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Identification & Distribution

The typical form (f. typica, see first picture below) of the adult two-spot ladybird has red elytra with a large black spot in the middle of each. The prothorax is black with a large white patch on each side and a small white patch on top. The black head may also have two small white spots. The 4-spot melanic form of Adalia bipunctata (form quadripustulata, see second picture below) is black with four red spots. The 6-spot melanic form (form sexpustulata, see third picture below) is black with sex red spots. Melanic forms of the 2-spot ladybird also have reduced white areas. The elytra are smooth before the tip (cf. the 10-spot ladybird, Adalia decempunctata in which the elytra have a transverse groove before the tip). The underside and the legs of Adalia bipunctata are black (cf. Adalia decempunctata which has brown legs).

Second image copyright Olei under a Creative Commons Attribution-Share Alike 2.5 Generic license.
Third image copyright Pavel Kirillov under a Creative Commons Attribution-Share Alike 2.0 Generic license.

Fourth instar larvae of Adalia bipunctata (see fourth picture above) have conspicuous orange to whitish tubercles on the first abdominal segment, and a pair of orange dorsal tubercles on abdominal segment four. The yellow associated with the dorsal tubercles on segment four may be restricted to the area between the tubercles. There are further orange to whitish spots down the dorsal midline and laterally on abdominal segment four. The outer tubercles on abdominal segments 5-8 are dark (cf. Adalia decempunctata which has those tubercles pale). Third instar larvae are grey-black with orange to whitish areas on the lateral apical margin of the pronotum and on the dorsolateral tubercles of the first abdominal segment; first and second instar larvae are black to grey with no coloured areas.

The two-spot ladybird is a widespread and common species which occupies a variety of habitats from mature lime or sycamore trees, to deciduous or coniferous woodland, orchards and crops. Adalia bipunctata is found throughout Europe, Asia (excluding the Indian subcontinent and Southeast Asia) and North America.

 

Biological Control of Aphids

Augmentative release of two-spot ladybird in greenhouses

The conventional wisdom is that ladybirds are incapable of regulating aphid populations under natural field conditions for reasons relating to their voracity, search efficiency, predation capacity, and reproductive rate. But it has been argued that the ability to regulate aphid populations is not essential if repeated (inundative) releases of ladybirds into a greenhouse are made. Such releases act as a 'biological pesticide' in aphid 'hotspots', and can be used to delay or prevent aphid outbreaks. Larvae of the 2-spot ladybird are commercially available (for example from Koppert biological systems ) for release for biological control in greenhouses.

Unfortunately, Adalia bipunctata seems to have a rather poor reputation for efficacy in biological control of aphids in greenhouses (see Omkar, 2005). For example, Hamalainen et al. (1977) attempted to use larvae of the coccinellids Coccinella septempunctata and Adalia bipunctata for controlling the rose aphid (Macrosiphum rosae) in small greenhouses, but this was unsuccessful because the predators did not remain on the rose plants.

Aphid control in the field

Despite the constraints to which Adalia bipunctata are subject, experimental work carried out on augmentative release of twin-spot ladybirds in the field in Switzerland gave promising results in control trials of rosy apple aphids (Dysaphis plantaginea) (Wyss et al., 1999). In a first experiment, eggs and larvae were released in spring in apple trees infested with five aphids at four different predator-prey ratios. In a second experiment, eggs and larvae were released at a predator-prey ratio of 5:1 on branches of apple trees naturally infested with aphids. In both experiments, the interaction with ants was taken into account and the releases were done at two different times in spring. The results showed that release of larvae significantly reduced the build-up of colonies of Dysaphis plantaginea. Significant reductions in aphid numbers were recorded at the two highest predator-prey ratios. Larvae were efficient just before flowering of apple trees at a time when growers normally have to spray their trees. Where ants were present the larvae of Adalia bipunctata were significantly less efficient. The effects of eggs of Adalia bipunctata were less reliable than larvae, and at the first date of release they did not hatch, probably as a consequence of bad weather conditions.

In a further set of experiments, Kehrli & Wyss (2001) compared the impact of releases of indigenous predators with that of insecticides to control the sexual forms of the Dysaphis aphids. Eggs and larvae of the two-spot ladybird beetle (Adalia bipunctata were released on 4-year old apple trees in various numbers at five different dates in autumn when sexuales of the aphids were present. Additionally, pyrethrum was sprayed at the same five dates to compare the effectiveness of these releases with that of a commonly applied insecticide. Releases of larvae before mid-October significantly reduced the deposition of overwintering eggs by aphids and consequently reduced the number of hatched fundatrices in spring, 1999. But applications of pyrethrum before mid-October were more effective than augmentative releases of larvae of Adalia bipunctata. The weather conditions in autumn seemed to have an impact on the autumn migration of the winged aphids back to their primary host. The authors felt prevention of egg deposition of aphids in autumn was a promising control strategy and deserved further exploration for practical use.

Although the jury is still out on whether two-spot ladybird can be used to suppress aphid populations acting alone, it doubtless makes an important contribution to the biological control of aphids, acting together with other mortality agents, in gardens, orchards and the natural environment. The two-spot ladybird can be found in any vegetation type, but does seem to show a preference for shrubs and trees over herbs.

Below are a few examples of where we have found two-spot ladybird predating aphids in southern England.

Predation of bird cherry-oat aphid

The two-spot ladybird is a common sight on bird cherry trees in spring predating the sometimes abundant bird cherry-oat aphid (Rhopalosiphum padi, see picture below) which has bird cherry (Prunus padus) as its primary host.

It is generally assumed that the wax, with which these aphids are covered, provides a degree of protection against predators - but this protection is only partial. Many aphids are consumed just after moulting, before the wax is secreted.

Predation of black cherry aphid

The black cherry aphid (Myzus cerasi) causes the young leaves of sweet cherry to curl, forming conspicuous leaf nests and damaging the tree. Very often the colonies are ant-attended, and the ants provide protection against coccinellids. The colony below was, however, not ant-attended and was being predated by fourth instar larvae of Adalia punctata, and by young coccinellid larvae, probably also two-spot ladybirds (see pictures below).

Predation of willow aphids

Willow aphids (Cavariella species.) also provide a ready meal for two-spot ladybirds (see picture below). The only defense that willow aphids seem to have is their cryptic coloration, blending in with the silvery-green willow leaves.

Predation of elder aphids

The picture below shows an adult two-spot ladybird predating elder aphids (Aphis sambuci). Compared with the willow aphids above, the elder aphid is rather well protected. It is frequently attended by ants which protect the aphid, and it also sequesters toxic compounds from its foodplants (Nedved & Salvucci, 2008).

Predation of black bean aphid

The picture below shows an adult two-spot ladybird, along with a larval coccinellid, predating black bean aphids (Aphis fabae) on cleavers (Galium aparine). As far as we know, Aphis fabae does not sequester any toxic compounds from its foodplant, but nevertheless feeding experiments suggest it is a 'low quality' food for coccinellid larvae (Hinkelman & Tenhumberg, 2013).

Aphis fabae may be protected by attending Lasius ants.

Others

In Ontario Hagley & Allen (1990) found that the most abundant predators on apple tree foliage were the reduviid Acholla multispinosa, the mirid Campylomma verbasci), the coccinellids Coccinella septempunctata and Adalia bipunctata, plus various coccinellid and chrysopid larvae. An efficacy index developed to assess the effectiveness of foliage-inhabiting predators indicated that those with the greatest potential were Chrysopa larvae and adult Coccinella septempunctata in 1987, and adult Coccinella septempunctata and Campylomma verbasci in 1988. The earwig, Forficula auricularia L., showed potential as a predator of Aphis pomi in 1989.

Blazhievskaya (1980) reports how the aphid Lachnus roboris overwinters in the egg stage - eggs are laid from late September onwards. The eggs darken after 5-6 days, and they are subject to attack by predators, particularly the coccinellid Adalia bipunctata. Out of 6281 eggs examined in autumn, 25% were destroyed by birds and coccinellids and 19% by fungal infections, while 56% appeared normal. However, only 30% hatched. It was concluded that winter mortality was a primary limiting factor.

 

Biology & Ecology

There is a substantial body of research that has been carried out the biology and ecology of the two spot ladybird, partly in recognition of its potential for biological control of aphid pests. Here we briefly review a few topics.

Searching efficiency and behaviour

Dixon (1970) examined the factors limiting the effectiveness of the two-spot ladybird as a predator of the sycamore aphid, Drepanosiphum platanoidis. Laboratory observations on the searching behaviour and efficiency in capturing aphid prey of the first instar larvae of Adalia bipunctata led to the suggestion that they would be unable to survive in the field unless the population density of young aphids on sycamore leaves exceeded two per 100 cm2. This minimum density value for survival of coccinellid larvae was confirmed by field observations from 1960 to 1968 on two sycamore trees where the intensity and success of predation by the coccinellid was related to the number of young aphids present. The proportion of prey taken by the coccinellids in the field did not increase as the prey population density rose, indicating that on their own they would be unable to regulate the prey population size.

Wratten (1973) studied the effectiveness of the two-spot ladybird as a predator of the lime aphid (Eucallipterus tiliae). First-instar larvae were found to require for survival an aphid density 4.4 times greater than that required by fourth-instar larvae. Ovipositing coccinellids laid eggs only when there were sufficient aphids for the survival of first-instar larvae, and did not lay proportionately more eggs at higher aphid densities. Calculations of the numbers of aphids removed by coccinellids indicated that the predator did not influence the timing or intensity of major peaks in aphid numbers, as larvae soon became satiated. However, the large numbers of larvae present after a peak inflicted a heavy mortality and accentuated the population decline, suppressing the production of aphid oviparae.

Hemptinne et al. (1996) found that adult males of the two-spot ladybird beetle did not show a functional response to increase in aphid abundance and consumed markedly fewer aphids than do the females. At high densities of prey, females spent more time in area-restricted search than when prey was scarce. Males were always less active than females and they did not respond to an increase in prey abundance by a change in searching behaviour. After a brief encounter with a female, a male showed area-restricted searching behaviour. This behaviour occurred in response to encountering a female's elytra and in particular to a sex pheromone present on or in the elytra. Males needed to encounter a female in order to respond to her presence, which indicated the pheromone is a contact pheromone. The searching behaviour of males appeared to be mainly directed towards locating females; that of females towards locating aphids.

Timms et al. (2008) compared the impact of predation by the generalist predator Adalia bipunctata and the pine specialist predator Aphidecta obliterata. In Petri dish trials, the larval stages of Aphidecta obliterata and all stages of Adalia bipunctata appeared to exhibit a Type II response to prey density, although Aphidecta obliterata adults showed a more linear response to prey density. There was no significant difference between the functional responses of the 3rd instars of the two coccinellid species, but there was a significant difference between the responses shown by the first instars, with Aphidecta obliterata larvae consuming more than those of Adalia bipunctata, especially at low densities, suggesting that the two species respond differently to an increase in prey density. Perhaps more importantly, the host plant affected the rate of consumption by adult Aphidecta obliterata as adults on Sitka spruce sections consumed significantly higher numbers of aphids than their counterparts on Norway spruce. The distributions of the two coccinellid species in the olfactometer were significantly affected by the presence of host plant material. Aphidecta obliterata adults were found in significantly higher numbers in the Sitka spruce chambers than the control chambers (those without plant material). Adalia bipunctata adults were found in significantly lower numbers in the Norway spruce chamber than the control chamber. The authors concluded that although Adalia bipunctata has a higher level of voracity than Aphidecta obliterata, the latter is more adapted to the spruce environment and the boom and bust population dynamics of its prey Elatobium abietinum.

Intra & Interspecific competition

Ladybirds commonly engage in cannibalistic behaviour. Egg cannibalism by first instars is considered advantageous to the cannibal, because it not only results in direct metabolic gain, but also a reduction in potential competitors. Roy et al. (2008) quantified the effect of cannibalism on the development rate and survival of Adalia bipunctata larvae through development to the adult stage. Larvae that had consumed a conspecific egg after hatching reached the adult stage 1.65 days earlier than those larvae that had not. Larval and pupal mortality was lower for cannibals compared to non-cannibals; only 46% of non-cannibalistic individuals reached the adult stage whereas 81% of cannibals pupated successfully. Egg cannibalism is undoubtedly advantageous to twin-spot ladybird larvae both in terms of faster development and increased survival.

Burgio et al. (2002) investigated interspecific competition between the exotic coccinellid Harmonia axyridis (see picture below) and the native species Adalia bipunctata in the laboratory by determining the consumption of interspecific eggs by fourth instar larvae and adult females. Intra-guild predation by Harmonia axyridis of eggs of the native species, Adalia bipunctata, was lower than the egg cannibalism by Harmonia axyridis, for both adults and larvae. Adalia bipunctata adult egg cannibalism was also significantly higher than intra-guild predation. For both cannibalism and intra-guild predation there was an inverse correlation was observed between egg consumption and aphid density. It was concluded that it was unlikely Harmonia axyridis will to have a negative impact on native species by intraguild predation of eggs.

Ware et al. (2002) reached the opposite conclusion in their study. They suggested that Harmonia axyridis had negatively impinged on indigenous species in its introduced range, and its recent arrival in Britain posed a threat for members of native aphidophagous guilds. They investigated the effects of different diets, designed to mimic possible conditions in the wild, on the survival, development, and adult size of Harmonia axyridis and Adalia bipunctata. Results demonstrated a skew in the consequences of intraguild predation between the two species: the supplementation of a limited aphid diet with non-conspecific eggs leads to a significant advantage for Harmonia axyridis in respect of all parameters assessed, but gives no benefit to Adalia bipunctata. Hence they concluded that intraguild predation of Adalia bipunctata by Harmonia axyridis will contribute to the spread and increase of Harmonia axyridis in Britain.

Olfactory responses

Aphidophagous ladybirds are reluctant to oviposit in patches of prey where larvae of the same species are present. This is adaptive, as larval cannibalism is a major threat to egg survival. Ladybirds avoid laying eggs in such patches by responding to a species specific oviposition deterring pheromone present in the tracks of larvae. Hemptinne et al. (2001) showed that the oviposition deterring pheromone consists of a mixture of alkanes, of which n-pentacosane is the major component (15.1%). These alkanes are likely to spread easily on the hydrophobic cuticle of plants and so leave a large signal. In addition, they are not quickly oxidized - and therefore provide a long lasting signal.

Insect predators often aggregate to patches of high prey density, and use prey chemicals as cues for oviposition. If prey have mutualistic guardians such as ants, however, then these patches may be less suitable for predators. Ants often tend aphids and defend them against predators such as ladybirds. Oliver et al. (2008) showed that ants can reduce ladybird performance. They showed that Adalia bipunctata ladybirds not only move away from patches with Lasius niger ants, but also avoid laying eggs in these patches. Furthermore, ladybirds not only respond to ant presence, but also detect ant semiochemicals and alter oviposition strategy accordingly. Ant semiochemicals may signal the extent of ant territories allowing aphid predators to effectively navigate a mosaic landscape of sub-optimal patches in search of less well-defended prey. Such avoidance probably benefits both ants and ladybirds, and the semiochemicals could be regarded as a means of cooperative communication between enemies. Overall, ladybirds respond to a wide range of positive and negative oviposition cues that may trade-off with each other and internal motivation to determine the overall oviposition strategy.

Despite their use of odour signals to reduce larval cannibalism, Blackman (1967) found that larvae and adults of Adalia bipunctata and Coccinella 7-punctata seemed unable to detect and avoid feeding on unsuitable or toxic aphids. For example larvae of Adalia bipunctata fed on the highly toxic Megoura viciae, when given the choice of a suitable aphid. Apparent preferences were not always for the most suitable food.

 

Defensive behaviour of coccinellids and aphids

Two-spot ladybirds secrete alkaloid (adaline)-rich defence fluid (reflex blood) in response to predator attack. de Jong et al. (1991) collected reflex fluid from individual ladybirds and weighed and measured the alkaloid content by gas chromatography. The amount of fluid produced built up rapidly following winter hibernation in animals feeding on aphids. The concentration of adaline in the fluid was highest in the first bleeding after winter hibernation. A large sample of beetles was reflex bled several times. There was significant variation among beetles in the amount of fluid produced and the concentration of the reflex blood. It is suggested that 2-spot ladybirds are Batesian mimics of 7-spot ladybirds - in other words 7-spot ladybirds are not palatable to predators, so the palatable 2-spot ladybirds have evolved to resemble 7-spot ladybirds to gain protection from predators.

Hajek & Dahlsten (1987) compared the defensive behaviour of three birch aphid species to larvae of Adalia bipunctata, the most common aphid predator on silver birch in northern California. Defensive behavior differed by aphid species. Euceraphis betulae was the most successful escapee. It was highly mobile and frequently walked away from coccinellid larvae. Betulaphis brevipilosa, a flattened, sessile species, was the least successful aphid at actively escaping from two-spot ladybird larvae, but could passively escape detection when coccinellid larvae walked over nymphs and did not perceive them. Active escape behavior was much safer for aphids than passive avoidance of detection. Both instars of Euceraphis betulae and fourth instars of Callipterinella calliptera escaped from coccinellid larvae more frequently when approached from the front, apparently using vision for pre-contact detection.

 

Male-killing bacteria

Male-killing bacteria, which are inherited through the female line and kill male progeny only, are known from five different orders of insect. Werren et al. (1994) showed that Western European populations of the two-spot ladybird bear a male-killing Rickettsia. However, Hurst et al. (1999) showed that the majority of the male-killing lines tested from Central and Eastern Europe do not bear this bacterium. Rather, male-killing is associated with a member of the genus Spiroplasma.

Acknowledgements

For coccinellid identification we have used Hackston and Martin (2016) for the key characteristics, together with UK Beetle Recording and the latest Wikipedia account for each species. For aphids we have made provisional identifications from photos of living specimens, along with host plant identity using the keys and species accounts of Blackman & Eastop (1994) and Blackman & Eastop (2006) supplemented with Blackman (1974), Stroyan (1977), Stroyan (1984), Blackman & Eastop (1984), Heie (1980-1995), Dixon & Thieme (2007) and Blackman (2010). We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure provided by Blackman & Eastop (2006).

Useful weblinks

References

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