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Ladybird Beetles: An Ecofriendly Agent for Pest Management
By: Omkar
Introduction
The insects belonging to order Coleoptera and family Coccinellidae are commonly termed as Ladybird beetles, Ladybeetles or Ladybirds. Majority of the ladybird beetles are predaceous in nature and are employed as biocontrol agents. However, some members of family Coccinellidae belonging to subfamilies Epilachninae and Coccinelinae are phytophagous in nature and are harmful to crop plants as pests.
The term ladybird has been coined from the species, Coccinella septempunctata Linnaeus (Plate-1) and the term lady refers to the virgin Mary because of the resemblance of scarlet elytral colour with Her Cloak. They are the most recognized and loved insects. The seven black spots present on the two elytra are supposed to represent the seven joys and sorrows of the mother Mary. They are called the beetles because they belong to the order Coleoptera and have the characteristic spherical body. Their forewings are thick and leathery, and provide protection to the functional and membranous hindwings. The ladybirds are most fascinating and various products of human use are designed of the ladybird shape. They are quite attracting to the children across the world. They have always been associated with good luck charms.
Life Cycle
Ladybirds are the ancient and successful group of insects that evolved in the lower Permian period, about 280 million years ago. They are holometabolous insects because their life cycle starts from the egg which gives rise to larva and passes through four larval stages. The final larval stage pupates and metamorphoses into an adult (Plate-1). Dixon (2000) described ladybirds as Aphidophagous and Coccidophagous; former have fast development and the latter have slow development which is possibly adaptive in nature. But both the groups have similar number of larval instars, except one coccidophagous species that has three larval instars rather than the usual four instars. The aphidophagous ladybirds generally lay eggs in clusters while coccidophagous ones lay eggs singly.
Food of Ladybirds
Food greatly influences the growth, development, survival, reproduction and progeny fitness of ladybirds. The diet of predaceous ladybirds includes aphids, coccids, psyllids, diaspids, pentatomids, aleyrodids, and other insects and acarines. However, the non-predaceous ladybird species feed on fungi, pollens, honey dew, etc. Thus, the dietary breadth in predaceous ladybirds is an outcome of the seasonal abundance and the synchrony of their potential prey. Moreover, attributes like morphology, chemistry and behaviour of the prey, the efforts involved in reaching the prey, the host plant architecture, and the level of threats or challenges imposed by intraguild predators and other natural enemies also affect the dietary breadth of predaceous ladybirds. While some ladybirds are stenophagic and have a narrow prey range, the others are euryphagic and depend on wide prey range. The former are further termed as specialists as they feed on monospecific or few prey species. However, the latter are termed as generalists or polyphagous, based on their broad dietary habits.
Among the accepted foods of ladybirds, only certain foods support both development and reproduction, and are termed as essential foods. The rest are meant only for their survival and are known as alternative foods. Further, the essential food is classified into optimal, adequate and marginal, based on the differences in their nutritive values and the ways by which these nutrients are assimilated and utilized post prey consumption. Rejected foods of ladybirds are unpalatable due to their intensive/aposematic colourations and/or presence of certain allelochemicals. Consequently, they are rejected even after encounters.
Certain prey species are also harmful to ladybirds, causing their mortality, and are termed as toxic prey species. One of the toxins is cyanoglycoside sambunigrin, producing hydrocyanate after enzymatic splitting. Another potentially toxic compound is alkaloid sambucine. Thus, the aphid species that have been found to be toxic for some ladybirds either cause gradual poisoning, or an acute toxicity in ladybirds. Further, ladybirds accept some prey species which worsen their life-history parameters, although they are not toxic, and they are considered as ‘problematic prey’. Moreover, the prey species selected by ovipositing females as food for their larvae are termed as ‘nursery prey’, and they are the species of prey on which the larvae are likely to develop the best in terms of survival and growth.
Ladybirds have a tendency for prey specialization, which could be both diet- and habitat- related. The concept of prey specialization elucidates that ladybirds reared on suboptimal diets for few generations specialize and condition themselves for the suboptimal diet. These conditioned ladybirds perform better on a suboptimal diet than those on the optimal diets. Moreover, the switching of prey after few generations of rearing on either suboptimal or optimal diets causes deterioration in their performance and fitness. Further, the prey specialization in ladybirds has been argued as a function of their size, or the size and density of their prey. The body size of ladybirds provides an important trade-off determining their dietary breadth and the prey specialization. Specialist ladybird species prefer the prey species that closely matches their size, have higher capture efficiencies and can easily reproduce at lower prey densities for longer duration of time. However, the generalist species adopt a one-size-fits-all strategy, which results in their lower capture efficiencies, making them difficult to sustain at lower prey density. Moreover, unlike the generalist species, whose adults frequently move between patchy prey habitats, the specialist species stay in the prey patches and lead a more sedentary life due to their greater tolerance of lower prey densities. They even start to reproduce earlier than the generalist species and also reproduce in the later stages of declining prey patch. Thus, their sedentary, stubborn and non-dispersing behaviour makes them the better biocontrol agents.
The analysis of how a ladybird responds to varying pest populations and how it affects pest management can be understood in terms of the functional response. It is the predator’s feeding response against the increasing prey density, and can be analytically explained by Holling’s type I (linear), type II (curvilinear), and type III (sigmoidal) responses. Ladybirds usually exhibit a type II functional response, where there is an initial increase in the rate of prey consumption with increase in prey densities up to a certain level and this rate decreases with a further increase in prey density. This happens due to satiation, as there is a threshold of prey consumption and the prey density dependent curve reaches an asymptote.
Numerous factors affect the functional response outcomes of ladybird predators. These include, generation time ratio of predator and prey, prey preference, prey switching, size disparity between prey and predator, prey density, predatory stage, walking speed of predator and prey, gender, intrinsic rate of increase of prey and predator, consumption rate, prey patchiness, predator patch allocation time, host plant, abiotic factors, and intra- and interspecific predator competition. Moreover, the functional responses of ladybird predators are interlinked with their numerical responses. The numerical response of a predator is its tendency to increase its number with increasing prey density; and can be both aggregative and reproductive numerical responses. In response to increasing prey density, predaceous ladybirds show aggregative numerical response by increasing the cumulative prey consumption; however, the rate of prey consumption decreases curvilinearly due to mutual interference. The reproductive numerical response is a consequence of the functional response in predaceous ladybirds; because the females lay high number of eggs at higher prey densities, which is a direct implication of their functional response.
Effects of abiotic factors on life attributes of Ladybirds
Temperature is the most crucial abiotic factor affecting ecological, functional, and behavioural attributes of predaceous ladybirds. It sets the limits of biological activity in form of low and high temperature thresholds. The developmental rate is almost zero at lower development threshold, which increases with temperature, reaches a peak value, and then decreases rapidly as the high thermal threshold is achieved. Not much lower development threshold variation occurs in ladybirds with similar dietary habits and this is also a reason for their successful establishment in different habitats and countries. At the optimum temperature, around 25–30 °C in most aphidophagous and coccidophagous ladybirds, the peak is attained at the youngest age of the ladybird demography. Thereafter, with further rises in temperature this peak is delayed and shortened. However, in acarophagous ladybirds the optimum temperature is 30–35 °C, resulting in high values for the demographic parameters, like the net reproductive rate and the intrinsic rate of increase in population.
Not only temperature, the light also affects the life attributes of ladybirds. Various variables of light, like intensity, quality (wavelength), and duration of exposure (photoperiod) significantly affect the development, reproduction and progeny fitness. Ladybirds have a wide range of tolerance limits to these variables. They are primarily diurnal insects and depend on visual cues and presence/absence of light to undergo various essential activities, like mating, moulting, and pupation. Most ladybirds are highly sensitive to light, particularly its photoperiod and wavelength. Short day-lengths with intensities of 1500 lux are beneficial for the reproductive activities. The likelihood of female ladybirds accepting the males increases in the dark because females are unable to evaluate visual criteria to select male and thereby mating rejection displays are also minimized. However, prolonged light days could have a negative effect on the physiology of ladybirds.
There exists a photoperiod-dependent bimodal or two-peak pattern in the development of certain ladybirds, where the first peak represents fast developing and the second shows slow-developing individuals in the same cohort of eggs. The slow-developing individuals are generally more in numbers in short day-lengths; however, long day-lengths promote fast developing adults having heavier body masses and more capable of producing quantitative progeny. White light is more suitable for essential activities compared with its red or blue components of the visible spectrum.
Predation by Ladybirds
A guild is formed by the association of predators that share a common food resource. However, when the ladybirds exploit a common food resource (extraguild prey) within a guild, they often attack each other. In this struggle, one becomes dominant (intraguild predator) and overpowers the other (intraguild prey). This exploitative competition of food resource is more advantageous to small-sized ladybirds as they have lesser food demands. As the density of extraguild prey decreases or the density of intraguild prey increases, the frequency of intraguild predation increases. On the contrary, an increase in extraguild prey density lowers the possibilities of intraguild predation. The relative size and stage, mobility of species, aggressive strategies, mandibular structure, degree of feeding and habitat specificity, defense strategies, and density of extraguild prey determine the outcome of intraguild predation.
Majority of the ladybirds attack, prey upon, and displace other members of their family in a limited food resource struggle. Invasion and establishment of aggressive species following displacement of indigenous ones may be an outcome of such interactions. The Harlequin ladybird, Harmonia axyridis (Pallas) is an invasive species that has a competitive advantage over the indigenous species, such as Coccinella septempunctata, Coleomegilla maculata DeGeer, Hippodamia variegata Goeze, and Adalia bipunctata L., due to its vast prey range, and higher predation and foraging potential. This invasive species frequently indulges in either interference competition or the apparent competition. Thus, it either interferes with other competitors or may compel the inferior indigenous species to become specialist predators of less preferred prey in nature.
Ladybirds also struggle with other group of insects outside the family for the common food resource. They usually co-occur with chrysopid (Neuroptera: Chrysopidae) larvae and share the limited food resources. However, intraguild interactions between them are asymmetrical which could have positive, negative, or neutral impacts on pest biocontrol. Both chrysopids and ladybirds use chemical defenses. The ladybird larvae, pupae, and adults all use chemical substances containing volatile hydrocarbons/alkaloids to deter predators.
Ladybirds and Biocontrol of Pests
The term biological control (=biocontrol) was introduced by Smith (1919) for the “topdown” action of natural enemies/biocontrol agents (viz., predators, parasitoids and pathogens) in maintaining the pest population density at a lower level than what may have occurred in their absence. Although several stories exist regarding the successful utilization of ladybirds as biocontrol agents, their use in biocontrol came into existence when the vedalia beetle, Rodolia cardinalis (Mulsant) was selected to control the population of scale insect, Icerya purchasi on citrus in California (USA) in the year 1889. Thereafter, numerous ladybird species were successfully used in the biocontrol of aphids, scale insects and mealybugs. The impact of ladybirds in terms of successful pest biocontrol is largely dependent on their voracity, prey specificity, intrinsic rate of increase, and the mean generation time ratio between prey and predator. Interestingly, the size of ladybirds attacking similar kind of prey does affect the biocontrol with the large-sized ladybirds being the better biocontrol agents. However, if the prey type is different, the small-sized ladybirds with specialization on that particular prey type are more promising biocontrol agents than the large but less specialized ladybirds.
A need for aphid biocontrol led to the introduction of 179 aphidophagous ladybird species in North America since 1900, but only 18 have successfully established. A few aphidophagous ladybirds took many years to establish. However, many established after accidental introduction, including Coccinella septempunctata, Harmonia axyridis, and Propylea quatuordecimpunctata (L.). The introduction and invasion of certain aphidophagous ladybird species has been implicated in the decline of some native species in the USA and elsewhere. A flightless form of Harmonia axyridis was also produced using a chemical mutagen followed by selective breeding. The Inundative releases of such flightless adults against aphids on glasshouse cucumbers were highly successful. However, in huge agriculture fields they were not successful because of their impaired foraging on account of being flightless. However, these adults had lower reproductive fitness and had fewer offspring despite ovipositing for a longer period.
Aphidophagous ladybirds are generally not considered as better biocontrol agents largely due to significant differences in their intrinsic rates of increase and mean generation time ratios, although the relative development rates of aphidophagous ladybirds are lower than those of aphids. However, aphid biocontrol could be benefitted if the prey is targeted early in the season, i.e. prey suppression initiation could be done when the aphid colony is young. The coccidophagous ladybirds are also successful biocontrol agents of both coccids and diaspids. Chilocorus nigritus (Fab.) is a highly successful and effective generalist predator of numerous species of Diaspididae with equal effects on some species of Coccidae and Asterolecaniidae. The Indian ladybird, Cryptolaemus montrouzieri Mulsant, is also a generalist predator of various scales and mealybugs and has been commercially exploited in both classical and augmentative biocontrol programs.
Moreover, certain specialist ladybirds belonging to genus Stethorus are potential biocontrol agents of tetranychid mites, especially at their high density. Similarly, the ladybird,Clitostethus oculatus (Blatchley) is credited for the biocontrol of whitefly, Aleurodicus dispersus (Russell) in Hawaii and India. While the specialists are better biocontrol agents than the generalists because of their selective feeding and persistence in the target prey habitats, the invasion of generalists in their resource space is an issue of serious concern as they become intraguild prey or they are forced to emigrate.
Conclusions
The predaceous ladybirds have a promising future in the biocontrol of insect pests of agricultural importance. While majority of ladybirds are generalists, some are specialists. There are lots of arguments on prey specialization in predaceous ladybirds. However, the resource partitioning and consistent exposure of a single prey type might have evolved the prey specialization in predaceous ladybirds. Moreover, the prey specialization in ladybirds is generally considered as a function of body size, prey size and prey density.
Mating and reproductive studies in ladybirds have provided knowledge on the optimal conditions pertaining to number and quality of mates to produce better progeny both in terms of quantity and quality. Similarly, the information pertaining to age, aging trajectories and age differences between mates not only increases the level of knowledge on ladybird physiology in general but may also help in mass multiplication of ladybirds by allowing mating with optimal age individuals. The intraguild interactions amongst the ladybird species distress the coexistence of ladybirds and displace many indigenous ladybird fauna, the biological invasions of certain dominant species could result in complete disappearance of numerous native ladybird species. Amongst the abiotic factors, temperature has a major impact on the ladybird’s life attributes. Moreover, the optimization of abiotic conditions, like temperature and light is a prerequisite for the augmentative rearing of ladybirds. There is an utmost need to understand the role of ladybirds in pest management through comprehensive ecological and ethological studies supported with laboratory experimentation, and glasshouse and field studies. The use of biocontrol agents in the suppression of pest populations minimizes the use of pesticides in agriculture. Biocontrol is an ecofriendly technique which is cost-effective in the long run and self-perpetuating, and would lead to sustainable agriculture.
Ladybird Research Laboratory, Department of Zoology,
University of Lucknow, Lucknow-226 007 (India) [email protected]
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