|Ladybird populations in 2011 - a record year|
|The dry and very warm
Spring, dry Summer, followed by a fine and warm Autumn,
meant that many invertebrates had their most successful
year since 2007. One of the most noticeable of all, to
the general public at least, was the number of Ladybirds
that were to appear in the latter part of the Summer.
Despite the fact that Ladybird numbers were indeed very high and that the breeding season was a very successful one, their very high numbers were as much down to reaching a four or five year cyclical peak, as the suitable conditions.
Ladybird numbers have been steadily building each year since 2007 as part of this cycle, whilst the same can also be said for many other invertebrates that had a bumper year.
populations and parasitism in 2011
As is possible with any invertebrates, Ladybirds are always susceptible to annual or cyclical variations within localised or national populations, with potential influencing factors including weather conditions, availability of food, parasitism and predation in the egg, larval and pupal stages. A build up in the numbers of any species, will always be counter-acted by greater instances of parasitism. It's the natural way of populations being maintained to a sustainable level. Instances of parasitism within the 7-spot Ladybird for example, were low in 2010 but increased greatly during 2011.
|We recorded the
percentage of parasitism within local populations of the
7-spot Ladybird during 2011 and found that one in five
adults were affected at all sites we visited. We have
still to record any similar parasitisation affecting our
other large ladybird species, including the Harlequin.
Even at sites where both species are commonly found, only
the 7-spot seems to have suffered from such parasitism.
Parasitisation of the 7-spot is due to the wasp Perilitus coccinellae, which is a parthenogenic endoparasitoid of adult ladybirds. After locating a suitable ladybird, the wasp lays a single egg between the ladybird's abdominal plates. On hatching, the larva eats the ladybird's fat store leaving the vital organs alone, but when ready to pupate, bites through the ladybird's six motor-neurones that control leg movement. When ready to pupate, the larva breaks out of the host's abdomen and spins a cocoon between the host's legs. The ladybird cannot move but is still alive and it is believed that the ladybird's bright colours and reflex bleeding helps protect the developing wasp until it emerges.
movement is regained once the wasp has emerged, but this
seems limited and the affected ladybird has an almost
drunken stagger when walking and is less able to grip and
it will struggle to find food.
We have noted a four or five year cyclical peak in the numbers of some Ladybirds and Shieldbugs before, with the last definite peak coming in Autumn 2007 and presently (November 2011) another peak appears to have been reached. 2011 was an excellent year for many of our Ladybirds, but Pine Ladybird (Exochomus quadripustulatus) numbers still remain relatively low since having a very good year in 2006, when several large aggregations were found at Holborn Hill Plantation near Budby, whilst numbers of the Orange Ladybird (Halyzia sedecimguttata) have yet to reach the numbers we recorded back in 2003.
Most of the smaller Ladybird species are generally recorded in small numbers on any one day, or at any one particular site. This is nothing new, but the 14-spot Ladybird (Proplylea quattuordecimpunctata) really had a bumper year in 2011 and we personally recorded it in record numbers. Newly hatched 14-spot adults reached peak numbers when they appeared in late July when 61 were counted on a short walk at Gamston Wood near Retford. The Adonis Ladybird (Hippodamia variegata) was another species to have a good year and single adults were found well away from their usual habitat, turning up at urban localities at Mapperley in Nottingham, on a house wall on Chesterfield Road in Mansfield and in a garden at Market Warsop. Another species to do well was Cream-spot Ladybird, recorded regularly throughout the late Spring and Summer.
Harlequin and 7-spot Ladybird population levels in 2011
Late July saw enormous numbers of 7-spot Ladybird (Coccinella septempunctata) emerge, when site counts would literally have numbered thousands at Rufford CP and at a brownfield site just outside Edwinstowe for example. At Rufford CP, the path underneath an avenue of Lime trees, was littered with hundreds of dead 7-spots that had been crushed underfoot, but few Harlequins were found at this time.
|Although breeding as
early in the year as the 7-spot, Harlequin numbers
remained very low throughout much of 2011, but we have
found this to be quite typical in any year since their
arrival in the county.
Their favoured habitat is in the tree canopy and this makes them less obvious. Even during tree top searches for the Hazel Pot Beetle at Sherwood Forest CP in May and July, only very small numbers were ever recorded.
It was only from October onwards that Harlequins really began to become more obvious, yet at all times throughout the year, their numbers never matched those of the 7-spot.
Despite being much more the common of the two species, large numbers of the 7-spot rarely recieve the public attention reserved for its more notorious relative.
|The reason is
probably down to preferred wintering habitat. During the
latter half of the year, groups of 7-spot Ladybirds can
be found clustered together on Thistles, Gorse, Brambles,
young Pines, coarse grasses and in leaf litter etc.
The graph produced above shows the highest monthly counts of both Harlequin and 7-spot during 2011. It was only during October and November that adult Harlequins were recorded in sufficient totals as to show on the graph. We actually recorded no Harlequins at all before April, which has proved to be quite typical every year since they first arrived in Nottinghamshire.
Early November 2011 saw us record some huge numbers of both 7-spot and Harlequin Ladybirds at many sites, but the best numbers came on a warm afternoon at Warsop Wood on 06/11/11, when conservative estimates of numbers of both species were minima of 3, 000 7-spot and 2, 000 Harlequin. The Harlequins were particularly attracted to the light coloured trunks of Beech trees on site (they are attracted to bright objects and surfaces) whilst 7-spots were found on virtually all types of vegetation at or near ground level. The ground vegetation and leaf litter was so full of 7-spot Ladybirds that it was never possible to find a square metre of ground that didn't have some 7-spots within it. Despite the huge numbers, it was still surprising when we rolled over a large section of felled Beech trunk and found a large number of Harlequins clustered in two groups.
|This was the
first time that we had recorded such a large aggregation
of adult Harlequins. When checked again a week later,
when their numbers had swelled to approximately 264
adults. Away from urban locations, the more usual
over-wintering site for adult Harlequins that we have
noted in previous years, has been under loose bark and
occasionally tucked into the unopened growth buds of
small Pines. When under loose bark, we have found them
associating with 2-spot Ladybirds, which share similar
over-wintering preferences. On Pines, they will mix with
The Harlequin Ladybird - a threat to our native species?
We have written many times about this recent addition to Nottinghamshire's fauna, yet still the mere mention of the Harlequin Ladybird on TV, radio and in the press, coincides with gloomy predictions on it's potential affects to other invertebrates. This now familiar Ladybird has been in the UK since 2004 and Nottinghamshire since 2006, yet from our own records, there is still no evidence to show that the forcasted potential demise of an estimated 1, 800 species of invertebrates, will be directly linked to the Harlequin's appearance or is still any nearer. Of the two species, we have always found the 7-spot to be the more aggressive (try moving a finger towards one on a hot day) and it seems to have been forgotten, that many 7-spot Ladybirds actually bit people during the big 'invasion' year of 1976.
One of the early concerns was that our resident Ladybirds would be most affected, but there is still no sign from our records of any real decrease in numbers that can be directly attributed to the Harlequin. If any of our resident species are to be affected, the most likely will be 2-spot, 10-spot, Kidney-spot and Orange Ladybirds as these are often found on the same trees favoured by the Harlequin. We have yet to see Harlequins in any numbers on Pine sp, so Cream-streaked, Striped and Eyed Ladybirds are unlikely to decline.
|The Harlequin has
suffered through the use of images showing large numbers
of over-wintering Harlequin Ladybirds, which have been
heavily used in anti-Harlequin propaganda since 2004. The
term 'swarm' has also been used by the press, in a manner
that quickly helped engage public bad feeling towards
them. Indeed, it was even deemed acceptable for the
public (and even some naturalists) to be actively
encouraged to kill them.
The Harlequin's variety of colouration, marking and spotting, coupled with much of the general public's inability to differentiate between the Harlequin and 7-spot Ladybird at a glance, suddenly meant that the one beetle actually liked and known by the public, suddenly became the victim of the other's notoriety, meaning that 7-spots were killed by people mistaking them for Harlequins.
Adult Harlequin Ladybirds have a strong preference for foraging on both Lime (Tilia sp) and Sycamore (Acer pseudoplatanus) two trees especially common in street planting schemes, but they can be found on a wide variety of other trees and shrubs.
|Where these trees occur
in more rural locations, Harlequin Ladybirds will often
be found in equal numbers. Large numbers of adults are
made more conspicuous by congregating on walls, gates,
fences and litter bins etc, mean that the Harlequin is
considerably more obvious to the general public in urban
localities, especially late in the year prior to
over-wintering. Over-wintering will occur in buildings
and houses if there is access and large aggregations can
sometimes build, yet an aggregation of perhaps 100
over-wintering Harlequins, merely just happen to be a
great deal more concentrated than 100 over-wintering
We have found that only late in the year when Harlequin numbers reach a peak and Aphid numbers decline with the onset of cooler conditions and leaf-fall, will any remaining larvae turn to cannibalism through starvation in order to reach the pupation stage. We have only recorded Harlequin larval cannibalism late in the year, with larvae in the process of pupation or newly formed pupae being affected. It is important to point out that we have also noted this within large numbers of 7-spot Ladybird larvae. It should be considered very probable, that the larger larvae of any Aphid eating Ladybird, will eat smaller larvae in times of food shortage.
partly defoliate to conserve water loss in extremely dry
years such as 1976 and 2011, or the late Autumn, is there
ever likely to a shortage of Aphids, inducing Ladybird
larvae to turn cannibalistic. By September though, most
of our native species have long since reached the adult
After hatching in late July 2011, most Ladybirds spent the rest of the year relatively dormant, often remaining as small groups in leaf litter or on vegetation till October and November, when activity seemed confined to sunning themselves or begin searching for more suitable over-wintering places. This seemed very surprising as the weather remained dry and warm till late in the year. Successful over-wintering should still see good numbers in the opening months of 2012, but if the same happens as in 2008, then Ladybird numbers are likely to decline suddenly next Spring. Much depends on the weather and other contributing factors affecting the populations of our well-loved native Ladybirds and the (we believe) still over-hated Harlequin.