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Aphid Predator (Coleoptera: Coccinellidae)
Larch ladybirdOn this page: Identification & Distribution Biological Control of Aphids: Classical biological control of spruce adelgids Predation of spruce aphids Biology & Ecology: Developmental rate and prey consumption Oviposition behaviour Overwintering Sex ratios & colour polymorphism
Identification & Distribution
Adult larch ladybirds (Aphidecta obliterata) are elongate oval in shape and are usually light tan brown in colour. They have a dark oblique patch of variable size on the rear third of each elytron and various other small blotches and spots (see first picture below). The oblique patch is sometimes completely absent (see second picture below). There are other colour varieties of Aphidecta obliterata, including one with entirely black elytra. The suture (where the elytra meet) is darkened. The pronotum is whitish beige with a brown W mark. The legs are brown.
Second image copyright S. Rae under a Creative Commons Attribution-Share Alike 2.0 Generic license. The fourth instar larva of Aphidecta obliterata (see third picture above) is light grey with dark grey tubercles. The marginal and lateral tubercles of abdominal segment 1 are orange. The tubercles bear rather short hairs with no spines.
Second image copyright S. Rae under a Creative Commons Attribution-Share Alike 2.0 Generic license.
The fourth instar larva of Aphidecta obliterata (see third picture above) is light grey with dark grey tubercles. The marginal and lateral tubercles of abdominal segment 1 are orange. The tubercles bear rather short hairs with no spines.
Aphidecta obliterata is found on conifers, especially larch (Larix), but also spruce (Picea), pines (Pinus) and firs (Abies). There it feeds on aphids and (perhaps less readily) on adelgids. It overwinters in bark crevices. This coccinellid is found throughout Europe and into Asia Minor, and has been introduced into North America.
Biological Control of Aphids
Classical biological control of spruce adelgids
In America Aphidecta has been used in a 'classical' biological control programme (in other words introduction of a natural enemy from an invasive organism's native country) in an attempt to control the balsam woolly adelgid (Adelges piceae). This adelgid was accidentally introduced from Europe to America around 1900. The first picture below shows two wax-covered feeding adult sistens of Adelges piceae on fir tree bark.
European firs, especially European silver fir (Abies alba), can support large adelgid populations, and yet remain relatively unharmed. North American species are much more sensitive to attack, and many trees have been killed. Asian firs seem intermediate in sensitivity; some are damaged and some are not.
Several species of predators have since been introduced to North America from other parts of the world, in the hope of controlling Adelges piceae. Six of them from Europe have become established in the USA, one of which was the larch ladybird (see first picture below). Attempts to introduce it in Eastern Canada were unsuccessful because of the excessively cold winters (Brown & Clark, 1959), but Amman (1966) describes its successful introduction to North Carolina in USA.
Second image copyright Francisco Welter-Schultes under a under the Creative Commons CC0 1.0 Universal Public Domain Dedication.
Another successfully introduced predator was the coccinellid beetle, Scymnus (Pullus) impexus (see second picture above). Others were Laricobius ericsoni (a derodontid beetle, a specialist adelgid predator) and Aphidoletes thompsoni, Cremanifania nigrocellulata and Leucopsis obscura (all flies).
Despite the successful introduction of predators, none unfortunately have achieved any detectable level of control, apparently because they feed on lifecycle stages of Adelges piceae that are unimportant in population regulation. Further information on the damage caused by Adelges piceae, and methods of control, are given by Ragenovich & Mitchell (2006), and the Invasive Species Specialist Group (2015).
Predation of spruce aphids
The larch ladybird occurs naturally throughout Europe, and contributes to the natural control of numerous species of conifer aphids. In southern England we have found it predating colonies of the mealy spruce aphid (Cinara costata), a colony of which is shown in the picture below.
Most colonies of this species are densely coated in wax which is often considered as defensive in purpose against predators. It did not seem to deter this larch ladybird, which consumed several aphids while we were watching - wax and all (see two pictures below).
Biology & Ecology
Developmental rate and prey consumption
Witter (1969) studied the developmental period and feeding behavior of Aphidecta obliterata on the balsam woolly adelgid, Adelges piceae. The coccinellid completed its developmental period (egg through pupal stage) in 53 days at 15°C and 75% RH. There was a straight-line relationship between larval size and the number of balsam woolly adelgid eggs consumed per capita per day. The average number of adelgid eggs consumed per capita during larval development was 1853, with a range from 1579 to 2456. In the 4th stage, larvae consumed more eggs per capita than they did in the 1st 3 stages combined. The consumption of prey per capita was reduced when predator density increased.
Parry (1992) found that Aphidecta obliterata was the main coccinellid species feeding on both Elatobium abietinum on Sitka spruce and Adelges cooleyi on Douglas fir during the pre-diapausal imaginal period. Coccinella septempunctata and Adalia decempunctata were found on Sitka spruce only following emergence from the pupa and rapidly dispersed as the aphid prey was absent. The superiority of Elatobium abietinum (see picture below) as a diet in comparison with Adelges cooleyi was shown by the higher dry weights of those adults from larvae feeding on the former.
A higher fat to dry-weight ratio on Sitka spruce was additional evidence supporting this diet superiority. A low respiration rate in comparison with post-diapause individuals indicated the necessity to conserve energy during the absence of prey in summer. Those coccinellids on Douglas fir exhibited a further respiratory rate depression in autumn which may be associated with overwintering energy conservation. There was no evidence of differential elytral melanic pigmentation between the two populations.
Timms et al. (2008) looked at whether the conifer specialist Aphidecta obliterata was better adapted to spruce than the generalist Adalia bipunctata. They 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 the aphid Elatobium abietinum.
Timms & Leather (2008) compared the responses of Aphidecta obliterata (considered a specialist) with that of Adalia bipunctata (considered a generalist) to prey limitation. They found no significant difference in the consumption of prey between the two coccinellid species. The results suggest that both of these coccinellids are well adapted to low-density-specific prey. There were no obvious differences between the two, which would tend to favour either species in an environment of limited prey.
Oliver et al (2006) looked at how the larch ladybird (Aphidecta obliterata) assessed the quality of potential oviposition sites. Gravid females were deterred from oviposition by larval tracks of the same species and the effect was density dependent. Females actively avoided searching in these contaminated areas. Tracks induced a significant effect on oviposition for up to three days. Tracks of the generalist coccinellid Adalia bipunctata or the chrysopid Chrysoperla carnea did not induce any oviposition response in larch ladybird females. Increasing aphid density induced increased oviposition rate in larch ladybird females. Extracts of conspecific egg-surface chemicals inhibited oviposition by larch ladybird female. In the field, cannibalism, competition and limited food availability represent the major threats to egg and larval survival. Patch quality assessment mechanisms enable females to lay eggs at sites where offspring survival is maximized.
Parry (1980) looked at the overwintering of Aphidecta obliterata in north east Scotland. He found that measured supercooling temperatures were lower in adults with empty guts (around -30°C) than in those with full guts (around -13 deg C), and were not affected by sex. Hibernating adults were able to supercool to between -8 and -18°C, this ability decreasing with time. No polyhydric alcohols (organic molecules, such as glycerol and ethylene glycol, that have more than 1 hydroxyl group) were isolated from the insects, and the water content was not related to supercooling ability, but it decreased significantly following a period of low precipitation or isolation in the absence of free water. A decrease in fat reserves was observed in adults prior to overwintering, while they remained constant during hibernation and decreased further during the post-hibernation period. Both pre-hibernation and hibernating adults showed a decreased respiration rate in comparison with post-hibernation adults, indicating the attainment of a true state of diapause. Lack of water, low temperature and possible fungal attack appeared to be significant mortality factors.
Sex ratios & colour polymorphism
Parry & Peddie (1981) found that male to female sex ratios of Aphidecta obliterata varied from 65:35 to 45:55 in samples from different areas immediately following adult emergence. During autumn the sex-ratios of samples from branches changed with time to give increased proportions of females which were inseminated prior to diapause. On branch samples in spring females outnumbered males by 3:1. No differential mortality occurred during overwintering beneath bark scales. Dark coloured elytra were commoner among male adults but light elytra were the most numerous in both sexes. A greater proportion of individuals with dark elytra survived until spring. Melanic pigmentation on both elytra and head capsules was greater in females. Mean length and width was greater in females.
Eichhorn & Graf (1971) noted that Aphidecta obliterata shows a pronounced sex dimorphism both in colour and size. Colour polymorphism of the elytra is sex-linked and restricted to females. It ranges from complete yellow (colour group I) to complete black (colour group V). The greater majority of female belong to colour group II. The sex-ratio is remarkably constant in the different areas, there being a variation in females between 60.7% and 65.4%. Aphidecta obliterata is a typical coccinellid, so far as the number of autosomes is concerned, but unusual in that the sex-determining system is XX/XO. Mated females of colour group IV/V (dark forms) produced about 6 times as many daughters of the colour group IV/V than females of colour group II. This means that the determination of the colour pattern is partly hereditary.