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Black Soldier Fly (BSF) Forums | Q&A's and Comments

Below are answers to the most frequent questions from our readers about Black Soldier Flies (BSF). If you have a question you would like us to comment on and share with other readers concerning recycling and BSF, please Contact Us. Once your question is received, it will be reviewed for general interest and, if appropriate, it will be posted below along with a brief answer to the question, or alternatively posted on our Blog page in soliciting comments from other readers of our website.

Q: Is it possible to freeze or store BSF egg clutches in the cold to prolong their viability during shipment?
A: Not at the present time. Egg clutches are very vulnerable to fungal spores, predators, desiccation when the humidity falls below approximately 50%, excess heat and and cold or freezing temperatures. Newborn larvae typically emerge from egg clutches within 4 to 5 days of deposition if stored and/or shipped under optimal conditions (RH > 50%, temperature ranging between approximately 21 up to about 40 C) with a small source of feedstock waste nearby for the newborn larvae to feed on upon emerging from the egg clutches.

Q: Which feedstock waste fed to BSF larvae on average has a better FCR (feed conversion ratio), manure or vegetable-food scrap waste?
A: The FCR for larvae growing off vegetable-food scrap waste (expressed as the ratio of dry weight larval yield versus dry weight of waste added to a bioreactor) is approximately 2-fold higher than that of manure. The FCR for manures in a well-managed farming operation averages around 0.12 to 0.15 compared to approximately 0.25 to 0.30 for vegetable-food scrap waste. This reflects the fact that vegetable-food scrap wastes have a higher nutritive value relative to manure in supporting the growth of larvae feeding off the waste residues.

Q: Is there anything to be gained by drying BSF larvae? It is worth extracting oil from the larvae?
A: For transport and storage of BSF larvae, and for larvae to be used as an additive in animal feeds, drying is necessary to prevent the larvae from spoiling once they have been harvested. In terms of recovering oil from harvested larvae as a biofuel, the extraction of the lipid fraction from an economic perspective is very dubious and should be viewed with a good deal of skepticism (see Is Biofuel from Black Soldier Fly Larvae (BSFL) Hype? | You Decide! ).

Q: How long can fermented food scrap be stored in a fermented state before it can no longer be added to a BSF bioreactor?
A: Food scrap waste stored in a fermented state can be stockpiled over a period of many months with no apparent negative outcomes as a source of nutrient on which BSF larvae feed. The pH of the waste tends to shift downward and plateau at about 3.5 at which point further its decomposition slows to a near halt. The amount of carbon turnover lost in reaching this state is very small, less than a few percent at most.

Q: Is it necessary to shred food scrap before cycling it through a BSF bioreactor?
A: Not necessarily. Food scrap that has been partially crushed or damaged can be added to a bioreactor without further shredding. Larvae cannot easily gain access to many fruits and vegetables, however, unless the outer skin (rind) has an opening through which they can enter. Fermented food scrap can be added to a bioreactor without shredding. Storing food scrap waste in a fermented state is one way of bypassing the need to shred waste using a shredder.

Q: How much and how frequently should one load food scrap into a bioreactor to sustain the growth of BSF larvae under steady-state conditions?
A: This depends upon the composition of the food scrap waste, and to a fair extent on the density of larvae seeded into the bioreactor. Optimal conditions can be determined empirically. For common food scrap made up of a mixture of carbohydrates, vegetables, fruit leftovers (including melon rinds and peelings), scraps of meat  and bone fragments (including fish offal), etc., a generally good rule of thumb in loading and maintaining a bioreactor is to aim for a loading rate of approximately 15 Kg per square meter bioreactor space per 3 day cycle.

Q: Why are there holes in the lids and walls of the secondary bioreactors (BR2s) described on DipTerra's website (see Propagating BSF Using “Box in a Box” Propagation Bioreactors )? Can adult BSF fly out from the holes in these bioreactors?
A: The holes in the walls of the BR2 units serve several purposes. The holes are large enough for adult flies to easily enter and exit the bioreactor even with the lids to the BR2s secured firmly in place. This helps maintain a dark, warm and humid environment above the waste inside the bioreactor which is attractive to females seeking places to deposit their egg clutches. In addition, the holes provide a means for venting and exchanging gases (carbon dioxide and oxygen) necessary for the healthy growth and survival of larvae feeding off waste inside the bioreactors, and they also provide easy exit routes for larvae seeking to temporarily exit the bioreactors, and for prepupae seeking to self-harvest free of the waste, unimpeded by the lids secured atop the bioreactors.

Q: How is the best way to get larvae growing on waste housed in a bioreactor - starting with pupae or eggs?
A: Waste to be used as larval feedstock can be seeded with eggs, seeded with young larvae, or you can start with prepupae/pupae set aside to pupate, emerge as adults and which after mating occurs, will seek out the waste housed in a bioreactor to deposit egg clutches in starting a new generation of larvae. Prepupae/pupae should not be added directly to the waste, but instead left in a bucket or bin that is dry and warm. Young larvae will immediately begin feeding and growing off waste when deposited into the waste. Egg clutches can be flicked atop and mixed into the waste. Any one of these three methods will work.

Q: How can egg clutches be transfered directly into a larval bioreactor and how many eggs should be added to a bioreactor at any given time?
A: Egg clutches can be picked up from any deposit site using a painter's spatula and flicked directly into waste housed in the bioreactor. Drag the spatula tip across the surface of the waste to release any eggs still retained on the spatula. Push the addition of eggs added to the bioreactor to the point where larvae begin to outgrow the supply of waste available for them to feed on upon emerging from their egg clutches. This limit will become evident when the larval density in the waste reaches the point where larvae begin prematurely exiting the bioreactor in search of more waste on which they can feed (see also, Steady-State Farming of BSF Larvae , Steady- State Harvest of BSF Egg Clutches in PBRs - Part I and Steady-State Harvest of BSF Egg Clutches in PBRs - Part II ).

Q: What is the average mass of a BSF egg? How can one estimate the approximate equivalence in larvae produced from a given mass of eggs?
A: The average mass of a larval egg is roughly 25 micrograms (mcg). One gram of fertile BSF eggs therefore, assuming a 100% survival and hatch rate would be the equivalent of approximately 40,000 larvae. Keep in mind, however, that not all egg clutches harvested are fertile, and that the viability of fertile larval eggs can also vary. Egg viability drops precipitously if eggs become exposed to fungi, to desiccation, freeze before larvae can emerge free of the egg clutch, or upon exposure to temperatures in excess of approximately 45 C.

Q: I am interested in farming BSF on a commercial scale. Where can I find bins or troughs for sale in starting up a commercial farming operation?
A: Scaled up commercial farming of BSF requires a very different design in processing larvae relative to the type of bin or trough used in raising BSF on a small scale. On a commercial scale BSF bioreactors used in farming BSF must be designed so that they can be accessed with heavy equipment. Several tons of biodegradable waste per day must be moved through the bioreactors efficiently and with minimal labor input. Bins fabricated out of fiberglass, polypropylene, ABS, polycarbonate, stainless steel, etc., whether purchased wholesale or custom built, are too expensive and ill-suited for use in scaling up BSF production. Far better BSF bioreactors suitable for processing and farming BSF on a commercial scale can be built on site using concrete. Additionally, bioreactors built with concrete can be fabricated at ground level making it much easier using front loaders and other heavy equipment to load and unload the bioreactors on a scale far exceeding the holding capacity of small scale bins and troughs.

Q: I have read on the internet that some people recommend mixing scrap paper and cardboard with food scrap to improve BSF larval growth and yield as is sometimes done in vermicomposting. What are the advantages of feeding larvae food scrap mixed with scrap paper relative to food scrap alone?
A: Larvae are not capable of feeding on or growing on paper waste alone. They lack the essential enzymes needed to breakdown cellulose, the principal component making up paper products. Additionally, many paper products contain toxic inks, heavy metals, dolomite clay additives of no nutritional value, chemical glues and plastic coatings, all of which should be kept out of the food chain and therefore out of the diet of larvae raised as an animal feedstock. Adding paper to larval processing bins and troughs mixed with food scrap furthermore causes an accelerated increase in the rate undigested debris accumulates in bins and troughs which works against efficient production of larvae over the long run. The short answer - no benefits, many negatives!

Q: One of your posts (see On the Road to Commercial Production of BSFL |Sorting Out the Chaff ) states that the FDA presently restricts the sale of insects formulated into or sold as animal feedstock. Where can I find more information on the law?
A: The FDA is authorized by congress to determine the safety of animal feed and food as described in The Defect Levels Handbook.  Additional information on regulatory issues concerning the US FDA’s position regarding insects in animal feedstocks can be found on the FDA’s website, Animal Food and Feeds . The Association of American Feed Control Officials (AAFCO) is a non-profit organization that sets standards for both animal feeds and pet foods in the United States. Generally, the FDA accepts the recommendations of AAFCO regarding the safety and use of additives in animal feeds and pet foods. The blog referenced was posted in August, 2014.  Since then, in 2017 the US FDA accepted AAFCO's recommendation concerning the safety and marketing of dried BSF larvae in feed formulations sold as a salmonid feedstock. In October, 2018, the FDA also accepted AAFCO's  recommendation regarding the safety and marketing of dried larvae in poultry feedstocks. Manufacturers wishing to market larvae for these purposes must however meet the standards for safe production and handling of larvae in accordance with guidelines overseen by the FDA as stipulated under the the Food Safety Modernization Act ( FSMA ) signed into law in 2011.

Q: Can you expose BSF larvae to the sun or do they prefer shaded areas or semi-exposed areas?
A: BSF larvae are photophobic (light fearing) and prefer to feed and grow in places shielded from direct sunlight. In open recycling bins they will burrow under layers of food scrap or waste material to escape direct exposure to light. Adult BSF prefer sunlight and warm temperatures to mate. Males hang out in shaded areas and mate with females in mid-flight. Females prefer to lay eggs in dark crevices hidden from direct sunlight. Recycling bins should be located in semi-exposed areas. Direct and prolonged exposure to sun can cause food scrap and compost recycler bins to overheat inducing heat stress among larvae driving them out of the bins.

Q: What type of food is ideal for BSF larvae? Is cat or dog food acceptable?
A: Ordinary food scrap trimmings, left over produce, vegetable waste, discarded fruit and any kind of produce in general, moldy breads, cakes, stale rolls, fish scrap (including bones), spoiled grains and cereals, spoiled yogurt, left over casseroles, etc. works well. Food scrap should be cut, shredded, mashed or broken up to increase the surface area and allow larvae better access to the discarded waste as it decays. Avoid cooking oils, heavy gravy and fats. Meats and bones are fine so long as they do not exceed about 10 to 15% of the total mix of waste. Cheeses or anything dense and heavy needs to be cut into thin sections or broken up into small chunks. Otherwise larvae and microbes will have a much harder time breaking them down. Cat or dog food could be used, but it is much to expensive. Any dry grains should be moistened as a mash or thick slurry with water before or after addition to a bin.

Q: How long does it take from receipt of BSF larvae before they become adult flies? How long is the BSF life cycle?
A: It typically takes about two weeks at around 85 F (~30 C), a little longer under cooler temperatures, for larvae to grow from newly hatched larvae to their sixth instar (final molting stage) at which point they emerge from decaying waste as prepupae seeking a dry dark place to pupate and transition into an adult BSF. It takes another roughly two weeks from the prepupae stage before they emerge from their puparia as adults. Over the next 4 to 5 days as adults they seek a mate before dying off. Females lay eggs near decaying waste which hatch in 3 to 4 days starting the cycle over again.

Q: What sort of feed conversion ratio (FCR) do you observe in scaling up production of BSF grown off of food scrap waste?
A: At DipTerra LLC we average a FCR [wet weight food scrap:dry weight larvae] of ~19:1. Assuming the moisture content of food scrap to be on average approximately 80% (total dry matter, 20%), this means that the conversion ratio of food scrap into insect biomass is ~ 26%. In round numbers 20 MT of food scrap (wet weight) yields about 1 MT of larvae (dry weight).

Q: The drain system in your Black Soldier Fly Food Scrap Recycler draws from the bottom of the recycler, but the drain pipe is about 3" above the bottom which would leave about 3" of liquid in the bottom. Is there a reason for this offset? Do you tilt the food scrap recycler to empty it or just capture the liquid (leachate fraction) that overflows from the drain port?
A: The design allows aerobic and anaerobic bacteria to work synergistically in combination with larvae in accelerating the decomposition of food scrap added to the recycler. There is no need to tilt the bin to drain the fluid. Collect the overflow from the bin in a milk jug or other container, dilute it 50 to 100 -fold in tap water, and add it to your garden and flower beds to improve your soil ecosystem in facilitating the growth of healthy plants around your house and in your garden (see Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil ).

Q: Are larvae not a good way to start a food scrap recycler?
A: It is perfectly fine and excellent to start your food scrap recycler with larvae. We sell pupae to help people get up and going with their recyclers who don't have access to larvae, and who want to prime their recycler with large numbers of larvae. Pupae are stable for shipment and can be seeded near your recycler (and inside a mosquito net enclosure if you are using netting to keep out unwanted flies and gnats). Each female adult BSF emerging from its puparium mates and lays upwards of 200 to 500 eggs. So pupa help jump start the larval population. It takes 3 to 4 weeks for the adults to emerge from their puparia depending upon the temperature of the environment around your recycler (see The Effect of Temperature on Emergence of Black Soldier Flies ). So it is really a matter of how much of a hurry one is to get the system going. Even without larvae or adults, decaying food scrap will attract adult BSF from the wild, and they, too, will lay eggs and hence get your system up and going. Houseflies and gnats can in certain situations be a nuisance  in competing initially for food scrap left in your recycler. They will soon be overcome by BSF larvae which tend to dominate once they get established in a recycler. We also sell a BSF starter kit which is a mixture of larval eggs, newly hatched and young larvae, prepupae and pupa. Bottom line - starting with larvae is perfectly fine.

Q: How do I download an eBook after paying for it?
A: When placing an order you will be asked to provide the email address where you wish to receive the eBook. After we receive your order, your eBook will be transmitted as a PDF file attachment to the email address you specified. To open up the eBook, place your mouse pointer on the file icon attached to the email and double click it. The file will open up and display on your computer screen. You can at this point print out a hard copy for your own personal use, or alternatively save the file on your computer hard drive, copy it to a memory stick or make a CD for viewing at a later time.

Q: How much should leachate from food scrap processed by Black Soldier fly larvae be diluted before applying it to lawns and gardens?
A: Generally most all plants will tolerate leachate diluted 50-fold or more in tap water applied either as a foliant spray or directly around the base of the plant. Avoid adding concentrated leachate directly to plants or roots without first testing whether the plant can tolerate direct application. Many perennials and larger shrubs do well with leachate added around their base undiluted. However, tender shoots and newly planted vegetables and flowers can be over powered by adding leachate without first diluting it 20-fold or more in tap water. Conditions can vary depending upon the quality and characteristics of your soil and the plants receiving an application. A 50-fold dilution can be prepared by mixing about ¼ cup of leachate into a gallon of tap water. Add enough to wet the soil but avoid over watering (e.g., flooding) the soil with leachate. Allow sufficient time between applications to observe how your plants are responding.  Avoid too frequent of applications in the same area. You can also apply the leachate in a sprayer attached to your garden hose and dial in the dilution, or apply it as a foliant spray. Ammonia in the leachate fraction recovered from processed food scrap is typically in a range of about 0.1 to 0.2% by weight of the undiluted leachate, and in combination with other byproducts can in some instances be too much for your tender plants to handle in undiluted form. It is fine to apply the leachate undiluted into soil in early spring before planting commences (see Amending Soil with Black Soldier Fly Processed Food Scrap Leachate and Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil ). In this situation, allow at least one to two weeks to pass before planting in soil treated in this manner. Mix the soil to a depth of six inches to one foot and water it well after you have added the leachate in this manner to ensure its even dispersal throughout the soil.

Q: Will BSF larvae damage my plants growing in my vegetable garden, especially if they escape from my food scrap recycling bin?
A: No! BSF larvae feed solely on dead decaying organic matter. They do not feed on living plants or root systems and instead seek out decaying organic debris, feeding on the organic byproducts, decaying cells and microbes present in waste materials susceptible to decomposition. BSF larvae are incapable of directly breaking down and digesting cellulose, the most abundant carbon material on earth. So long as a plant is living and not undergoing decay, BSF larvae have no means of growing off of the plant. Once a plant dies, microbes capable of breaking down cellulose provide a means for BSF larvae to access the nutrients in the plant.

Q: Are adult BSF harmful, for example, do they spread diseases?
A: No! They do not feed on organic matter, and they do not pick up and transmit infectious bacteria. They are classified by the USDA as a non-pest insect. Unlike the common housefly, adult BSF do not fly from one waste pile to another as they seek out a mate during their adult life cycle. They furthermore do not bore into plants in laying eggs. The live no longer than a few days to upwards of a week while seeking a mate, and after mating, lay their eggs near decaying waste, usually in nooks and crannies protected from direct sunlight, then die off, leaving the next generation to then hatch and carry on the same life cycle.

Q: Do I need tilted ramps inside larvae incubators to help the larvae self-harvest?
A: No!  They make BSF bins cumbersome, add extra expense to the assembly of the bin and are unnecessary.  Pre-pupa have no problem climbing vertical walls.  They can be grown in totes, buckets, barrels or composting bins and they will still crawl out.  You can facilitate their exit, if you are concerned and want to enhance their ability to climb by sanding the inside of your bin ith a coarse sand paper using horizontal strokes.  To harvest  larvae, place a collection container underneath the bin. Position the container so that it lies in vertical alignment with the edge of the bin. Pre-pupa drop into the container on crawling over the edge of the lip of the bin.

Q: How can I retain larvae harvested in a container so that they don't wander off?
A: If you simply want to harvest clean larvae and intend to empty your container frequently, add about 1-2 inches of water in to the "catch" container.  Larvae crawling out of the bin will fall into the water, float and remain alive for many days.  Collection of larvae is very simple.  Use a fine mesh fish net from a pet store or a butterfly net.  Alternatively, place small pebbles, bark chips, wood shavings, ample layers of straw, small twigs, etc. inside the harvesting container. Larvae dropping into the container will burrow beneath the cover material looking for a hiding place away from light and remain there until they emerge as adults or are then collected from the container. The larvae are looking for a dark hiding spot. Make it easy for them - they will stay out of sight while remaining where you want them.

Q: What is the maximum density of larvae one can realize in a BSF bin?  
A:  If a nursery is used to produce large numbers of larvae which are added continuously to bins, larval densities of upwards of half a million larvae or more per square meter (~47,000 /sq. ft.) can be easily achieved. However, it is important to keep in mind that the number of larvae expressed per unit volume of waste, or surface area, is not very relevant as an index in tracking larval production. Larvae do not distribute evenly in the waste they feed on. Additionally, the number of larvae in waste can vary substantially depending upon the age of the larvae, temperature, the nutritional nature of the waste, and other environmental conditions. Even though seeding young larvae continuously into waste will elevate the overall larval count, it is possible to overpopulate waste with larvae. Adding too many larvae in waste can lead to overcrowding and competition for space causing the larvae to grow less rapidly and also inducing young larvae to prematurely self-harvest. A better guide in maximizing and assessing larval production is to monitor larval mass recovered per unit time relative to the mass of waste processed. Larval production in bin/trough layouts generally average conversion rates of waste into larval biomass (on a dry weight basis) when managed carefully in the range of 25%.

Q: What are the best food sources for growing larvae and what types of  feed-stocks and materials should I avoid?  
A: Food waste rich in vegetal, carbohydrates and fruit scraps are especially ideal for growing BSF larvae.  Also good are fruit pulp and vegetal trimmings.  Green leaves and grass trimmings are OK, but dry grass, hay, straw and dry leaves are not.  Do not use more than 15-20%  of the total residues added to bins in the form of cheeses or animal entrails (they will stink).  Avoid manure, pet poop and human excrement (they stink and pose additionally a health hazard).  Do not add plastic  or bags in BSF incubators even if they are labelled biodegradable, compostable, etc.  Materials rich in cellulose, especially lignin-rich materials such as sawdust, mulch, wood shavings, twiggs, roots, dry leaves, straw, corn stover, etc. are not degraded efficiently by larvae and tend to take much longer to breakdown. The latter, as a general rule, are better processed in compost piles.

Q: What sort of production of larvae might I expect from my BSF bin?
A: For a well managed one square meter bin, larvae fed with coarsely ground food waste at a daily rate of ~ 1 lb/sq.ft. (~5 kg/square meter) at an external temperature of ~ 86 F (30 C) the average production of larvae is ~ 1 lb (~0.5 kg) per day.

Q: What is the efficiency of converting food waste into BSF larvae?  
A: Food conversion efficiency depends on many factors. In the case of BSF larvae the best conversion efficiency is ~ 2. On a wet weight basis larval yield is about 10% of the weight of food scrap added to a processing bin. Food waste is typically about 70-80% water while that of larvae is about  55% water.

Q: Is it true that adult Black Soldier Flies do not eat or drink?
A: They do not need to eat to mate and they lack mouth parts as adults necessary for feeding on food. It is not true about drinking. They need to drink water. When they are dehydrated the flies will not mate.

Q: The Black Soldier Flies in my nursery do not lay eggs - what is wrong?
A: Direct solar (natural) light exposure for at least a couple of hours a day is very important in encouraging mating of BSF.  Keep your nursery temperature at least 77 F (25 C). Do not crowd cages with shelves, bushy artificial plants and boxes. Adults need space to fly and mate.  Adults need too drink water to keep from dehydrating but will drown if water is left around in pans or buckets. Use a mister or make certain that the feeding bin is kept moistened so that they always have access to water.

Q: If I extract fats from BSF larvae to make biodiesel, can I still use the residue that is left over?
A: The residue is rich in proteins, sugars, organic nitrogen, phophorus, calcium and vitamins.  After removing all solvents used for lipid extraction, the residue can be used as feed for chicken or fish. Chitin is also present and in large quantities merits consideration as raw material in the pharmaceutical and food industries. 

Q: Do BSF larvae accumulate hazardous chemicals?
A: There is no evidence for the accumulation hazardous chemicals such as heavy metals, hormones, pescticides, and antibiotics on larvae raised on food scrap wastes, nor would hazardous materials be expected in presenting larvae with feedstocks meant for human consumption. There is some risk of heavy metal contamination, pesticides, pharmaceuticals, etc. on larvae harvested from sewage sludge, or any other waste materials known to have been contaminated with hazardous materials. The market and subsequent use of larvae must be assessed relative to what the larvae are fed and the environment in which they are raised. 
Q: Is it true that BSF larvae eliminate pathogenic microbes such as E.coli?
A: Due to the feeding activity of BSF larvae on microbes, including E.coli, decreases in numbers have been observed, but they are not totally eliminated.

Q: What do I need to grow BSF larvae in Oregon, Washington and Canada or other temperate zones experiencing cold winters?
A: You need heat and sunlight.  Grow BSF during the summer outdoors, and indoors in the winter. House them in a space that has plenty of access to natural light and which can be maintained warm throughout the colder winter months.

​​Q: When I add the liquid (leachate) recovered from my food scrap recycling bin to my plants it inhibits instead of stimulating plant growth - why is that?
A: Almost all liquid (leachate) fractions recovered from composted waste, fermenting food scrap, or even BSF processed food scrap, contain small amounts of transient phytotoxic (plant toxin) compounds. A very common class of known phytoxic compounds are the volatile fatty acids such as acetic acid, propanoic, butyric, caproic and longer chain fatty acids produced by anaerobic microorganisms present in the treated waste.  Additionally, trace amounts of alcohols are often present, particularly ethanol, and to a lesser extent propanol and butanol - all known phytoxic compounds.  You can read more about these issues in our blogs " Recycling Food Scrap? | What"s with the Stink? " and " Are Waste Products in Your Food Scrap Inhibiting Plant Growth? ". As a general rule, dilute the liquid fraction upwards of 50 fold in tap water (about 1/3rd cup liquid mixed into 1 gallon of tap water). Apply this to your plants. You should observe a markedly improved response of your plants following this treatment of the liquid recovered from your food scrap bin.

Q: I am having difficulties in getting eggs to hatch after females have deposited them in cardboard flutes. What's wrong?
A: We do not recommend using cardboard flutes for collecting eggs because of the high humidity and presence of molds often carried on cardboard. It is very easy for the eggs to get contaminated with molds that will quickly overtake and kill young larvae as they emerge from the eggs. Additionally, molds destroy the eggs before they hatch. Cardboard cannot be easily sterilized once it picks up mold spores. For all of these reasons, we strongly recommend using plastic lids having fine grooves or tight corners molded or cut into the plastic, or solid wood blocks with narrow grooves cut into the surface as better collector ports for larval eggs. Female BSF will hunt out and deposit eggs in almost any tight, narrow hole or groove in proximity to decaying organic matter. A groove or hole approximately 1 to 2 mm across, or even slightly wider, is plenty of room for encouraging egg laying females to leave clutches in the tight grooves. Eggs tend to darken as they age to a deeper yellow-orange color, so nonviable eggs left deposited over time will darken up and ultimately turn dark brown as they age through oxidation of pigments left behind in the eggs.  Another, although less common cause of nonviable eggs is dessication - make certain that the eggs are not left out where they become dry since this will quickly kill viable eggs.