi-species testing protocols that evaluate control measures against at least two Callosobruchus species simultaneously, rather than extrapolating from single-species trials [19] .

Conclusion

The main contribution of this review is the consolidation of scattered findings on oviposition behaviour and host-seed selection into a unified, species-comparative framework for the five major Callosobruchus pest species. By synthesising 56 peerreviewed publications from 2010 to 2025, this work provides a comparative reference that was previously unavailable in the literature.

The supporting evidence can be summarised along three axes.

On the sensory hierarchy axis, all five species employ a conserved sequence of olfactory, tactile, and chemosensory assessments when selecting oviposition sites, but the ranking of cues within that sequence is species-specific. On the host preference axis, cowpea remains the most universally preferred host for C. maculatus, but C. chinensis and C. subinnotatus show strong preferences for mung bean and Bambara groundnut, respectively, driven by different trait weightings. On the environmental modulation axis, temperature and humidity act as rate multipliers on oviposition intensity, with peak activity consistently recorded between 28°C and 32°C and between 65% and 75% relative humidity. The significance of these findings for the field is twofold. First, they confirm that oviposition behaviour is not a monolithic phenomenon within the genus species-level differences in sensory priorities are large enough to affect the expected efficacy of control measures designed around a single species model. Second, the hierarchical decision structure itself suggests multiple intervention points: volatile-based traps or repellents can target the long-range location stage, seed coat surface modifications can disrupt the short-range assessment stage, and varietal resistance breeding can make seeds intrinsically less acceptable for egg deposition.

Several open questions remain. The chemical identity of the full volatile blend driving oviposition attraction in species other than C. maculatus is still poorly characterised. The neurobiological mechanisms linking antennal reception of host cues to the motor programme of oviposition are understood only in outline. The genetic architecture of oviposition preference whether single genes of large effect or many genes of small effect underlie hostseed discrimination has barely been explored. And the evolutionary dynamics of oviposition preference shift in response to novel host legumes or changing storage environments remain largely theoretical. Addressing these gaps would strengthen the foundation for behavioural manipulation strategies aimed at suppressing bruchid oviposition at its source. The most promising near-term opportunity lies in integrating volatile-based disruption with seed surface modification and genetic resistance breeding into a multi-layered defence that targets different stages of the oviposition decision hierarchy and remains effective against mixed-species bruchid assemblages in real-world grain storage systems.

Acknowledgement

AcknowledgementsThe authors acknowledge and express gratitude to the concerned authorities for their support and cooperation during this research.

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