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Identification & Distribution

All adult viviparae of Monelliopsis pecanis are alate. These alatae (see first picture below) are yellow with reddish eyes, and have the dorsum either unmarked or with a few dark spots. The antennae are pale apart from the apical parts of segments and the section of antennal segment III bearing secondary rhinaria which are dark (cf. Monelliopsis bisetosa, which has the antennae mainly dark except at the bases of segments). The rhinaria-bearing section of antennal segment III is only slightly swollen: up to 1.5 times the basal diameter of the segment (cf. Monellia caryella, which has the rhinaria-bearing section conspicuously swollen: it is more than 1.5 times the basal diameter of the segment). The subcoxa (= coxa base) of the hind leg has no transverse mark (cf. Monelliopsis bisselli, whose subcoxa of hind leg usually has a dark transverse streak). The distal ends of the hind-femora (and to a lesser extent the fore-femora) are marked with conspicuous black patches. The tibiae are mainly pale (cf. Monelliopsis bisetosa, which has dark tibiae).

Note: Monelliopsis pecanis was first named and described by Bisell (1983). It was previously confused with the similar Monelliopsis nigropunctata which is now known to seldom occur on Carya illinoiensis, and not at all on Carya aquatica. See also Quednau (2003).

First image above copyright Domingo Zungri under a Creative Commons Attribution 3.0 License.
Second image above copyright Louis Tedders, USDA under a Creative Commons Attribution 3.0 License..

The alatae of Monelliopsis pecanis usually have 2-5 spinopleural hairs on each of abdominal tergites I-V in spring, and 4-6 spinopleural hairs on each segment in midsummer; in summer a few of the hairs may have dark bases, especially on tergite V (cf. Monelliopsis nigropunctata, which has 4-6 spinopleural hairs on each of abdominal tergites I-V in spring, and 5-8 spinopleural hairs on each segment in midsummer; most hairs have dark bases, so Monelliopsis nigropunctata usually has more dark spots per segment than Monelliopsis pecanis). The spinopleural hairs on abdominal tergites I-V are often hardly raised above the surface, especially those on tergites I & II (cf. Monelliopsis pleurialis, in eastern North America, which has the spinopleural hairs on tergites I & II arising from low processes). The body length of the adult Monelliopsis pecanis alate is 1.2-1.7 mm.

Immature Monelliopsis pecanis (see second picture above) are pale yellow-green and have long capitate hairs.

Image above copyright H.C. Ellis, University of Georgia, Bugwood.org under a Creative Commons Attribution 3.0 License.

The yellow pecan aphid lives and feeds on the leaves of pecan (Carya illinoiensis, see picture above) and bitter pecan (Carya aquatica), and does not host alternate. Populations are concentrated along the main leaf veins. Sexual forms develop in autumn, and the oviparae deposit overwintering eggs in bark crevices of pecan. Monelliopsis pecanis is indigenous to USA and Mexico, but has been introduced to Egypt, Sicily and South Africa.

 

Biology & Ecology

Life cycle

Entoweb of Oklahoma State University provide information on the life cycle of Monelliopsis pecanis. This species overwinters as eggs deposited in bark crevices along the trunk and limbs. Nymphs hatch in April, move to the new spring foliage, and begin to feed. The nymphs mature in about a week and become fundatrices, which produce large numbers of young asexually. These in turn produce many generations (15 to 30) of alate viviparous females each year (see picture below of alate vivipara giving birth to nymphs).

Image above copyright Domingo Zungri under a Creative Commons Attribution 3.0 License.

Beginning in September, wingless oviparae (see picture below) and winged males develop from some of the nymphs produced.

Image above copyright Louis Tedders under a Creative Commons Attribution 3.0 License.

These mate and the oviparae lay black overwintering eggs.

Natural enemies

Edelson & Estes (1987) carried out a survey of the seasonal abundance and distribution of predators and parasitoids associated with Monelliopsis pecanis and Monellia caryae in Georgia State, USA. The most abundant predators amongst the lacewings Chrysopa rufilabris, Chrysopa quadripunctata (see picture below), Micromus posticus and Coniopteryx simplicior.

Image above copyright Robert Webster under a Creative Commons 4.0 International Licence.

Amongst the coccinellids, the commonest were Olla abdominalis, Coleomegilla maculata, Hippodamia convergens, and Allograpta oblique. One parasitoid species, Aphelinus perpallidus, and one hyperparasitoid, Alloxysta schlingeri, were identified from aphid mummies. Predators were most abundant at times of greatest aphid abundances.

van den Berg & Maritz (1996) surveyed the predators of the non-native Monelliopsis pecanis in the eastern parts of South Africa. The coccinellids Cheilomenes propinqua and Scymnus ? morelleti were the most important predators of the yellow pecan aphid. Although these predators reduced yellow pecan aphid numbers, they were unable to maintain populations of the aphid at economically acceptable levels. The introduction of other natural enemies, especially host-specific parasitoids, was recommended.

Avila-Rodriguez et al. (2016) looked at the population dynamics and parasitism of Monellia caryella, Monelliopsis pecanis and Melanocallis caryaefoliae in pecan orchards with different insect pest management programs in northern Mexico. Monellia caryella and Monelliopsis pecanis showed two periods of peak population in May-June and September. Melanocallis caryaefoliae showed only one population peak in September. The lowest populations of aphids occurred in the pecan orchard with organic management. Parasitism was zero in Monelliopsis pecanis, very low in Monellia caryella (0.3%) and greater in Melanocallis caryaefolia (25.9%). The highest level of parasitism was in the pecan orchard with a conventional management program. Aphid parasitoids and hyperparasitoids belonged to the Aphelinidae, Eulophidae, Encyrtidae, Torymidae and Braconidae families.

 

Other aphids on the same host

Monelliopsis pecanis has been recorded on 2 species of hickories (Carya aquatica, Carya illinoiensis).

 

Damage and control

Damage

The most noticeable damage caused by yellow pecan aphids results from their deposits of honeydew on the foliage. Honeydew is a sticky fluid excreted by aphids, and it supports the growth of sooty mould fungi. This fungus reduces the amount of sunlight reaching the photosynthetic cells of the leaflets. Heavy spring infestations can injure a tree's vascular system and reduce chlorophyll development and leaflet size. Some studies show aphids also impair overall tree growth. Heavy late summer infestations can defoliate trees. If defoliation occurs too early in the season, from any source, nut size and fill can be adversely affected. In addition, if the leaf loss occurs too early, then the nut set for the next year can also be affected and winter injury could occur. For more information see Entoweb.

Control

There has been considerable research into biological and integrated control of pecan aphids because of repeated cases of the development of insecticide resistance necessitating excessively frequent insecticide applications.

Ellington et al. (1995) report on several biological and cultural control methodologies for the control of pecan aphids. Traditionally the coccinellid Hippodamia convergens has been collected from hibernating aggregations (see picture below) and released in agricultural systems for pecan aphid control. Unfortunately they cannot be reared artificially and they tend to disperse when released, albeit in the more northern parts of the pecan range Dr William Reid notes that the aphids are generally adequately controlled by this coccinellid without making additional making supplementary releases.

Image above copyright John Rusk under a Creative Commons Attribution 2.0 Generic License.

Harmonia axyridis (an introduced Asian species) looks much more promising for releases. Spiders may also have a part to play in aphid control especially those of genus Aysha which overwinter under the bark and may consume aphids and their eggs in late winter and early spring. Decreased use of insecticidal methods for control of other pecan pests like the pecan nut casebearer (Acrobasidis nuxorella) and their replacement by e.g. Bacillus thuringiensis would increase the number of insect predators. The degree of orchard groundcover may determine the abundance and hence effectiveness of natural enemies. For example Bugg (1991) demonstrated that in pecan orchards cover cropped with rye and vetch, the mean density of aphidophagous coccinellids was six times greater than in unmowed resident vegetation and 87 times greater than in mowed grass and weeds. The hope was that a combination of cultural and biological control techniques may reduce aphids to the point at which insecticides may be required infrequently.

Kunkel & Cottrell (2007) looked at the oviposition response of the chrysopid lacewing Chrysoperla rufilabris to pecan aphids and potential attractants to increase chrysopid numbers on pecan. The lacewing laid more eggs on seedling trees infested with the Monellia caryella than on seedlings infested with Melanocallis caryaefoliae. Lacewing larvae consumed pecan aphids irrespective of species. One of the attractant/food spray treatments applied to trees that were tested significantly increased green lacewing oviposition for three of the five treatment dates over both years. They concluded that combinations of attractants and food sprays could indeed be used to enhance green lacewing populations in orchards.

University of California provide information on insecticidal control of yellow pecan aphids which may be appropriate in the southern part of their range: Aphid numbers should be monitored often as they can increase rapidly and exceed economic thresholds quickly. Scouting for yellow pecan and black margined aphids should begin in May and continue every 4 to 5 days throughout shoot and nut development. The undersurface of 5 compound leaves on at least 10 random trees throughout the orchard should be sampled to give a total sample of at least 50 compound leaves. An insecticide should be applied before June 1 if honeydew is accumulating. Between June 1 to August 15, an insecticide should be applied if the total number of aphids exceeds an average of 20 per compound leaf. Between August 15 to leaf fall, an insecticide should be applied if the total number of aphids exceeds an average of 10 per compound leaf.

Insecticides that preserve beneficial insects will decrease aphid populations over time. One such insecticide long-used for controlling yellow pecan aphid is the systemic imidacloprid. Although it is traditionally applied to the soil early in the season, foliar application is recommended later in the season, because imidacloprid is slowly taken up by the roots and translocated throughout the plant. By the time imidacloprid builds up to high enough levels in the leaves, aphids are already present and have damaged leaves. Foliar application may also help to slow the development of resistance, because aphids are not subjected to sublethal doses of imidacloprid, which may be the case with soil application. Researchers have some evidence of resistance to imidacloprid in California, Texas and New Mexico. Furthermore, growers have observed what appears to be imidacloprid resistance in Arizona.

Acknowledgements

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used 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

  • Avila-Rodriguez, V. et al. (2016). Identification, population dynamics and parasitism of aphids in pecan in the Comarca Lagunera, Mexico. 85, 249-255. Full text

  • Bissell, T.L. (1983). A new species of aphid, genus Monelliopsis on pecan. Journal of the Georgia Entomological Society 18, 71-77.

  • Bugg, R.L. (1991). Cover crops and control of arthropod pests in agriculture. pp 157-163 In :W.L. Hargrove (ed). Cover crops for clean water. Soil and Water Conservation Soc., Ankey, Iowa. Full text

  • Edelson, J.V. & Estes, P.M. (1987). Seasonal abundance and distribution of predators and parasites associated with Monelliopsis pecanis Bissell and Monellia caryella (Fitch) (Homoptera: Aphidae). Journal of Entomological Science 22(4), 336–347. Abstract

  • Ellington, J.J. et al. (1995). Biological Control of Pecan Insects in New Mexico. Hort Technology 5(3), 230-233. Full text

  • Kunkel, B.A. & Cottrell, T.E. (2007). Oviposition response of green lacewings (Neuroptera: Chrysopidae) to aphids (Hemiptera: Aphididae) and potential attractants on pecan. Environmental Entomology 36 (3), 577-83. Full text

  • Quednau, F.W. (2003). Atlas of the Drepanosiphine aphids of the world. Part ii: Panaphidini: Panaphidina. Memoirs of the American Entomological Institute 72, 1-301. Book review

  • van den Berg, M.A. & Maritz, M. (1996). Predators of the yellow pecan aphid, Monelliopsis pecanis (Homoptera: Aphididae), in the eastern parts of South Africa. African Plant Protection 2(2), 123-125. Full text