[MAIPC] end of field season thoughts from the SLF frontline

[Richard Gardner]

Thoughts on 2018research on the Spotted lanternfly, Lycormadelicatula, in Berks County PA

Richard Gardner 

Nov. 14, 2018

 

  From field research I have been doing this year the Spotted lanternfly, Lycorma delicatula, is an insect of ecotones. Locally we havefour distinct ecosystems: urban, suburban, rural and forest. Three of theseecosystems are primarily ecotones: urban, suburban and rural.  To this point the three most common foodplants in order of preference appear to be Ailanthusaltissima, Vitis sp. and Celastrus orbiculatus. Acersaccharinum, an ornamental tree common near where I live,appears to be another food source when A. altissima and Vitis sp.are not available.

  Urban ecosystems tend to befragmented with few if any forested areas more than 100 yards across. Mostly, theyare a series of vacant lots, small hedgerows between properties, utility right-of-ways and similar disturbed areaswhere plants grow. Additionally, there are domesticated trees planted bymunicipal authorities and landowners. Manmade surfaces abound where SLF eggscan be deposited and vehicles to transport SLF across the landscape. Thedistances between parts of the ecotone appear to be easily traversed by SLF withouthuman help since they are often short. Therefore, this appears to the mosthighly infested of the four local ecosystems.

  Suburban ecosystems are lessfragmented than urban areas but have similar characteristics in having vacantlots, disturbed areas between properties and utility right-of-ways with fewdeeper forested areas. Landowners and local government bodies plantdomesticated plants, like urban governments, but on larger tracts of land. Thelargest difference is that there tends to be more space between buildings andlarger patches of land where plants can grow with fewer manmade surfaces andvehicles. Still the distance between parts of this ecotone are relativelyshort.

  Rural ecosystems have more openspace and larger blocks of trees, yet with the same patchwork of hedgerows,abandoned tracts of land, utility right-of-ways and similar as urban andsuburban ecosystems. The biggest differences are that the hedgerows can bedeeper/longer, there are small forests scattered across the landscape with manyfewer vehicles and manmade surfaces and the distances between parts of theecotone are further.

  Forested ecosystems tend to belarge areas of deep forests with longer and fewer edges even though roads,trails and utility right-of-ways run through them. This is critical becausemost of the plants that the SLF feeds on appear to be ecotone plants, notplants of the deep forest. I seldom find A. altissima, Vitis sp.and C. orbiculatus more than a few yards deep in forests, except wherean ecotone was created by geological features, fallen trees or human disturbance.I have yet to find SLF on any of the forest trees beyond the edges of anecotone. Therefore, this appears to be the least heavily infested of the localecosystems I have investigated.

  Each ofthese ecotones has different challenges in SLF control. Urban areas haveclosely spaced ecotones separated by roads of varying width and utility actingas minor boundaries for SLF spread and more people which apparently enhance SLFspread. Suburban and rural areas have decreasing numbers of roads with decreasingtraffic loads and fewer people making the spread of SLF slower. Forested areasare the slowest for the spread of SLF because there are fewer people tofacilitate its spreading and food sources tend to be further apart.

 To determinewhich woody plants are susceptible to SLF predation, analysis of thenutritional content of their sap needs to be done. Use Ailanthus altissimaas a baseline since from observation it is the plant with the heaviestinfestation and the one it feeds on in its original home. First test qualitativelyfor overall sap components of A. altissima. Then test quantitively for totalsugars, proteins, fats, specific sugars and micronutrients. Compare this datato data from either specific species SLF may be using as an energy source ormembers of their families. Using sugar content as the primary test of plantdesirability it can be assumed plants with the highest sugar content are preferredfood.

 Another partof this is to run the same quantitative tests on the waste SLF produces on A.altissima to determine the amount of sugar and/or other nutrients in thewaste, comparing it to the same from other potential food sources. The higherthe sugar content in the waste, potentially the higher the sugar content in thetree because apparently the excess sugar will be in the waste produced by theSLF.

 A morecomplex and accurate predictor of plant preference is the analysis of theutility a plant has for the SLF. Utility is the amount of benefit an organismderives from a specific resource. U = (pU-c)/T. Utility = (potentialUtility-cost)/Time. Potential utility is the maximum utility which can beobtained with no cost. Costs can be related to the sugar concentration of thesap (either too low or too high to use without additional energy expenditure), adifferent primary sugar than Ailanthus, sap viscosity and potentialtoxins in the sap which need to be dealt with, hardness of the bark, thicknessof the bark or noxious/toxic chemicals in the bark. Time can either be by lifestage from egg to senescence, end of a (the) reproductive cycle or a discreteunit of time such as minutes, hours or days. Environmental factors such as airtemperature, bark temperature, humidity, amount of direct/indirect sunlight onthe food source, state of the food source – bud break, full growth, dormancyand the amount of rain – flood, drought and time from most recent rainfall maychange the utility values. The higher the quality of the food and the greaterease of access, the more utility it has. Hence, the higher the U value, themore energy for growth and reproduction.

 This mayhave a gender component as it is generally accepted that in most species maleshave a much lower reproductive cost than females. Therefore, males may be ableto use a resource of lower quality or less of a high-quality resource thanfemales because of their lower breeding cost. If this is true, then it helpsensure his progeny and the reproductive viability of the species by reservingeither higher quality resources or more of a higher quality resource forfemales to maximize their reproductive success.

 Egg layingis an aspect which is confounding me. There appear to be mixed strategies ofsingle females laying eggs and covering them relatively far from other femalessuch as different trees/surfaces and group egg laying either contiguous to ornear each other. This becomes more complicated because it appears that one SLFfemale may lay eggs close to the eggs of another female with the second femalecovering both sets of eggs. Then there are the eggs which are not covered whichadds another dimension to the puzzle. The large communal egg masses are muchless common than egg masses randomly scattered on a single tree or across thelandscape on a variety of plants and surfaces. So far, I have found eggs onwhite birch, black birch, pignut, choke cherry, wild grape, silver maple, boxelder, oak sp. and most commonly Ailanthus.

 All the egglaying strategies can be reduced to game theory in the same way determiningfood sources is. The biggest mistake is to assume that what we see in this areais not reflective of where the SLF originated. Egg masses scattered around alandscape may ensure lower egg predation in the home habitat. Whereas, eggmasses on a food source ensures that hatching nymphs have a readily availablefood source. Large masses of eggs in a small area may ensure that if eggpredation occurs, some of the eggs will survive. The problem with assigningvalues to variables such as predation and proximity to food is that we do notknow what the conditions are in the original habitat. When the SLF becameestablished here the variables changed. What was a good strategy in Asia, maybe a neutral or negative strategy here. Or, the strategy is good here for differentreasons than in Asia. The scattering of the eggs across the landscape in Asiamay have avoided predation, but here allows for the efficient movement ofmultiple generations of SLF across our landscape. The one constant is that theegg laying and other survival strategies are rapidly evolving to meet the newchallenges offered by our ecology as it is different than the home ecology ofthe SLF.

Ailanthus has been isolated from the SLF since themid-1700’s when seeds were brought from China to Paris. Next the tree went toLondon before coming to Philadelphia after the end of the American RevolutionaryWar in 1784. As often happens, when a defense is no longer needed it willeither cease to exist or exist at a very low level. It will be exciting towatch the changes in Ailanthus over time with the reintroduction of thisthreat to it and the possibility that the tree by itself will control the SLFby bringing back or reinventing defense mechanisms to this specific threat.*

 A finalpoint is that the SLF was introduced in this country only a few generations ago,perhaps four generations, but most probably several more. What will happen inthe next several years is hard enough to guess. What may happen beyond that isbeyond our ability to comprehend at the present time. That the SLF we areseeing are derived from one to a few parents is important. The fewer parentsthe more limited the gene pool. This means that the SLF does not have the fullgenetic toolbox of where it came from to deal with multiple new challenges suchas predators, disease and foods (which may be toxic) in its new home. Therelays our greatest hope – that the SLF will encounter a challenge which willeither control it or hopefully eradicate it.

 

*The wild (European)parsnip Pastinacasativa L.apparently decreased its defenses when introduced to the European NorthAmerican colonies in the early 1600’s due to the lack of a principal herbivore- the parsnip webworm, Depressaria pastinacella. Defenses built back up with the accidentalreintroduction of D. pastinacella in the late 1800’s. (Increase in toxicity of an invasive weedafter reassociation with its coevolved herbivore, Arthur R.Zangerl and May R. Berenbaum, PNAS October 25,2005 102 (43) 15529-15532.)

 

 
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