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Large walnut aphid, Dusky-veined walnut aphidOn this page: Identification & Distribution Biology & Ecology Competition Other aphids on Juglans regia Damage & Control
Identification & Distribution:
All Panaphis juglandis viviparae are alate. The fourth-instar nymph (first picture below) has transverse rows of brown patches on the dorsal abdomen. The adult alate (second picture below) is large and stout-bodied. It has a darkened head and thorax, and a yellow abdomen with dark brown transverse bands. The forewing veins are fuscous-bordered. The siphunculi are short and truncate. The body length of the Panaphis juglandis alate is 3.5-4.3 mm.
Panaphis juglandis live on the upper sides of leaves of European walnut (Juglans regia), in rows along the veins, and are sometimes ant-attended. Sexual forms occur in September-October. The species is rarely found together with the small walnut aphid (Chromaphis juglandicola ), apparently because Panaphis juglandis is adversely affected by the rain of honeydew from Chromaphis juglandicola. Panaphis juglandis occurs in Europe, and parts of central Asia, Pakistan and was introduced into the western USA.
Biology & Ecology:
Both adult and immature Panaphis juglandis live and feed on the upper side of the leaf and have aposematic coloration - contrasting blocks of yellow and dark brown - to dissuade predators.
Nymphs have large dark spots on a bright yellow background (see first picture below). The adult alate has bold yellow and dark brown stripes (see second picture below).
Aposematic coloration is typical of aphids that live in very exposed situations, rather than those that feed on the leaf underside. There is no point having such coloration if it is not clearly visible to potential predators. It is unclear whether the aphids are really distasteful to vertebrate predators (juglone, which is found in walnuts, is toxic to other plants but not, as far as we know, to herbivorous insects), or whether they are imitating a very common warning pattern used by many organisms (known as Mullerian mimicry).
The same colour pattern can be either aposematic or cryptic, depending on the distance and angle of viewing. We have suggested this for the aphids Tuberolachnus salignus and Macrosiphoniella absinthii (see InfluentialPoints blog ), and the same probably applies for the nymphal coloration of Panaphis juglandis. From a distance the line of nymphs could appear simply as a yellowish discoloration of the leaf caused by a leaf miner or plant disease. Such an effect is enhanced by the black spots which break up a nymph's visual outline -
One notable feature of large walnut aphid colonies is that they are concentrated along the main leaf vein, with rarely any along the side veins (see pictures below).
They mostly follow a very regular head-to-tail distribution on the leaf lamina on one side, or usually both sides, of the mid-vein. Just occasionally a few nymphs will also sit on top of the vein (see picture below), rather than beside it.
Feeding from the main leaf vein presumably optimises the feeding flow from the phloem, and perhaps some protection from wind.
Given the importance of walnut as a crop in some parts of the world, there has been some research into the aphid's ecology and control. Olson (1975) carried out some of the earliest work on Panaphis juglandis when he looked at the effect that photoperiod and temperature had on morph determination. He found that sexual forms of this aphid appear only in autumn when day length decreases and temperatures begin to drop. Initially, a few sexual females appear along with the parthenogenetic females. As the season progresses and temperatures continue to get cooler, the production of the parthenogenetic females is inhibited, and male aphids then appear along with the sexual females.
Jaskiewicz & Kmiec (2007) studied the population dynamics of aphids colonizing walnut trees in relation to weather conditions in the years 2003-2005 in Lublin, Poland. The weather in spring apparently had no effect on the date of hatching of the larva of the fundatrix, which was usually observed in the third 10-days' period of May. On the other hand, the dry period, heat waves (above 30°C) and stormy rainfalls affected the drop in the aphid population observed in July and August. The maximum population of Panaphis juglandis was found in the second 10-days' period of June, while that of Chromaphis juglandicola was at the turn of June and July. Chromaphis juglandicola was dominant in 2003, whilst Panaphis juglandis was dominant in 2004.
Karczmarz (2012) carried out a similar study two years later, with the additional aim of comparing numbers in three habitat types: garden, park and street-side. The greatest number of aphids was reported in the year 2007 in the park location. The course of weather conditions significantly affected the spring and autumn presence of aphids. A warm spring with not very intense precipitation enhanced the development of Panaphis juglandis. However, torrential rains and air temperatures exceeding 30°C in summer limited the numbers of this aphid. In all the study years the highest mean fecundity was that of the fundatrices, and the lowest was found in the females of sexual generation. The most fecund parthenogenetic females were observed in spring 2006 in the street location. The highest number of eggs was reported in 2008 in the park location.
There is no mention in the literature of Panaphis being attacked by fungal pathogens (Entomophthora), but we have found specimens apparently killed by fungus as shown in the picture above.
Atlihan et al. (2015) monitored the seasonal abundance of the aphid species Chromaphis juglandicola (small walnut aphid) and Panaphis juglandis and their parasitoids and predators at two sites in Lake Van Basin in Turkey from 2005 to 2006. Populations of Chromaphis juglandicola remained low at both sites in both years, but populations of Panaphis juglandis began to develop in early summer and reached their highest level, which was considered above the economic threshold, in mid-summer. Populations then declined and fluctuated at a low level until the end of the season. Predators belonging to Coccinellidae, Anthocoridae, Miridae, Chrysopidae and Hemerobiidae were present. Adalia fasciatopunctata revelierei, Chrysoperla carnea and Orius spp. were the most abundant predators. Trioxys pallidus was the only parasitoid parasitizing both aphid species in walnut orchards and it appeared to be effective in suppressing populations of the aphids to low levels. In Lake Van Basin, pesticides are usually applied to suppress the codling moth (Laspeyresia pomonella) which is the major pest in walnut orchards. This practice causes not only an extra cost and residue problems, but also negatively impacts the natural biological control agents. Management programmes for controlling the codling moth should take Panaphis juglandis and its natural enemies into consideration.
Although several authors report that Panaphis juglandis may be attended by ants (e.g. Karczmarz, 2012 ), we never found a colony attended. With unattended colonies, leaves close to the colony are often covered with honeydew which attracts a variety of insects - including ants - which glean the available nutrients. The pictures below show (first) a horse fly (Tabanus sp.) feeding on honeydew and (second) a Lasius ant feeding on honeydew, in each case from Panaphis juglandis colonies.
Wani & Ahmad (2014a) & Wani & Ahmad (2014b) examined how the two species of walnut aphids Chromaphis juglandicola and Panaphis juglandis interact on walnut. Panaphis juglandis is much larger than the Chromaphis and lives entirely on the upper surface of leaves. Chromaphis juglandicola, on the other hand, feeds on lower surface of leaves (see picture of adult alate below).
Both aphids obtain their nutrition from the phloem tissue. In walnut orchards of Kashmir valley these two aphid species are not usually found together on same trees or orchards. Walnut orchards infested with one species usually do not have other. In laboratory studies, using potted walnut seedlings, competition with Chromaphis juglandicola proved to be an important factor limiting population growth rate of Panaphis juglandis, which survived an average of 12 days when competing for nutrients. The preference of Panaphis juglandis for upper leaf surface subjects it to a constant rain of honey dew and debris from colonies of Chromaphis juglandicola on the lower surface of higher leaflets. Contamination of upper leaf surface with Chromaphis juglandicola honeydew also helps limit the population of Panaphis juglandis, which survived an average of eight days when exposed to Chromaphis juglandicola honeydew, but an average of 20 days when not exposed to either honeydew or competition for nutrients.
The two aphid species Panaphis juglandis and Chromaphis juglandicola have recently been reported (Magnussen & Hansen, 2014 ) on leaves of common walnut (Juglans regia) in a botanical garden in Oslo, Norway. The two species have not previously been reported from Norway. Such distributional change may well be a consequence of ongoing climate change.
Other aphids on the same host
Blackman & Eastop list 8 species of aphid as feeding on walnut (Juglans regia) worldwide, and provide formal identification keys. Of those aphid species, Baker (2015) lists 3 as occurring in Britain: Chromaphis juglandicola, Myzus persicae and Panaphis juglans.
Only Chromaphis juglandicola (see picture above of alatiform fourth instar nymph) and Panaphis juglandis are specific to Juglans regia.
Damage and control
Walnut (Juglans regia) is one of the most important nut crops in the world. Walnut trees are susceptible to variety of pests and diseases including two aphids, Panaphis juglandis and Chromaphis juglandicola. In some areas, such as Kashmir in India, the walnut aphids are the most serious pests (Wani & Ahmad, 2014c ). Their feeding reduces tree vigour, nut size, yield, and quality. In addition to direct feeding damage (such as the direct leaf feeding damage, shown below) they excrete copious amounts of honeydew that falls onto nuts, leaves and shoots.
Honeydew supports growth of the black sooty mould fungus. This fungus reduces light penetration to the leaf surface reducing its photosynthetic capacity. Being black, it also absorbs heat to predispose nuts to sunburn and subsequent kernel quality loss due to high temperatures. High populations of aphids may also cause leaf drop, exposing more nuts to sunburn. If heavy populations are allowed to develop (i.e. > 15 aphids per walnut leaflet) and remain for as little as 14 days uncontrolled, current season's nut quality is reduced.
Akkopru et al. (2015) evaluated walnut cultivar resistance to the dusky-veined walnut aphid. They collected life table data of this aphid reared on five cultivars of walnut under field conditions. The raw data of the developmental time, survival rate, and fecundity was analyzed using the age-stage, two-sex life table to account for the variable developmental rate and stage differentiation among individuals. Bootstrapping was used for the estimation of standard errors of developmental time, longevity, fecundity, and other parameters. The long immature developmental time, short adult longevity, short reproduction period, and low fecundity resulted in the key population growth parameters (net reproduction rate, intrinsic rate of increase, and finite rate of increase) being the lowest when aphids were reared on the Fernor cultivar. Those reared on 'Akça I' exhibited the highest population growth parameters. It was concluded that cultivar 'Fernor' has the highest level of resistance to Panaphis juglandis.